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Controlled Nanoscale Electrohydrodynamic Patterning Using Mesopatterned TemplateClick to copy article linkArticle link copied!
- Swarit DwivediSwarit DwivediMultiscale Computational Fluid Dynamics Laboratory, Department of Chemical Engineering, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal 721302, IndiaMore by Swarit Dwivedi
- Raj NarayananRaj NarayananMultiscale Computational Fluid Dynamics Laboratory, Department of Chemical Engineering, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal 721302, IndiaMore by Raj Narayanan
- Rahul ChaudharyRahul ChaudharyMultiscale Computational Fluid Dynamics Laboratory, Department of Chemical Engineering, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal 721302, IndiaMore by Rahul Chaudhary
- Rabibrata MukherjeeRabibrata MukherjeeInstability and Soft Patterning Laboratory, Department of Chemical Engineering, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal 721302, IndiaMore by Rabibrata Mukherjee
- Arnab Atta*Arnab Atta*E-mail: [email protected]. Phone: +91 3222 283910 (A.A.).Multiscale Computational Fluid Dynamics Laboratory, Department of Chemical Engineering, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal 721302, IndiaMore by Arnab Atta
Abstract
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We report the path for a possible fabrication of an array of nanogrooves, by electro-hydrodynamic instability-mediated patterning of a thin polymer film using a patterned stamp with much larger features. Using a predictive computational model based on finite element method, we find the route to control the coalescence of initial instabilities that arise with the onset of spatially varying DC electric field generated through topographical patterns in the top electrode. These quasi-steady structures are shown to evolve with the electrostatic and geometric nature of the two-electrode system and are of a stable intermediate during the process of feature replication, under each electrode feature. We identify conditions to obtain nanogrooves for a range of operating conditions. Such simulations are likely to guide experiments, where simultaneous optimization of multiple parameters to fabricate features with lateral dimension smaller than that of the electrode patterns is challenging.
Copyright © 2018 American Chemical Society
1. Introduction
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In a time when miniaturization is the need of hour, rapid and effective techniques of synthesizing nano/mesoscale patterns are in high demand. These structures are used in various fields of nanobiotechnology, (1) lab-on-a-chip devices, (2) microfluidics, (3) hydrophobic and self-cleaning surfaces, (4) bulk heterojunction solar cells, (5,6) organic light-emitting diodes, (7) and so on. Consequently, researchers have turned to various novel patterning techniques to engineer surfaces and structures that are not easily manufactured by conventional top-down lithographic methods. (8) However, lithographic techniques are generically expensive, meticulous, and often has limitations either in terms of throughput or resolution. (9) Recently, spontaneous instability-mediated morphological evolution in ultrathin films, particularly in conjugation with lateral confinement, is being considered as a viable nonlithographic mesofabrication technique for soft materials. (10) Such instabilities result because of applications of thermally excited capillary waves engendered by van der Waals force-mediated disjoining pressure and are thus limited only in ultrathin films (h ≤ 100 nm). On the other hand, much thicker films can be destabilized by the action of an externally applied field, such as an electric field or a thermal gradient. Although the stability and dynamics of fluid–fluid interfaces subject to an externally applied electric field was studied first by Swan (11) in 1897, it was only around the year 2000 when Schäffer et al. (12) pioneered the concept of electric-field-mediated instability for patterning thin polymer films, which has been termed as electro hydrodynamic (EHD) lithography. EHD instability-mediated patterning, which uses a capacitor system with thin/ultrathin films of polymers, provides excellent ability of morphology control, in comparison with direct embossing-based techniques. (8,13−15) Apart from thin films of homopolymers, films of various functional materials such as conductive polymers, block copolymers, inorganic thin films, and so on have been patterned by EHD instability. (16−19) Fabrication of high aspect ratio structures was accomplished by Lv et al. (20,21) in their recent studies.
The destabilizing effect of electric forces on the free surface of a thin film has been well-explored by numerous researchers. (22−30) A significant advantage of using EHD is the accomplishment of control over the length scale of the surface patterns. (31−33) However, required external source of energy, breakdown of dielectric polymer at high voltages, and involvement of several influencing parameters are challenges to this technology. (8,34) Schäffer et al. (12) found that careful control of EHD instabilities can be exploited to generate well-defined pillars because they could replicate a patterned top electrode on a sub-micrometer lateral dimension. Various experimental observations with a flat top and bottom electrode show the evolution of isotropic structures with a dominant wavelength that corresponds to the fastest growing mode of instability, which governs the spatiotemporal evolution and the final morphology of the features. Theoretically, an expression for the fastest growing mode of instability can be obtained from a linear stability analysis, (12,35,36) the expression for which is
where γ, ψ, ϵ0, ϵp, and Ep are interfacial tension, applied voltage, permittivity of free space, relative permittivity of polymer film to the bounding fluid, and electric field intensity at the polymer–air interface, respectively.
(1)
It can be seen that the characteristic wavelength is a function of both the geometry and electrostatic parameters of the system, which is an interplay between the electrostatic and surface tension forces. Additional control and ordering of the instability features including possible miniaturization can be achieved by using a topographically patterned top electrode, where patterns are seen to replicate below each protrusion of the patterned top electrode. Under a patterned top electrode, the gradient in electric field strength because of the presence of heterogeneity strongly influences the evolution process, and consequently the pattern periodicity (Lp) and geometry of the top electrode determine the final morphology, in addition to initial film thickness (h0), gap spacing (d), and field strength. Several theoretical and numerical studies have been carried out to mathematically elucidate the patterning process as well as to identify the conditions for the formation of ordered patterns on single and/or multilayer thin films. (2,34,37−51) With a nonlinear 3D model for polymer/substrate morphology, Verma et al. (52) identified the ideal conditions necessary for pattern replication and indicated that the number density of the replicated patterns can be altered by varying the applied voltage or tuning the mean film thickness, periodicity, and depth of grooves on the top electrode. Moreover, they predicted the possible formation of secondary structures between two parallel ridges, which was also experimentally substantiated by Harkema. (53) The formation of secondary structures has been related to the depletion of liquid during the formation of primary continuous ridges. (54) Atta et al. (55) showed the self-assembly of ordered nanopillars on the introduction of marginal chemical heterogeneity on the surface of electrode assembly. Electrically heterogeneous patterned electrodes have also been used for the fabrication of re-entrant structures which are beyond the fabrication capability of imprint lithography. (56) Recently, researchers have also investigated EHD patterning of prepatterned surfaces in search of novel and miniaturized structures. (57,58)
On the basis of a careful review of published literature, we identify that there is no clear study that elaborates the morphological evolution of stable intermediate structures, particularly capturing the transition of single to multiple pillars under a solitary protrusion of the top electrode. In this study, we report an intermediate structure in case of an insufficient peak splitting during the transitional regime from single to multiple pillars under one protrusion that results in a toothlike cavity, termed as nanogroove. We identify the relation between width of the protrusion (w) and λc, as the effect of Lp on λc is already well-perceived. A numerical model based on finite element method is employed to investigate the influence of filling factor, initial film thickness, applied voltage, protrusion height, and electrode spacing on the obtained quasi-steady-state patterns. These results are of paramount importance in designing, optimization, and fabrication of ordered nanostructures, which particularly will be able to guide toward the desired morphology avoiding undesired coalescence of patterns. Fabrication of such nanoscale structures using a stamp with a much larger lateral dimension is the key novelty of this work, particularly from the standpoint of nanofabrication.
2. Mathematical Formulation
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2.1. System Description
Figure 1 shows the schematic of the EHD assembly, where the bottom electrode is coated with ultrathin polymeric film of initial thickness of h0, and the confined space above the film is occupied by air as the bounding fluid. The electrode assembly with variable d(x) because of protrusion at the top electrode results in heterogeneous electric field, where as the flat plate assembly produces homogeneous electric field in lateral direction. Along with gap spacing (d), other parameters that define a heterogeneous system, such as electrode width (w), pattern height (p), and stamp periodicity (Lp) are depicted in Figure 1b. Figure 1c represents a 3D structure of the physical system with columnlike top-electrode protrusions. The interfacial perturbation with the onset of electric field by applying a DC potential of ψ at the top electrode and keeping the bottom as ground results in film thickness h(x,t) as a function in space and time domain. This thin-film system is considered to be isothermal, incompressible (ρ2 = 1000 kg/m3), Newtonian (μ = 1 Pa·s), and perfect dielectric (ϵ2 = 2.5) with air–polymer interfacial tension (γ) as 0.038 N/m.
Figure 1
Figure 1. Schematic of electrode assembly with polymeric liquid film denoting the geometrical parameters and top electrode as (a) flat plate and (b) patterned stamp with square protrusions, and (c) 3D representation of patterned stamp with column protrusions electrode assembly.
2.2. Governing Equations and Boundary Conditions
2.2.1. Fluid Flow
The governing equation for this incompressible Newtonian fluid is defined by the set of Navier–Stokes (eq 2) and continuity equation (eq 3), coupled with Maxwell equations to account for the presence of external electric field.
where fi represents the body forces and the subscript denotes the phases (1 for air and 2 for polymeric film).
(2)
(3)
2.2.2. Boundary Conditions
At the upper and lower electrodes, no slip boundary conditions are applied.
(4)
At the interface, continuity of velocity is assumed.
(5)
The interfacial velocity (ω) under an infinitesimal perturbation is related to the film thickness by the following kinematic boundary condition. (59)
(6)
To simplify and solve the governing equations for this system, we assume (a) two fluids as immiscible with no interphase mass transfer across the interface, (b) the film is sufficiently thick that continuum assumption is valid but thin enough so that the effect of gravity can be neglected, and (c) inertial effects will be negligible because of ultrathin film condition. These simplifications along with the consideration of long-wave approximation (60) give rise to the temporal evolution of thin-film height that can be described by the following expression (61)
where
(7)
(8)
In eq 8, ψ0, ψel, and ψLW represent atmospheric pressure, electrostatic pressure, and disjoining pressure because of Lifshitz–van der Walls (LW) interactions, respectively. Moreover, ψel and ψLW can be estimated from the following equations (36,55)
where AL is the effective Hamaker constant for interactions between the free surface of the film and lower solid surface through the fluid film. In our case, ψLW can be neglected because of the presence of dominant electrostatic force. Detailed derivation of the aforementioned equations can be found elsewhere. (35,38,52,55)
(9)
(10)
3. Computational Model
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As shown in Figure 2, a 2D model in COMSOL Multiphysics (62) has been developed to realize the evolution of the structure because of the electrostatic force and surface tension at the interface of the liquid polymer. The 2D computational model exemplifies columnlike physical system, as shown in Figure 1c in three dimensions. DC potential is implemented between the bottom (boundary 2) and the top electrode (boundaries 5–21). Other boundary conditions for the solution are (a) no slip at boundaries 2, and 5–21; (b) symmetry on boundaries 1, 3, 22, and 23; (c) initial fluid interface at boundary 4; and (d) pressure point constraint at points a and b, which are the two ends of the air film interface. The polymer–air interface is tracked by using conservative form of the level-set method (eq 11), (62) where a phase is characterized by a level-set function φls, which is 0 in case of air and is 1 for polymer. Consequently, the interface is captured by contour of level function φls = 0.5.
where γls determines the amount of reinitialization of the level-set function and the parameter ϵls specifies thickness of the interface. A suitable value for γls (0.1) is the maximum magnitude of the velocity field in the model, and ϵls = hc/2, where hc is the maximum element size across the interface.
(11)
Figure 2
Figure 2. Schematic of computational domain, boundaries, and points used in this work.
Furthermore, the assumptions of film and air as perfect dielectric media with zero net charge in the bulk results in equations governing the force exerted on the interface. This is resolved using the following Laplace equation.
where ϵ0 is the dielectric permittivity of the vacuum, ψ is the voltage, and ϵr is the dielectric constant of the air or polymer, defined as ϵr = 1 + 1.5φls. The level-set function smooths the material property such as density, viscosity, and dielectric constant jump across the interface as follows:
(12)
(13)
(14)
(15)
Two components of the body-force term fi in eq 16 are volumetric forces because of surface tension, Fst = pelδn⃗, and electrostatic force, Fel = pelδ, where the electrostatic pressure pel = γκδn⃗, γ is the surface tension, δ is the numerical interpretation of Dirac delta function that is concentrated at the interface and in level set method, and δ = 6|φls(1 – φls)||∇φls| provides a smooth transition along the interface. n⃗ is the unit outward normal to the interface, and κ is surface curvature estimated as: κ = ∇·n⃗.
4. Results and Discussion
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In case of a homogeneous electric field, EHD-induced patterning is analytically described by characteristic wavelength derived from linear stability analysis of the system. In a system of two flat electrodes (Figure 1a), the gradient of electric pressure at extreme corners is responsible for the initial perturbation of interface, which consequently propagates creating a pressure gradient along the interface. This mechanism is essentially responsible for the formation of nanopatterns in case of a flat electrode assembly. However, in case of a topographically patterned top electrode (Figure 1b), difference in electrical pressure is locally generated over the interface at the level of each feature on the top electrode, and this variation subsequently triggers a flow of polymeric fluid, which manifests in rising morphologies below the protrusions of the top electrode. Fluid drains from the domain between two protrusions toward the region directly under these features. Initially, electric pressure gradient present over the interface and beneath the corner of protrusion is responsible for driving this fluid, which we denote as inflow gradient. An interesting but intuitive morphology is observed when the width of the protrusion is large. Similar to flat plate assembly, a secondary instability is detected, which rises because of the ripple created by the gradient of electrical pressure. For complex systems with patterned top electrode, there is no single characteristic λ owing to varying gap spacing. In such systems, on moving from bottom electrode to the top, we find two gap spacings, d1 and d2, which are minimum and maximum separation distances between top and bottom electrodes. Characteristic wavelength of instability corresponding to d1 and d2 are denoted by λd1 and λd2, respectively. From a number of simulations, it is evident that secondary instability is first observed in between λd1 and λd2, where two separate pillars form under one protrusion, and this leads to a novel and intriguing morphological structure having nanogrooves on the top. In such cases, ripple created by the inflow gradient tries to rip the rising pillar, although it is not strong enough to snap completely. These structures attain quasi-steady state as confirmed by the simulations in our study. In an attempt to estimate this morphological transition and its relation to influencing parameters, we have identified five different electrode widths that are on the basis of characteristic λ (eq 1), as: λd1, λ(d1+davg)/2, λdavg, λ(davg+d2)/2, and λd2 for electrode spacing (d) as d1, (d1 + davg)/2, davg, (davg + d2)/2, and d2, respectively, where davg = (d1 + d2)/2.
4.1. Mechanism of Structural Evolution
Understanding of structural evolution in case of transitional structure not only provides better understanding about the evolution process but also reveal the key parameters that control the transition from single to multiple pillars under one protrusion. Although the flow of polymer is a function of instantaneous electric field distribution, it can be shown that initial electric field distribution can be a fair metric for predicting the final morphology, and the subsequent dynamics. Maximum (Emax) and minimum (Emin) field strengths across the interface are just beneath the center of protrusion and the groove of top electrode, respectively (Figure 3). The inflow gradient drives the fluid from outside toward protrusion and is proportional to (Emax2 – Emin2) across the interface.
Figure 3
Figure 3. Initial electric field intensity distribution on a 30 nm polymeric film under protruded top electrode having w = λd1 = 214 nm, and p = 20 nm for 70 V, d2 = 100 nm, and Lp = 350 nm.
Figure 4 demonstrates the progressive evolution of a 30 nm liquid film under the applied DC potential of 70 V using a 214 nm (λd1) × 20 nm protruded top electrode and gap spacing (d2) of 100 nm. Instabilities can be seen rising from the edges because of the presence of the inflow gradient, which pulls polymeric fluid from outside of the protrusion inward at the edges, as shown in Figure 4b. Thereafter, these instabilities derive the polymeric fluid toward the center of the protrusion in the horizontal direction and vertically upward because of the electrostatic pressure (Figure 4c). Although inflow gradient drives the flow toward the spatial region beneath the protrusions, incoming fluid does not get equally distributed in the whole region creating two bulges, similar to what is observed under a flat top electrode as is evident from Figure 4c. This growth attains a quasi-steady state, when the rising front of a growing pillar touches the top electrode (Figure 4d) and subsequently remains unaltered (Figure 4e).
Figure 4
Figure 4. Temporal evolution of a 30 nm polymeric film under protruded top electrode having w = λd1 = 214 nm, and p = 20 nm for ψ = 70 V, d2 = 100 nm, and Lp = 350 nm at (a) 0, (b) 2, (c) 6, (d) 15, and (e) 100 μs.
4.2. Influence of Filling Factor
The filling factor, defined as the ratio of w/Lp, is a key parameter in systems with topographically patterned top electrode that limits the patterning range. Its effect on the final morphology can be observed by either changing w or Lp at a time. Figure 5 shows the variation in the final morphology when w is varied from λd1 to λd2 keeping ψ = 70 V, h0 = 30 nm, Lp = 350 nm, d2 = 100 nm, and p = 20 nm. It is evident from Figure 5a that single pillar is observed when the width of top electrode is 210 nm (<λd1). Further increase in the width of protrusion, a pillar with nanogroove is observed at w = λd1 = 214 nm (Figure 5b), and for w = λ(d1+davg)/2, the grooves become deeper that touch the ground electrode (Figure 5c). Two distinct pillars are observed at λdavg (Figure 5d), and further increase in w results in even more distant pillars under one protrusion at w = λd2 (Figure 5f). This development is consistent with our observations of initial electric-field distribution (Figure 6), where it is apparent that the gradient of electric field is maximum at the corners of the protrusion and vanishes on approaching the center. The region of low-electric-field gradient increases with an increase in protrusion width (w). Therefore, the final morphology is seen to shift from a high degree of coalescence (single pillar) to partial coalescence (nanogroove structures) to no coalescence (multiple pillars) (Figure 5). The parameters are selected in such a way that periodicity value is sufficiently larger than the electrode width. For comparable values, one may encounter two major issues. First, fabricating within tens of nanometers with accuracy is a problem, and second, the system will start behaving similar to a flat plate because of very small distance between two protrusions. The gap region with low electric field strength will be negligible compared to area under protrusion, and results similar to a flat plate might be obtained. Figure 7 demonstrates the filling factor variation by changing the pattern periodicity Lp from 250 to 400 nm, keeping voltage at 70 V, film thickness h0 = 30 nm, electrode spacing (d2) = 100 nm, and pattern height (p) at 20 nm. At a fixed electrode width w, the inflow gradient increases with Lp and the system shows higher degree of deviation from a flat electrode system (Figure 8). It can be understood that at higher Lp values, formation of multiple pillar is more favorable as compared to single pillar because of enhanced flow of material resulting from high inflow gradient, which alleviates the nanogroove formation. Therefore, it is worth noting that we show fabrication of structures that are much smaller than features on the stamp, and we circumvent the issue of nanoscale patterning by a mesopatterned stamp, which otherwise involves more complex and costly fabrication procedure.
Figure 5
Figure 5. Quasi-steady-state patterns obtained for ψ = 70 V, h0 = 30 nm, p = 20 nm, d2 = 100 nm, and Lp = 350 nm at (a) w < λd1, (b) w = λd1, (c) w = λ(d1+davg)/2, (d) w = λdavg, (e) w = λ(davg+d2)/2, and (f) w = λd2.
Figure 6
Figure 6. Initial electric-field-intensity distribution on a 30 nm polymeric film under protruded top electrode having w = λc, p = 20 nm for 70 V, d2 = 100 nm, and Lp = 350 nm.
Figure 7
Figure 7. Quasi-steady-state patterns of a 30 nm polymeric film under a protruded top electrode having w = 225 nm, and p = 20 nm at 70 V, and d2 = 100 nm at (a) Lp = 250 nm, (b) Lp = 300 nm, (c) Lp = 350 nm, and (d) Lp = 400 nm.
Figure 8
Figure 8. Initial electric-field-intensity distribution for different Lp on a 30 nm polymer film under protruded top electrode (w = 225 nm, p = 20 nm) at 70 V and d2 = 100 nm.
4.3. Influence of Initial Film Thickness
It can be realized from linear stability analysis (eq 1) that the values of characteristic wavelength decrease with increasing h0. To elucidate the sole effect of film thickness, here, we have shown the sole effect of film thickness by (a) keeping all other parameters fixed including the filling factor (w/Lp = 0.6) and electrode width as w = λ(d1+davg)/2 for all cases (Figure 9) and by (b) maintaining w constant (225 nm) along with all of the other parameters fixed for all h0 (Figure 10). For the first case, Figure 9 illustrates quasi-steady-state structures with electrode width corresponding to λ(d1+davg)/2. Separate pillars are observed in case of thinner film (Figure 9a). With increasing h0, tendency of nanogroove structures and single pillar increases. The nanogroove structure is observed for h0 = 40 nm (Figure 9d). This can be attributed to the fact that on increasing film thickness, interface comes closer to top electrode and enhanced distributed electrical pressure acts on the same. Figure 10 describes the second case, where morphologies are obtained on the variation of film thickness keeping the electrode width constant (w = 225 nm) along with all of the other system parameters. We observed a single pillar under the protrusion in case of h0 = 25 nm (Figure 10a), which is in line with our expectation as w = 225 nm < λd1 = 229 nm (Figure 11). In case of h0 = 30 nm (Figure 10b), the nanogroove structure is observed as λd1 = 213 nm < w = 225 nm < λ(d1+davg)/2 = 240 nm. Similarly, we noticed separated polymeric pillars when h0 = 35 nm (Figure 10c) as λ(d1+davg)/2 = w = 225 nm. Thereafter, pillar separation increases in case of 40 nm thick polymeric film (Figure 10d) as w = 225 nm > λ(d1+davg)/2 = 208 nm. Figure 11 shows the characteristic λ defined for different film thicknesses and summarizes the results obtained through identification of three zones where single, nanogroove, and separate pillars are more likely to be observed.
Figure 9
Figure 9. Quasi-steady-state structure obtained at ψ = 70 V under a protruded top electrode having w = λ(d1+davg)/2 nm, and p = 20 nm with d2 = 100 nm and filling factor = 0.6 for (a) h0 = 25 nm, (b) h0 = 30 nm, (c) h0 = 35 nm, and (d) h0 = 40 nm.
Figure 10
Figure 10. Quasi-steady-state structure obtained at ψ = 70 V under a protruded top electrode having w = 225 nm and p = 20 nm with d2 = 100 nm and Lp = 350 nm for (a) h0 = 25 nm, (b) h0 = 30 nm, (c) h0 = 35 nm, and (d) h0 = 40 nm.
Figure 11
Figure 11. Morphological phase diagram of single, nanogroove, and separate pillars for different electrode widths and initial film thicknesses.
4.4. Influence of Electric Potential
Quasi-steady-state structures of 30 nm polymeric film at varying electric potential for a fixed Lp and h0 is shown in Figure 12. At higher potential, the inflow gradient increases and with increasing voltage, the likelihood of formation of nanoscale groove increases. For example, at 60 V, Lp = 350 and 30 nm initial film thickness, nanogrooved structure is not observed, but it becomes prevalent with increasing the electric potential to 70 V keeping the other parameter fixed. It should be noted that there is an upper limit of applied voltage for a particular system above which the dielectric breakdown point of polymer has to be taken into account.
Figure 12
Figure 12. Quasi-steady-state structure of a 30 nm polymeric film under a protruded top electrode having w = 225 nm and p = 20 nm with d2 = 100 nm and Lp = 350 nm at (a) ψ = 60 V, (b) ψ = 65 V, (c) ψ = 70 V, (d) ψ = 75 V, and (e) ψ = 80 V.
4.5. Influence of Protrusion Height and Electrode Spacing
An increase in protrusion height (p) effectively reduces the gap between the top electrode and the interface, which increases the perturbing force. Accordingly, the characteristic wavelength values decrease while the ease of groove formation is enhanced. In Figure 13a, the formation of a single pillar is observed when the protrusion height (p) is 10 nm with ψ = 70 V, w = 214 nm, d2 = 100 nm, and Lp = 350 nm. By keeping all other operating parameters fixed, we observed the nanogroove structures when p was increased to 20 nm (Figure 13b). Two separate pillars are detected at p = 30 and 40 nm (Figure 13c,d). On the contrary, with increasing electrode spacing (d), exactly opposing behavior is found as the electrode-film interface gap increases with increasing d2 and the formation of nanogrooved structures becomes more difficult (Figure 14).
Figure 13
Figure 13. Quasi-steady-state structure of a 30 nm polymeric film at 70 V for different protrusion heights (p) (a) 10 nm, (b) 20 nm, (c) 30 nm, and (d) 40 nm with w = 214 nm, d2 = 100 nm, and Lp = 350 nm.
Figure 14
Figure 14. Quasi-steady-state structure of a 30 nm polymeric film at 70 V with w = 214 nm, p = 20 nm, and Lp = 350 nm for (a) d2 = 90 nm, (b) d2 = 100 nm, and (c) d2 = 110 nm.
5. Conclusions
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In this article, a 2D computational model based on finite element method has been utilized to study the thin film dynamics under the action of an external electric field in the presence of a topographically patterned top electrode. It is shown that by applying a heterogeneous electric field, nanogrooved structures can be produced through controlled ripple formation at the fluid–air interface. Under topographically patterned top electrode having width corresponding to λc, spatiotemporal variation of the morphological patterns beneath a single protrusion is reported. Effect of filling factor, applied voltage, initial film thickness, protrusion height, and electrode spacing on the formation of structures are systematically characterized. The study is significant in elucidating the sensitivity of several independent parameters such as λc, Lp, h0, and ψ on EHD instability in a thin polymeric film. Systematic investigation indicates λd1 < w < λ(d1+davg)/2 as a first approximation for nanogroove structures, and this study can further be used to optimize the process parameters for desired morphology. It also suggests a recipe for miniaturization with desired nanostructures under appropriate condition in the form of possible fabrication of nanogrooves starting from a patterned stamp with a much wider lateral dimension. Such miniaturization of feature dimension is not possible in any of the direct embossing-based patterning techniques.
Author Information
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- Arnab Atta - Multiscale Computational Fluid Dynamics Laboratory, Department of Chemical Engineering, and , Indian Institute of Technology Kharagpur, Kharagpur, West Bengal 721302, India;
http://orcid.org/0000-0001-9052-6097;
- Swarit Dwivedi - Multiscale Computational Fluid Dynamics Laboratory, Department of Chemical Engineering, and , Indian Institute of Technology Kharagpur, Kharagpur, West Bengal 721302, India; Present Address: Department of Chemical Engineering, Monash University, Clayton Campus, 3168 Melbourne, Australia;http://orcid.org/0000-0002-4639-5576
- Rabibrata Mukherjee - Instability and Soft Patterning Laboratory, Department of Chemical Engineering, and , Indian Institute of Technology Kharagpur, Kharagpur, West Bengal 721302, India;http://orcid.org/0000-0001-7031-9859
References
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This article references 62 other publications.
- 1Duncan, R. The dawning era of polymer therapeutics. Nat. Rev. Drug Discovery 2003, 2, 347– 360, DOI: 10.1038/nrd1088Google Scholar1The dawning era of polymer therapeuticsDuncan, RuthNature Reviews Drug Discovery (2003), 2 (5), 347-360CODEN: NRDDAG; ISSN:1474-1776. (Nature Publishing Group)A review. As we enter the twenty-first century, research at the interface of polymer chem. and the biomedical sciences has given rise to the first nano-sized (5-100 nm) polymer-based pharmaceuticals, the 'polymer therapeutics'. Polymer therapeutics include rationally designed macromol. drugs, polymer-drug and polymer-protein conjugates, polymeric micelles contg. covalently bound drug, and polyplexes for DNA delivery. The successful clin. application of polymer-protein conjugates, and promising clin. results arising from trials with polymer-anticancer-drug conjugates, bode well for the future design and development of the ever more sophisticated bio-nanotechnologies that are needed to realize the full potential of the post-genomic age.
- 2Pais, A.; Banerjee, A.; Klotzkin, D.; Papautsky, I. High-sensitivity, disposable lab-on-a-chip with thin-film organic electronics for fluorescence detection. Lab Chip 2008, 8, 794– 800, DOI: 10.1039/b715143hGoogle Scholar2High-sensitivity, disposable lab-on-a-chip with thin-film organic electronics for fluorescence detectionPais, Andrea; Banerjee, Ansuman; Klotzkin, David; Papautsky, IanLab on a Chip (2008), 8 (5), 794-800CODEN: LCAHAM; ISSN:1473-0197. (Royal Society of Chemistry)The authors report a high-sensitivity, disposable lab-on-a-chip with a thin-film org. light-emitting diode (OLED) excitation source and an org. photodiode (OPD) detector for on-chip fluorescence anal. A NPB/Alq3 thin-film green OLED with an active area of 0.1 cm2 was used as the excitation source, while a CuPC/C60 thin-film OPD with 0.6 cm2 active area was used as a photodetector. A novel cost-effective, cross-polarization scheme was used to filter out excitation light from a fluorescent dye emission spectrum. The excitation light from the OLED was linearly polarized and used to illuminate a microfluidic device contg. a 1 μL vol. of dye dissolved in ethanol. The detector was shielded by a second polarizer, oriented orthogonally to the excitation light, thus reducing the photocurrent due to excitation light leakage on the detector by approx. 25 dB. The fluorescence emission light, which is randomly polarized, is only attenuated by ∼3 dB. Fluorescence signals from Rhodamine 6G (peak emission wavelength of 570 nm) and fluorescein (peak emission wavelength of 494 nm) dyes were measured in a diln. series in the microfluidic device with emission signals detected by the OPD. A limit-of-detection of 100 nM was demonstrated for Rhodamine 6G, and 10 μM for fluorescein. This suggests that an integrated microfluidic device, with an org. photodiode and LED excitation source and integrated polarizers, can be fabricated to realize a compact and economical lab-on-a-chip for point-of-care fluorescence assays.
- 3Stone, H. A.; Kim, S. Microfluidics: Basic issues, applications, and challenges. AIChE J. 2001, 47, 1250– 1254, DOI: 10.1002/aic.690470602Google Scholar3Microfluidics: basic issues, applications, and challengesStone, H. A.; Kim, S.AIChE Journal (2001), 47 (6), 1250-1254CODEN: AICEAC; ISSN:0001-1541. (American Institute of Chemical Engineers)A review with many refs. The objective of this article is to highlight some avenues of research and development in micro-fluidics.
- 4Wicks, Z. W., Jr.; Jones, F. N.; Pappas, S. P.; Wicks, D. A. Organic Coatings: Science and Technology; John Wiley & Sons, 2007.Google ScholarThere is no corresponding record for this reference.
- 5Hoppe, H.; Niggemann, M.; Winder, C.; Kraut, J.; Hiesgen, R.; Hinsch, A.; Meissner, D.; Sariciftci, N. S. Nanoscale Morphology of Conjugated Polymer/Fullerene-Based Bulk- Heterojunction Solar Cells. Adv. Funct. Mater. 2004, 14, 1005– 1011, DOI: 10.1002/adfm.200305026Google Scholar5Nanoscale morphology of conjugated polymer/fullerene-based bulk-heterojunction solar cellsHoppe, Harald; Niggemann, Michael; Winder, Christoph; Kraut, Juergen; Hiesgen, Renate; Hinsch, Andreas; Meissner, Dieter; Sariciftci, Niyazi SerdarAdvanced Functional Materials (2004), 14 (10), 1005-1011CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)The relation between the nanoscale morphol. and device properties for conjugated polymer/fullerene bulk-heterojunction plastic solar cells, was studied. Complementary measurements on solid-state blends of poly[2-methoxy-5-(3,7-dimethyloctyloxy)]-1,4-phenylenevinylene (MDMO-PPV) and the sol. fullerene C60 deriv. 1-(3-methoxycarbonyl) propyl-1-Ph [6,6]C61 (PCBM), spin-cast from toluene or chlorobenzene solns., were made. The characterization of the nanomorphol. was carried out by SEM and AFM, while solar-cell devices were characterized by I-V and spectral photocurrent measurements. The morphol. was manipulated by annealing to increase the extent of phase sepn. in the thin-film blends and to identify the distribution of materials. Photoluminescence measurements confirm the demixing of the materials by the thermal treatment. The photoluminescence of PCBM clusters with sizes of up to a few hundred nanometers indicates a photocurrent loss in films of the coarser phase-sepd. blends cast from toluene. For toluene-cast films the scale of phase sepn. depends strongly on the ratio of MDMO-PPV to PCBM, as well as on the total concn. of the casting soln. Small beads of 20-30 nm diam. were obsd. in blend films cast from both toluene and chlorobenzene and these are attributed to MDMO-PPV.
- 6Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient planar heterojunction perovskite solar cells by vapour deposition. Nature 2013, 501, 395– 398, DOI: 10.1038/nature12509Google Scholar6Efficient planar heterojunction perovskite solar cells by vapour depositionLiu, Mingzhen; Johnston, Michael B.; Snaith, Henry J.Nature (London, United Kingdom) (2013), 501 (7467), 395-398CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)Many different photovoltaic technologies are being developed for large-scale solar energy conversion. The wafer-based 1st-generation photovoltaic devices have been followed by thin-film solid semiconductor absorber layers sandwiched between 2 charge-selective contacts and nanostructured (or mesostructured) solar cells that rely on a distributed heterojunction to generate charge and to transport pos. and neg. charges in spatially sepd. phases. Although many materials have been used in nanostructured devices, the goal of attaining high-efficiency thin-film solar cells in such a way has yet to be achieved. Organometal halide perovskites have recently emerged as a promising material for high-efficiency nanostructured devices. Nanostructuring is not necessary to achieve high efficiencies with this material: a simple planar heterojunction solar cell incorporating vapor-deposited perovskite as the absorbing layer can have solar-to-elec. power conversion efficiencies of over 15 per cent (as measured under simulated full sunlight). Perovskite absorbers can function at the highest efficiencies in simplified device architectures, without the need for complex nanostructures.
- 7Sperling, L. H. Introduction to Physical Polymer Science; Wiley-Blackwell, 2005; Chapter 13, pp 687– 756.Google ScholarThere is no corresponding record for this reference.
- 8Wu, N.; Russel, W. B. Micro- and nano-patterns created via electrohydrodynamic instabilities. Nano Today 2009, 4, 180– 192, DOI: 10.1016/j.nantod.2009.02.002Google Scholar8Micro- and nano-patterns created via electrohydrodynamic instabilitiesWu, Ning; Russel, William B.Nano Today (2009), 4 (2), 180-192CODEN: NTAOCG; ISSN:1748-0132. (Elsevier Ltd.)A review. Intense interests on surfaces with micro- and nano-patterns originated from abundant applications in optics, electronics, tissue engineering, biomedical sensors, etc. Searching for ways to create small patterns in both org. and inorg. materials at reduced operation time and cost is one of the challenges in nanotechnol. This paper reviews recent advances in patterning of thin films by using electrohydrodynamic instabilities, a simple and cost-effective bottom-up method. Challenges and future directions are also discussed from the point of view of both fundamental understanding and technol. applications of the pattern formation induced by the elec. field.
- 9Mukherjee, R.; Sharma, A. Instability, self-organization and pattern formation in thin soft films. Soft Matter 2015, 11, 8717– 8740, DOI: 10.1039/c5sm01724fGoogle Scholar9Instability, self-organization and pattern formation in thin soft filmsMukherjee, Rabibrata; Sharma, AshutoshSoft Matter (2015), 11 (45), 8717-8740CODEN: SMOABF; ISSN:1744-683X. (Royal Society of Chemistry)The free surface of a thin soft polymer film is often found to become unstable and self-organizes into various meso-scale structures. In this article we classify the instability of a thin polymer film into three broad categories, which are: category 1: instability of an ultra-thin (<100 nm) viscous film engendered by amplification of thermally excited surface capillary waves due to interfacial dispersive van der Waals forces; category 2: instability arising from the attractive inter-surface interactions between the free surface of a soft film exhibiting room temp. elasticity and another rigid surface in its contact proximity; and category 3: instability caused by an externally applied field such as an elec. field or a thermal gradient, obsd. in both viscous and elastic films. We review the salient features of each instability class and highlight how characteristic length scales, feature morphologies, evolution pathways, etc. depend on initial properties such as film thickness, visco-elasticity (rheol.), residual stress, and film prepn. conditions. We emphasize various possible strategies for aligning and ordering of the otherwise isotropic structures by combining the essential concepts of bottom-up and top-down approaches. A perspective, including a possible future direction of research, novelty and limitations of the methods, particularly in comparison to the existing patterning techniques, is also presented for each setting.
- 10Bhandaru, N.; Das, A.; Mukherjee, R. Confinement induced ordering in dewetting of ultra-thin polymer bilayers on nanopatterned substrates. Nanoscale 2016, 8, 1073– 1087, DOI: 10.1039/c5nr06690eGoogle Scholar10Confinement induced ordering in dewetting of ultra-thin polymer bilayers on nanopatterned substratesBhandaru, Nandini; Das, Anuja; Mukherjee, RabibrataNanoscale (2016), 8 (2), 1073-1087CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)We report the dewetting of a thin bilayer of polystyrene (PS) and poly(methylmethacrylate) (PMMA) on a topog. patterned nonwettable substrate comprising an array of pillars, arranged in a square lattice. With a gradual increase in the concn. of the PMMA soln. (Cn-PMMA), the morphol. of the bottom layer changes to: (1) an aligned array of spin dewetted droplets arranged along substrate grooves at very low Cn-PMMA; (2) an interconnected network of threads surrounding each pillar at intermediate Cn-PMMA; and (3) a continuous bottom layer at higher Cn-PMMA. On the other hand the morphol. of the PS top layer depends largely on the nature of the pre-existing bottom layer, in addn. to Cn-PS. An ordered array of PMMA core-PS shell droplets forms right after spin coating when both Cn-PMMA and Cn-PS are very low. Bilayers with all other initial configurations evolve during thermal annealing, resulting in a variety of ordered structures. Unique morphologies realized include laterally coexisting structures of the two polymers confined within the substrate grooves due to initial rupture of the bottom layer on the substrate followed by a squeezing flow of the top layer; an array of core-shell and single polymer droplets arranged in an alternating order etc., to highlight a few. Such structures cannot be fabricated by any stand-alone lithog. technique.
- 11Swan, J. W. Stress and other effects produced in resin and in a viscid compound of resin and oil by electrification. Proc. R. Soc. London 1897, 62, 38– 46, DOI: 10.1098/rspl.1897.0077Google ScholarThere is no corresponding record for this reference.
- 12Schäffer, E.; Thurn-Albrecht, T.; Russell, T. P.; Steiner, U. Electrically induced structure formation and pattern transfer. Nature 2000, 403, 874– 877, DOI: 10.1038/35002540Google Scholar12Electrically induced structure formation and pattern transferSchaffer, Erik; Thurn-Albrecht, Thomas; Russell, Thomas P.; Steiner, UllrichNature (London) (2000), 403 (6772), 874-877CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)The interaction of an elec. field with matter is well understood. Common expts. demonstrate the forces on dielec. materials in elec. fields, such as a dielec. liq. which is drawn into the air gap of a plate capacitor when a voltage is applied across the plates. As electrostatic interactions are relatively strong and long-range, they can be used to control structures on length scales which are difficult to manipulate in any other manner. One challenge in this context is the creation of lateral features below the diffraction limit of light. Various techniques for the manuf. of lithog. layers with submicrometre structures have been proposed, such as particle lithog., lithog. using scanning near-field microscopes, and printing or embossing structures onto surfaces. Here we describe a particularly simple addn. to this collection of submicrometre replication techniques: the electrostatic transfer of a master pattern into a polymer film. While elec. fields have been used before to orient the microphases in block copolymer melts, the work presented here focuses on the use of elec. fields to control and organize polymers laterally to replicate a master pattern.
- 13Morariu, M. D.; Voicu, N. E.; Schäffer, E.; Lin, Z.; Russell, T. P.; Steiner, U. Hierarchical structure formation and pattern replication induced by an electric field. Nat. Mater. 2003, 2, 48– 52, DOI: 10.1038/nmat789Google Scholar13Hierarchical structure formation and pattern replication induced by an electric fieldMorariu, Mihai D.; Voicu, Nicoleta E.; Schaeffer, Erik; Lin, Zhiqun; Russell, Thomas P.; Steiner, UllrichNature Materials (2003), 2 (1), 48-52CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)Several techniques based on soft lithog. have emerged to replicate micrometer-sized patterns. Similar to most other lithog. methods, these techniques structure a single layer of photoresist. For many applications, however, it is desirable to control the spatial arrangement of more than one component. With traditional methods, this requires an iterative, multistep procedure, making the replication process more complex and less reliable. Here, a replication process is described where multiple materials are processed simultaneously. Using a bilayer formed by two different polymers, electrohydrodynamic instabilities at both polymer surfaces produce a hierarchic lateral structure that exhibits two independent characteristic dimensions. A lateral modulation of the elec. field enables replication with a resoln. down to 100 nm. This approach might provide a simple strategy for large-area, sub-100-nm lithog.
- 14Xiang, H.; Lin, Y.; Russell, T. P. Electrically induced patterning in block copolymer films. Macromolecules 2004, 37, 5358– 5363, DOI: 10.1021/ma049888sGoogle Scholar14Electrically Induced Patterning in Block Copolymer FilmsXiang, Hongqi; Lin, Yao; Russell, Thomas P.Macromolecules (2004), 37 (14), 5358-5363CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)Thin films of block copolymers were manipulated on two different length scales simultaneously by use of an elec. field. The copolymers are asym. diblock copolymers of polystyrene-block-poly(2-vinylpyridine) (PS-b-PVP), polystyrene-block-polyisoprene (PS-b-PI), and polystyrene-block-polybutadiene (PS-b-PBD). Electrostatic pressure generated at the surface of a block copolymer film between two electrodes with an air gap sepg. the surface of the block copolymer film and the upper electrode produced, as in homopolymer films, an array of hexagonally ordered columns, tens of microns in size, that spanned between the two electrodes. Within each column the diblock copolymer microphase sepd. into hexagonally packed cylindrical microdomains, tens of nanometers in size. The orientation of these microdomains was controlled by the interfacial energy of each block with the surfaces of the electrode and the direction of the applied field. Microdomain alignment parallel to and normal to the applied field could be controlled by the strength of the interfacial interactions. Thus, block copolymers can be controlled on two different length scales simultaneously and the method is a new route toward fabricating functional nanoscopic structures in thin polymer films.
- 15Lee, S. H.; Kim, P.; Jeong, H. E.; Suh, K. Y. Electrically induced formation of uncapped, hollow polymeric microstructures. J. Micromech. Microeng. 2006, 16, 2292– 2297, DOI: 10.1088/0960-1317/16/11/007Google Scholar15Electrically induced formation of uncapped, hollow polymeric microstructuresLee, Sung Hun; Kim, Pilnam; Jeong, Hoon Eui; Suh, Kahp Y.Journal of Micromechanics and Microengineering (2006), 16 (11), 2292-2297CODEN: JMMIEZ; ISSN:0960-1317. (Institute of Physics Publishing)Uncapped, hollow polymeric microstructures were fabricated on a silicon substrate using elec. field induced stretching and detachment. Initially, square or cylinder microposts were generated using a solvent-assisted capillary molding technique, and a featureless electrode mask was positioned on the top of the microstructure with spacers maintaining an air gap (∼20 μm). Upon exposure to an external elec. field (1.0-3.0 V μm-1), the hollow microstructures were destabilized and stretched by the well-known electrohydrodynamic instability, resulting in contact of the top polymer surface with the mask. Subsequently, detachment of the capping layer occurred upon removal of the mask due to larger adhesion forces at the polymer/mask interface than cohesion forces of the polymer. These hollow microstructures were tested to capture the budding yeast, Saccharomyces cerevisiae, for shear protection.
- 16Zhou, S.; Zheng, H.; Li, G.; Liu, J.; Liu, S. Formation of controllable polymer micropatterns through liquid film electro-dewetting. Appl. Surf. Sci. 2018, 436, 839– 845, DOI: 10.1016/j.apsusc.2017.12.070Google Scholar16Formation of controllable polymer micropatterns through liquid film electro-dewettingZhou, Shangru; Zheng, Huai; Li, Guoliang; Liu, Jie; Liu, ShengApplied Surface Science (2018), 436 (), 839-845CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)Controllable polymer micropatterns, served as indispensable function structures, are extensively required in many micro/nano scientific areas and engineering applications. Exploring advanced methods of fabricating micropatterns is always a research hotspot. In this article, we introduce a novel method of patterning polymer by the electro-dewetting induced by corona discharge. For the first time, it is obsd. exptl. that liq. polymer on conductive/non-conductive patterned substrates, spontaneously converges from non-conductive areas to conductive areas under the action of ion wind. Taking advantage of such a flow phenomenon, controllable polymer micropatterns including microbump arrays and microwell arrays are fabricated successfully. Their sizes range from hundreds of microns to millimeters. Micropattern surfaces present an ultra-smooth characteristic, with roughness in the nanometer range.
- 17Rickard, J. J. S.; Farrer, I.; Oppenheimer, P. G. Tunable nanopatterning of conductive polymers via electrohydrodynamic lithography. ACS Nano 2016, 10, 3865– 3870, DOI: 10.1021/acsnano.6b01246Google Scholar17Tunable Nanopatterning of Conductive Polymers via Electrohydrodynamic LithographyRickard, Jonathan James Stanley; Farrer, Ian; Goldberg Oppenheimer, PolaACS Nano (2016), 10 (3), 3865-3870CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)The authors introduce advanced yet simple electrohydrodynamic lithog. (EHL) for patterning conductive polymers directly on a substrate with high fidelity. The generality of this method is demonstrated by structuring thin polypyrrole films via elec.-field-induced instabilities, yielding well-defined conductive structures with feature sizes ranging from tens of micrometers to hundreds of nanometers. Exploitation of a conductive polymer induces free charge suppression of the field in the polymer film, paving the way for accessing scale sizes in the low submicron range. The authors show the feasibility of the polypyrrole-based structures for field-effect transistor devices. Controlled EHL pattering of conductive polymer structures at the micro and nano scale demonstrated in this study combined with the possibility of effectively tuning the dimensions of the tailor-made architectures might herald a route toward various submicron device applications in supercapacitors, photovoltaics, sensors, and electronic displays.
- 18Goldberg-Oppenheimer, P.; Kabra, D.; Vignolini, S.; Hüttner, S.; Sommer, M.; Neumann, K.; Thelakkat, M.; Steiner, U. Hierarchical orientation of crystallinity by block-copolymer patterning and alignment in an electric field. Chem. Mater. 2013, 25, 1063– 1070, DOI: 10.1021/cm3038075Google Scholar18Hierarchical Orientation of Crystallinity by Block-Copolymer Patterning and Alignment in an Electric FieldGoldberg-Oppenheimer, Pola; Kabra, Dinesh; Vignolini, Silvia; Huttner, Sven; Sommer, Michael; Neumann, Katharina; Thelakkat, Mukundan; Steiner, UllrichChemistry of Materials (2013), 25 (7), 1063-1070CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)Electron and hole conducting 10-nm-wide polymer morphologies hold great promise for org. electro-optical devices such as solar cells and light emitting diodes. The self-assembly of block-copolymers (BCPs) is often viewed as an efficient way to generate such materials. Here, a functional block copolymer that contains perylene bismide (PBI) side chains which can crystallize via π-π stacking to form an electron conducting microphase is patterned harnessing hierarchical electrohydrodynamic lithog. (HEHL). HEHL film destabilization creates a hierarchical structure with three distinct length scales: (1) micrometer-sized polymer pillars, contg. (2) a 10-nm BCP microphase morphol. that is aligned perpendicular to the substrate surface and (3) on a mol. length scale(coating process) (0.35-3 nm) PBI π-π-stacks traverse the HEHL-generated plugs in a continuous fashion. The good control over BCP and PBI alignment inside the generated vertical microstructures gives rise to liq.-crystal-like optical dichroism of the HEHL patterned films, and improves the electron cond. across the film by 3 orders of magnitude.
- 19Voicu, N. E.; Saifullah, M. S. M.; Subramanian, K. R. V.; Welland, M. E.; Steiner, U. TiO2 patterning using electro-hydrodynamic lithography. Soft Matter 2007, 3, 554– 557, DOI: 10.1039/b616538aGoogle Scholar19TiO2 patterning using electro-hydrodynamic lithographyVoicu, Nicoleta E.; Saifullah, M. S. M.; Subramanian, K. R. V.; Welland, Mark E.; Steiner, UllrichSoft Matter (2007), 3 (5), 554-557CODEN: SMOABF; ISSN:1744-683X. (Royal Society of Chemistry)An increasing no. of technologies require the fabrication of micro- and nanostructures over large areas. Soft lithog. methods are gaining in popularity for the manuf. of low-cost micrometer and sub-micrometer structures. Increasingly, these methods developed to structure org. resists can also be used to pattern inorg. materials. Here we introduce a simple lithog. technique that is able to pattern ceramic TiO2 micro- and nanostructures with high fidelity. Our method makes use of an electrohydrodynamic (EHD) film instability that is controlled by a laterally modulated elec. field. A spin-coated film of a stabilized metal alkoxide precursor material was patterned using EHD lithog. followed by a heat treatment at 400 °C to yield cryst. TiO2 micropatterns. Our technique is rather general and can be extended to a no. of single- and multicomponent oxide systems.
- 20Lv, G.; Liu, Y.; Shao, J.; Tian, H.; Yu, D. Facile fabrication of electrohydrodynamic micro-/nanostructures with high aspect ratio of a conducting polymer for large-scale superhydrophilic/superhydrophobic surfaces. Macromol. Mater. Eng. 2018, 303, 1700361, DOI: 10.1002/mame.201700361Google ScholarThere is no corresponding record for this reference.
- 21Lv, G.; Zhang, S.; Shao, J.; Wang, G.; Tian, H.; Yu, D. Rapid fabrication of electrohydrodynamic micro-/nanostructures with high aspect ratio using a leaky dielectric photoresist. React. Funct. Polym. 2017, 118, 1– 9, DOI: 10.1016/j.reactfunctpolym.2017.06.014Google Scholar21Rapid fabrication of electrohydrodynamic micro-/nanostructures with high aspect ratio using a leaky dielectric photoresistLv, Guowei; Zhang, Shihu; Shao, Jinyou; Wang, Guolong; Tian, Hongmiao; Yu, DemeiReactive & Functional Polymers (2017), 118 (), 1-9CODEN: RFPOF6; ISSN:1381-5148. (Elsevier Ltd.)A leaky dielec. photoresist was designed and prepd. for rapid fabrication of high-aspect-ratio micro-/nanostructures via electrohydrodynamic patterning (EHDP). The rheol. behavior and elec. properties of the photoresists were systematically studied, since the structure formation in EHDP essentially originates from the flow and deformation of the polymeric film actuated by an applied elec. field. The photoresists exhibit the suitable rheol. behavior with a low viscosity of 2.4-157.7 mPa s, controllable elec. cond. of 5.0 × 10- 6 - 7.2 × 10- 4 S m- 1, as well as high homogeneity, minor surface tension of ∼30 mN m- 1, favorable wettability and film-forming property on substrate and an extremely large redn. in the contact angle (down to 1.64°) of electrowetting on dielec. (EWOD). The EHDP results showed that a higher elec. cond. of the photoresists can lead to a higher filling height, a smaller characteristic wavelength and a shorter patterning time, while a lower viscosity can also lead to a shorter patterning time, which is accordance with the theor. prediction. The patterning time of the photoresists cannot be too short because the following rapid ripening and coalescence of the formed microstructure will damage the high fidelity of the final pillar arrays.
- 22González, A.; Castellanos, A. Nonlinear electrohydrodynamic waves on films falling down an inclined plane. Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys. 1996, 53, 3573– 3578, DOI: 10.1103/physreve.53.3573Google ScholarThere is no corresponding record for this reference.
- 23Wu, N.; Pease, L. F.; Russel, W. B. Electric-field-induced patterns in thin polymer films: weakly nonlinear and fully nonlinear evolution. Langmuir 2005, 21, 12290– 12302, DOI: 10.1021/la052099zGoogle ScholarThere is no corresponding record for this reference.
- 24Tseluiko, D.; Blyth, M. G.; Papageorgiou, D. T.; Vanden-Broeck, J.-M. Electrified falling-film flow over topography in the presence of a finite electrode. J. Eng. Math. 2010, 68, 339– 353, DOI: 10.1007/s10665-010-9377-9Google ScholarThere is no corresponding record for this reference.
- 25Kim, H.; Bankoff, S. G.; Miksis, M. J. The effect of an electrostatic field on film flow down an inclined plane. Phys. Fluids A 1992, 4, 2117– 2130, DOI: 10.1063/1.858508Google Scholar25The effect of an electrostatic field on film flow down an inclined planeKim, Hyo; Bankoff, S. G.; Miksis, Michael J.Physics of Fluids A: Fluid Dynamics (1992), 4 (10), 2117-30CODEN: PFADEB; ISSN:0899-8213.A study of the interaction of an electrostatic field with a thin liq. film flowing under gravity down an inclined plane is presented. First, the effect of the elec. field on the stability of the film flow is examd. Next, several limits of the equations of motion are investigated anal., and then compared with an explicit numerical calcn. of the equations of motion. Also, applications of these calcns. to a proposed electrostatic liq. film space radiator are discussed.
- 26Uma, B.; Usha, R. A thin conducting viscous film on an inclined plane in the presence of a uniform normal electric field: Bifurcation scenarios. Phys. Fluids 2008, 20, 032102, DOI: 10.1063/1.2896300Google Scholar26A thin conducting viscous film on an inclined plane in the presence of a uniform normal electric field: Bifurcation scenariosUma, B.; Usha, R.Physics of Fluids (2008), 20 (3), 032102/1-032102/17CODEN: PHFLE6; ISSN:1070-6631. (American Institute of Physics)A theory for two dimensional long and stationary waves of finite amplitude on a thin viscous liq. film down an inclined plane in the presence of uniform elec. field at infinity is investigated. A set of exact averaged equations for the film flow system is described and linearized stability anal. of the uniform flow is performed using normal-mode formulation and the crit. condition for linear instability is obtained. The linearized instability for the permanent wave equation, consistent to the second order in ε, is examd. and the eigenvalue properties of the fixed points are classified in various parametric regimes. Numerical integration of the permanent wave equation as a third-order dynamical system is carried out. Different bifurcation scenarios leading to multiple-hump solitary waves or leading to chaos are exhibited in the parametric space. (c) 2008 American Institute of Physics.
- 27Tseluiko, D.; Blyth, M. G.; Papageorgiou, D. T.; Vanden-Broeck, J.-M. Electrified viscous thin film flow over topography. J. Fluid Mech. 2008, 597, 449– 475, DOI: 10.1017/s002211200700986xGoogle ScholarThere is no corresponding record for this reference.
- 28Tseluiko, D.; Blyth, M. G.; Papageorgiou, D. T.; Vanden-Broeck, J.-M. Effect of an electric field on film flow down a corrugated wall at zero Reynolds number. Phys. Fluids 2008, 20, 042103, DOI: 10.1063/1.2909660Google Scholar28Effect of an electric field on film flow down a corrugated wall at zero Reynolds numberTseluiko, D.; Blyth, M. G.; Papageorgiou, D. T.; Vanden-Broeck, J.-M.Physics of Fluids (2008), 20 (4), 042103/1-042103/19CODEN: PHFLE6; ISSN:1070-6631. (American Institute of Physics)The effect of an elec. field on a liq. layer flowing down an inclined, corrugated wall at zero Reynolds no. is investigated. The layer is taken to be either a perfect conductor or a perfect dielec. The region above the layer is assumed to be a perfect dielec. Steady flow down a wall with small-amplitude sinusoidal corrugations is considered, and it is shown how the elec. field can be used to control the amplitude of the free-surface deflection and the phase shift between the free surface and the wall profile. Steady flow over walls with large amplitude sinusoidal corrugations or other-shaped indentations is studied by using the boundary-element method. Results for flow into a wide rectangular trench are compared to previous model predictions based on the lubrication approxn. For a perfect-conductor film, the results confirm that the height of the capillary ridge, which appears above a downward step, monotonically decreases as the elec. field strength increases. Solns. for a perfect-dielec. film with relative permittivity larger than unity are similar to those for a perfect-conductor film, although the height of the capillary ridge nonmonotonically varies with the elec. field strength. The behavior of the solns. for a perfect-dielec. film with relative permittivity less than unity is qual. different. The height of the capillary ridge monotonically increases as the elec. field strength increases. Flows into narrow trenches and over narrow mounds are also computed. (c) 2008 American Institute of Physics.
- 29Lv, G.; Zhang, S.; Shao, J.; Tian, H.; Wang, G.; Yu, D. Preparation, properties, and efficient electrically induced structure formation of a leaky dielectric photoresist. RSC Adv. 2016, 6, 82450– 82458, DOI: 10.1039/c6ra17957fGoogle Scholar29Preparation, properties, and efficient electrically induced structure formation of a leaky dielectric photoresistLv, Guowei; Zhang, Shihu; Shao, Jinyou; Tian, Hongmiao; Wang, Guolong; Yu, DemeiRSC Advances (2016), 6 (85), 82450-82458CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)A leaky dielec. photoresist was designed and prepd. by doping a sol. conductive polypyrrole into a low-viscosity photocurable resin (perfect dielec.) to achieve efficient elec. induced structure formation (EISF). The comprehensive properties of both the leaky dielec. photoresist and its cured film were systematically investigated. It was found that the leaky dielec. photoresist is homogeneous and stable in both the liq. state and solid state after curing, as a result of the low mol. wt. and the bulky side groups of polypyrrole. The leaky dielec. photoresist retains the Newtonian nature of the photocurable resin with low viscosity, displays a significant increase in the elec. cond. with an increase in the polypyrrole loading, and shows favorable wettability on a silicon substrate. Meanwhile, the cured film is still transparent, thermally stable and featureless, following the increase in the polypyrrole loading. It is worth noting that by using a low-viscosity leaky dielec. photoresist, pillar arrays can be rapidly fabricated over large areas at ambient temp. via EISF onto a featureless template. The resulting patterned film is hydrophobic with an apparent contact angle of 109°, even though the cured film is hydrophilic with an intrinsic contact angle of 64°.
- 30Wu, D.; Sun, Y.; Teh, K. S.; Zhu, Y.; Luo, Y.; Deng, L.; Zhao, L.; Luo, G.; Zhao, Y.; Wang, L.; Sun, D. Investigation of electrohydrodynamic behaviors from open planar solution under rod-induced electrospinning. J. Phys. D: Appl. Phys. 2017, 50, 455602, DOI: 10.1088/1361-6463/aa8ddbGoogle Scholar30Investigation of electrohydrodynamic behaviors from open planar solution under rod-induced electrospinningWu, Dezhi; Sun, Yu; Teh, Kwok Siong; Zhu, Yuchao; Luo, Yihui; Deng, Lei; Zhao, Libo; Luo, Guoxi; Zhao, Yang; Wang, Lingyun; Sun, DaohengJournal of Physics D: Applied Physics (2017), 50 (45), 455602/1-455602/10CODEN: JPAPBE; ISSN:0022-3727. (IOP Publishing Ltd.)Understanding and controlling the electrohydrodynamic (EHD) behaviors of polymer solns. are of great importance to the design and fabrication of micro/nano devices and sensors. In this work, rod-induced electrospinning (RIES) was used as a versatile technique to study and characterize the EHD behaviors of polyethylene oxide soln. during the electrospinning process. In RIES, a grounded insulated rod moves back and forth several millimeters above an open bath of polymer soln. subjected to high voltage to induce the formation of Taylor cones that lead to subsequent polymer jet emissions from the soln. surface. Using a phase field method, we performed finite element anal. to explore the EHD behaviors of air-polymer interface in RIES process to better understand fundamentals of this process. Pertinent theories such as phase field theories and elec. field theories were briefly discussed to provide relevant background to the techniques used in this work. We investigated the RIES process and the effect of the diam. of the induction rod, applied voltage and soln. cond. on the formation of the jets. The numerical results of the formation and merging process of the jets based on different exptl. parameters are in good agreement with exptl. observations and provide a better understanding of the RIES process to improve the throughput of nanofibers.
- 31Dickey, M. D.; Gupta, S.; Leach, K. A.; Collister, E.; Willson, C. G.; Russell, T. P. Novel 3-D structures in polymer films by coupling external and internal fields. Langmuir 2006, 22, 4315– 4318, DOI: 10.1021/la052954eGoogle Scholar31Novel 3-D Structures in Polymer Films by Coupling External and Internal FieldsDickey, Michael D.; Gupta, Suresh; Leach, K. Amanda; Collister, Elizabeth; Willson, C. Grant; Russell, Thomas P.Langmuir (2006), 22 (9), 4315-4318CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)A route to produce novel three-dimensional structures in thin films is demonstrated. Such structures are most difficult to produce in a simple manner without the use of multiple fabrication steps. Here, we show the generation of 3-D cage-type structures using a combination of electrohydrodynamic instabilities and dewetting in a polymer/polymer/air trilayer. Removal of one of the components by use of a selective solvent or by degrdn. of one of the components reveals the formation of a three-dimensional structure, where one polymer is encased in the other. Thus, by coupling an external field with a surface field inherent to the polymers, a novel fabrication strategy is shown that has clear applications in microfluidics and microelectromech. systems with extensions to patterned surfaces and structured fluids, like block copolymers.
- 32Lyutakov, O.; Hüttel, I.; Prajzler, V.; Jeřábek, V.; Jančárek, A.; Hnatowicz, V.; Švorčík, V. Pattern formation in PMMA film induced by electric field. J. Polym. Sci., Part B: Polym. Phys. 2009, 47, 1131– 1135, DOI: 10.1002/polb.21718Google Scholar32Pattern formation in PMMA film induced by electric fieldLyutakov, Oleksiy; Huettel, Ivan; Prajzler, Vaclav; Jerabek, Vitezslav; Jancarek, Alexander; Hnatowicz, Vladimir; Svorcik, VaclavJournal of Polymer Science, Part B: Polymer Physics (2009), 47 (12), 1131-1135CODEN: JPBPEM; ISSN:0887-6266. (John Wiley & Sons, Inc.)Micro-sized patterns were created on thin poly(Me methacrylate) (PMMA) films by the effect of external field, perpendicular to the film surface. The PMMA film, prepd. by spin-coating onto Si wafer, was heated to the fluid temp. (275 °C) and a linear pattern was created by the effect of elec. field produced by a strip electrode. In another expt., a round pattern was created as a result of local laser heating of the PMMA film under homogeneous elec. field. The created patterns were analyzed by optical microscopy and profile meter. The dependence of the form and size of the created patterns on the intensity of the elec. field, exposure time, and initial film thickness was examd. Wave guiding property of a linear pattern, produced by the above technique, was examd. in a simple expt. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1131-1135, 2009.
- 33Wu, N.; Kavousanakis, M. E.; Russel, W. B. Coarsening in the electrohydrodynamic patterning of thin polymer films. Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys. 2010, 81, 026306, DOI: 10.1103/physreve.81.026306Google Scholar33Coarsening in the electrohydrodynamic patterning of thin polymer filmsWu, Ning; Kavousanakis, Michail E.; Russel, William B.Physical Review E: Statistical, Nonlinear, and Soft Matter Physics (2010), 81 (2-2), 026306/1-026306/13CODEN: PRESCM; ISSN:1539-3755. (American Physical Society)Periodic pillarlike microstructures can be created from initially flat polymer films via the electrohydrodynamic instabilities. Those patterns, however, are metastable. Our exptl. observations show that the av. pillar size increases slowly after linear growth. Major coarsening events then take place over times several orders of magnitude longer than the linear growth time. For all fill ratios, a logarithmic time dependence of the av. pillar size can be identified, i.e., 〈S〉.varies.ln t. Thicker films, however, have faster coarsening rates than thinner films. Linear stability anal. of the pseudosteady states reveals two major coarsening mechanisms, collision and Ostwald ripening, which can also be identified from exptl. images. We then reduce the original partial differential equation (PDE) into a pair of ODEs, which govern the interaction between pillars due to the above two coarsening mechanisms. From this, a logarithm scaling law is obtained for both low and high fill ratios and the coarsening rate is slower for lower fill ratios, consistent with exptl. observations. We also find that arrays with more uniform sizes tend to start coarsening later, but they coarsen faster than more "disperse" arrays, which could be possibly utilized in expts. for controlling the onset and speed of coarsening. The logarithm scaling in the electrohydrodynamic coarsening phenomenon, which differs from coarsening in spinodal decompn. and dewetting of thin liq. films, is due to the significant nonlinear effect of Maxwell stresses and geometric confinement on the disjoining pressure at both top and bottom electrodes.
- 34Lau, C. Y.; Russel, W. B. Fundamental limitations on ordered electrohydrodynamic patterning. Macromolecules 2011, 44, 7746– 7751, DOI: 10.1021/ma200952uGoogle ScholarThere is no corresponding record for this reference.
- 35Schäffer, E.; Thurn-Albrecht, T.; Russell, T. P.; Steiner, U. Electrohydrodynamic instabilities in polymer films. Europhys. Lett. 2001, 53, 518– 524, DOI: 10.1209/epl/i2001-00183-2Google Scholar35Electrohydrodynamic instabilities in polymer filmsSchaffer, E.; Thurn-Albrecht, T.; Russell, T. P.; Steiner, U.Europhysics Letters (2001), 53 (4), 518-524CODEN: EULEEJ; ISSN:0295-5075. (EDP Sciences)We have studied the influence of elec. fields on highly viscous polymer films. An electrohydrodynamic (EHD) instability causes a wave pattern with a characteristic wavelength λ, leading to an array of polymer columns which span the gap of a capacitor device. When represented as a master curve, the data is quant. described by an EHD model, without any adjustable parameters. Our results suggest that EHD expts. using polymer films are well suited to study non-equil. pattern formation in quasi-two-dimensional systems.
- 36Gambhire, P.; Thaokar, R. Electrokinetic model for electric-field-induced interfacial instabilities. Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys. 2014, 89, 032409, DOI: 10.1103/physreve.89.032409Google Scholar36Electrokinetic model for electric-field-induced interfacial instabilitiesGambhire, Priya; Thaokar, RochishPhysical Review E: Statistical, Nonlinear, and Soft Matter Physics (2014), 89 (3-A), 032409/1-032409/9CODEN: PRESCM; ISSN:1539-3755. (American Physical Society)Technol. based on elec.-field-induced instabilities on thin polymer film surfaces has emerged as a promising candidate for soft lithog. Typically, the instability is modeled using the perfect dielec. (PD) or the leaky dielec. (LD) model. These assume the elec. diffuse layer to be infinitesimally large or small, resp. In the present work we conduct stability anal. assuming a PD-electrolyte soln. interface. The concn. of ions and, hence, the diffuse layer thickness is in general assumed to be of the same order as the electrolyte film thickness. The PD-LD models are then realized as limiting cases of the ratio of the double layer thickness to the film thickness.
- 37Lu, W.; Kim, D. Thin-film structures induced by electrostatic field and substrate kinetic constraint. Appl. Phys. Lett. 2006, 88, 153116, DOI: 10.1063/1.2195095Google ScholarThere is no corresponding record for this reference.
- 38Pease, L. F., III; Russel, W. B. Linear stability analysis of thin leaky dielectric films subjected to electric fields. J. Non-Newtonian Fluid Mech. 2002, 102, 233– 250, DOI: 10.1016/s0377-0257(01)00180-xGoogle ScholarThere is no corresponding record for this reference.
- 39Wu, N.; Russel, W. B. Electrohydrodynamic instability of dielectric bilayers: Kinetics and thermodynamics. Ind. Eng. Chem. Res. 2006, 45, 5455– 5465, DOI: 10.1021/ie0510876Google Scholar39Electrohydrodynamic Instability of Dielectric Bilayers: Kinetics and ThermodynamicsWu, Ning; Russel, William B.Industrial & Engineering Chemistry Research (2006), 45 (16), 5455-5465CODEN: IECRED; ISSN:0888-5885. (American Chemical Society)The kinetic and thermodn. properties are studied of a bilayer patterning process induced by an electrohydrodynamic instability. A parametric map is constructed depending on the dielec. contrast and ratio of two film thicknesses that describes the conditions under which hexagonally ordered pillars or holes can form when the viscosity of the upper layer is negligible. The distinct formation of arrays of pillars and holes results from the nonlinear interactions among different modes and, hence, is governed by the kinetics. The dynamic structures of pillars or holes continue to evolve to decrease the system's free energy. During this evolution, individual pillars or holes coalesce in a coarsening process until a thermodynamically stable state is reached in the form of a localized pillar, hole, or a roll structure. The selection of the pillar or hole at the final steady state represents a thermodn. preference that can be predicted qual. without solving the fully nonlinear partial differential equation.
- 40Yeoh, H. K.; Xu, Q.; Basaran, O. A. Equilibrium shapes and stability of a liquid film subjected to a nonuniform electric field. Phys. Fluids 2007, 19, 114111, DOI: 10.1063/1.2798806Google Scholar40Equilibrium shapes and stability of a liquid film subjected to a nonuniform electric fieldYeoh, Hak Koon; Xu, Qi; Basaran, Osman A.Physics of Fluids (2007), 19 (11), 114111/1-114111/22CODEN: PHFLE6; ISSN:1070-6631. (American Institute of Physics)Stresses induced by a spatially nonuniform elec. field acting on an initially flat fluid-fluid interface can be exploited beneficially to pattern polymer microstructures without the use of resists, exposure, development, and etching, but cause undesirable nonuniformity in film thickness in precision coating processes. The equil. shape of an interface sepg. a liq. film from an ambient fluid subjected to a uniform elec. field is flat so long as the field strength is below a crit. value. A nonuniform elec. field, however, results in the deformation of the interface no matter how small its strength, an important difference which previous theor. studies have not addressed satisfactorily. Hence, whereas under a uniform field loss of stability occurs via a bifurcation from the flat film soln., under a nonuniform field destabilization may occur at a turning point at which the film profile already exhibits a finite-amplitude deformation. This deficiency in understanding is remedied here by analyzing a model problem in which a gas overlying a perfect dielec. liq. film is sandwiched between two electrodes wherein the top electrode is grounded and the elec. potential of the bottom electrode varies sinusoidally with distance measured along it. The equil. shapes and stability of the liq.-gas interface are detd. directly in the present work by simultaneously solving the augmented Young-Laplace equation governing the shape of the free surface and the Laplace equation governing elec. potentials theor. by domain perturbation anal. and numerically by finite element anal. For small nonuniformities in the elec. field, anal. solns. are reported for the profile of the free surface. The computational predictions are shown to be in excellent accord with these small-deformation results. Moreover, computations are used to extend the investigations into the nonlinear regime where nonuniformities in the elec. field and deformations of the free surface are large, and loss of stability may occur. The variation of the equil. shapes and the limits of stability with the governing dimensionless groups are investigated thoroughly. It is shown that the rich response exhibited by the system can be rationalized by interrogating the computed solns. and scrutinizing the balance of stresses due to the normal component of the elec. field, which are destabilizing, and those due to its tangential component, which are stabilizing.
- 41Sarkar, J.; Sharma, A.; Shenoy, V. B. Electric-field induced instabilities and morphological phase transitions in soft elastic films. Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys. 2008, 77, 031604, DOI: 10.1103/physreve.77.031604Google Scholar41Electric-field induced instabilities and morphological phase transitions in soft elastic filmsSarkar, Jayati; Sharma, Ashutosh; Shenoy, Vijay B.Physical Review E: Statistical, Nonlinear, and Soft Matter Physics (2008), 77 (3-1), 031604/1-031604/10CODEN: PRESCM; ISSN:1539-3755. (American Physical Society)We investigate the morphol. transitions of surface patterns induced in a soft elastic film in the presence of an applied elec. field by the linear stability anal. and simulations. The surface patterns emerge beyond a crit. strength of the elec. field and the pattern length scale is always nearly three times the film thickness, regardless of the precise pattern morphol. and other phys. parameters. Interestingly, the simulations show that the precise pattern morphol. depends strongly on the film and the field parameters and can be classified into three broad morphol. phases: columns, stripes, and cavities. By tuning the elec. field and the gap distance, we show that transitions from one morphol. phase to another can be induced as described by a morphol. phase diagram for this phenomenon. We also study the conditions under which the transitions can be "glassy" or "hysteretic.". In addn. to uncovering the rich physics underlying these nearly two-dimensional morphol. phase transitions, our simulations also suggest expts. and applications of this phenomenon in mesopatterning. In particular, simulations demonstrate the possibility of controlling the pattern morphol. and alignment by using the elec.-field induced instability in soft elastic films. Unlike the short range adhesive interactions which cannot be varied, the use of an elec. field allows far greater flexibility in modulation and control of the pattern morphol. and its height.
- 42Li, B.; Li, Y.; Xu, G.-K.; Feng, X.-Q. Surface patterning of soft polymer film-coated cylinders via an electric field. J. Phys.: Condens. Matter 2009, 21, 445006, DOI: 10.1088/0953-8984/21/44/445006Google Scholar42Surface patterning of soft polymer film-coated cylinders via an electric fieldLi, Bo; Li, Yue; Xu, Guang-Kui; Feng, Xi-QiaoJournal of Physics: Condensed Matter (2009), 21 (44), 445006/1-445006/8CODEN: JCOMEL; ISSN:0953-8984. (Institute of Physics Publishing)Using the linear stability anal. method, we investigate the surface wrinkling of a thin polymer coating on a cylinder in an externally applied elec. field. It is demonstrated that energy competition between surface energy, van der Waals interactive potential energy and electrostatic interaction energy may lead to ordered patterns on the film surface. The anal. solns. are derived for the crit. conditions of both longitudinal and circumferential instabilities. The wavelengths of the generated surface patterns can be mediated by changing the magnitude of the elec. field. Our anal. shows that the surface morphol. is sensitive to the curvature radius of the fiber, esp. in the micrometer and nanometer length scales. Furthermore, we suggest a potential approach for fabricating hierarchical patterns on curved surfaces.
- 43Roberts, S. A.; Kumar, S. Electrohydrodynamic instabilities in thin liquid trilayer films. Phys. Fluids 2010, 22, 122102, DOI: 10.1063/1.3520134Google Scholar43Electrohydrodynamic instabilities in thin liquid trilayer filmsRoberts, Scott A.; Kumar, SatishPhysics of Fluids (2010), 22 (12), 122102/1-122102/15CODEN: PHFLE6; ISSN:1070-6631. (American Institute of Physics)Expts. by Dickey et al. and Leach et al. show that novel pillar shapes can be generated from electrohydrodynamic instabilities at the interfaces of thin polymer/polymer/air trilayer films. In this paper, we use linear stability anal. to investigate the effect of free charge and ac elec. fields on the stability of trilayer systems. Our work is also motivated by our recent theor. study which demonstrates how ac elec. fields can be used to increase control over the pillar formation process in thin liq. bilayer films. For perfect dielec. films, the effect of an ac elec. field can be understood by considering an equiv. dc field. Leaky dielec. films yield pillar configurations that are drastically different from perfect dielec. films, and ac fields can be used to control the location of free charge within the trilayer system. This can alter the pillar instability modes and generate smaller diam. pillars when conductivities are mismatched. The results presented here may be of interest for the creation of complex topog. patterns on polymer coatings and in microelectronics. (c) 2010 American Institute of Physics.
- 44Bandyopadhyay, D.; Sharma, A.; Thiele, U.; Reddy, P. D. S. Electric-field-induced interfacial instabilities and morphologies of thin viscous and elastic bilayers. Langmuir 2009, 25, 9108– 9118, DOI: 10.1021/la900635fGoogle ScholarThere is no corresponding record for this reference.
- 45Gambhire, P.; Thaokar, R. M. Electrohydrodynamic instabilities at interfaces subjected to alternating electric field. Phys. Fluids 2010, 22, 064103, DOI: 10.1063/1.3431043Google Scholar45Electrohydrodynamic instabilities at interfaces subjected to alternating electric fieldGambhire, P.; Thaokar, R. M.Physics of Fluids (2010), 22 (6), 064103/1-064103/16CODEN: PHFLE6; ISSN:1070-6631. (American Institute of Physics)Instabilities at the interface of two immiscible fluids, either perfect or leaky dielecs., subjected to alternating elec. fields, is studied using a linear stability anal. in the limit of the electrode spacing being large compared to the wavelength of the perturbation. The Floquet anal. of the stability of this system indicates a significant effect of the frequency on the value of smax, the growth rate of the fastest growing instabilities and ETaylor, the min. field required to excite an instability. It is seen that alternating fields act to damp the system instabilities compared to the d.c. (dc) case. Moreover, the growth rate of the instabilities can be tuned from that of leaky dielec. fluids subjected to dc fields, in the low frequency limit, to that of perfect dielecs. in the high frequency limit. It is also obsd. that for a leaky dielec.-leaky dielec. interface, the a.c. (ac) fields can induce instabilities in a system which is stable at zero frequency, by increasing the frequency of the applied voltage. (c) 2010 American Institute of Physics.
- 46Reddy, P. D. S.; Bandyopadhyay, D.; Sharma, A. Self-Organized Ordered Arrays of Core–Shell Columns in Viscous Bilayers Formed by Spatially Varying Electric Fields. J. Phys. Chem. C 2010, 114, 21020– 21028, DOI: 10.1021/jp106253kGoogle ScholarThere is no corresponding record for this reference.
- 47Srivastava, S.; Reddy, P. D. S.; Wang, C.; Bandyopadhyay, D.; Sharma, A. Electric field induced microstructures in thin films on physicochemically heterogeneous and patterned substrates. J. Chem. Phys. 2010, 132, 174703, DOI: 10.1063/1.3400653Google ScholarThere is no corresponding record for this reference.
- 48Ghosh, A.; Bandyopadhyay, D.; Sharma, A. Electric field mediated elastic contact lithography of thin viscoelastic films for miniaturized and multiscale patterns. Soft Matter 2018, 14, 3963– 3977, DOI: 10.1039/c8sm00428eGoogle ScholarThere is no corresponding record for this reference.
- 49Tian, H.; Shao, J.; Hu, H.; Wang, L.; Ding, Y. Role of space charges inside a dielectric polymer in the electrohydrodynamic structure formation on a prepatterned polymer (ESF-PP). RSC Adv. 2016, 6, 77275– 77283, DOI: 10.1039/c6ra14479aGoogle Scholar49Role of space charges inside a dielectric polymer in the electrohydrodynamic structure formation on a prepatterned polymer (ESF-PP)Tian, Hongmiao; Shao, Jinyou; Hu, Hong; Wang, Li; Ding, YuchengRSC Advances (2016), 6 (81), 77275-77283CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)Electrohydrodynamic structure formation on a prepatterned polymer (ESF-PP) can duplicate structures identical to the initial geometry, but with a higher aspect ratio, under the influence of a spatially modulated elec. field. In this process, a voltage is applied between a flat template and a flat substrate, sandwiching a prepatterned polymer and an air gap so as to generate an electrohydrodynamic (EHD) force at the air-polymer interface. Subsequently, the prepatterned polymer can be non-uniformly pulled upwards, causing deformation in its micro/nano-structure. Until now, most of the research into ESF-PP has explored various dielec. polymers, which are all considered to be the perfect dielecs. because of their low elec. cond. However, the assumption of a perfect dielec. typically creates discrepancies between theor. anal. and exptl. results in terms of the polymer motion and the final morphol. This phenomenon can be attributed to ignoring the action of the small no. of free space charges within dielec. polymer motion (although the elec. cond. of the dielec. polymer may be even lower than that of deionized water), which emphasizes the importance of the influence of space charges inside the dielec. polymer on deformation. This paper explored the role of free space charges by making a comparison between the perfect dielec. polymer and the leaky dielec. polymer on the progressive development, the surface topog. and the aspect ratio from exptl. tests and numerical simulations, and a discussion of the effect of the different elec. conductivities. Results showed that the free charges inside the dielec. polymer can lead to a larger EHD force because of the addnl. Coulomb force, even at a low cond. of 10-7 S m-1, thus demonstrating the ability to duplicate a mushroom-like structure with a high aspect ratio, which has wide applications in superhydrophobicity, dry adhesion, nanogenerators, etc.
- 50Tian, H.; Shao, J.; Ding, Y.; Li, X.; Hu, H. Electrohydrodynamic micro-/nanostructuring processes based on prepatterned polymer and prepatterned template. Macromolecules 2014, 47, 1433– 1438, DOI: 10.1021/ma402456uGoogle ScholarThere is no corresponding record for this reference.
- 51Tian, H.; Shao, J.; Ding, Y.; Li, X.; Liu, H. Simulation of polymer rheology in an electrically induced micro- or nano-structuring process based on electrohydrodynamics and conservative level set method. RSC Adv. 2014, 4, 21672– 21680, DOI: 10.1039/c4ra00553hGoogle Scholar51Simulation of polymer rheology in an electrically induced micro- or nano-structuring process based on electrohydrodynamics and conservative level set methodTian, Hongmiao; Shao, Jinyou; Ding, Yucheng; Li, Xiangming; Liu, HongzhongRSC Advances (2014), 4 (42), 21672-21680CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)An elec. induced structuring process, such as an electrohydrodynamic (EHD) approach for fabricating polymeric micro-/nano-structures in various micro-/nano-devices, was performed by applying a voltage to an electrode pair consisting of a planar or structured template and a polymer-coated substrate sandwiching an air gap, resulting in either periodic columns or template-modulated structures. Anal. approaches were explored to characterize this micro-/nano-structuring process, based on a linear thermodn. instability of the polymer film combining capillary waves and electrostatic forces, leading to a definition of "most unstable wavelength" in relation to various process variables such as external voltage, polymer film thickness, and so on. For math. simplicity, the linear stability anal. was only carried out to demonstrate an initiation of the polymer structuring under elec. induction by an infinite planar template, and cannot numerically visualize the evolution of the polymer structure which grows from an initially flat film upwards to the template underside. Therefore, a numerical modeling of such a process, which is capable of demonstrating a full-cycle evolution of the polymer structuring, is desirable to provide an in-depth insight into this elec. induced structuring technique. This paper presents a detailed numerical formulation for simulating the rheol. behavior for this structuring process based on EHD equations and a conservative level set approach. Firstly, a numerical simulation is performed to demonstrate the dynamic evolution of periodic structures for a planar (non-structured) template and compared with the linear instability anal. to validate the effectiveness of the proposed numerical modeling. Then simulations are performed to numerically visualize the evolution of a polymer structure induced by a structured template, with a subsequent discussion about the influences of some crit. process variables, such as voltage, air gap, polymer thickness, depth of template patterns, and so on, on the electrohydrodynamic rheol. of the polymer.
- 52Verma, R.; Sharma, A.; Kargupta, K.; Bhaumik, J. Electric field induced instability and pattern formation in thin liquid films. Langmuir 2005, 21, 3710– 3721, DOI: 10.1021/la0472100Google Scholar52Electric Field Induced Instability and Pattern Formation in Thin Liquid FilmsVerma, Ruhi; Sharma, Ashutosh; Kargupta, Kajari; Bhaumik, JaitaLangmuir (2005), 21 (8), 3710-3721CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)Electrostatic field induced instability, morphol., and patterning of a thin liq. film confined between two electrodes with an air gap are studied on the basis of nonlinear 3D simulations, both for spatially homogeneous and heterogeneous fields. In addn. to the spinodal flow resulting from the variation of field because of local thickness changes, a heterogeneous imposed field also moves the liq. from the regions of low field to high field, thus allowing a more precise control of pattern. Hexagonal packing of liq. columns is obsd. for a spatially homogeneous elec. field, which is in accord with the e-field expts. on thin polymer films (Schaffer et al. Nature 2000, 403, 874). For a large liq. vol. fraction in the gap, φ ≥ 0.75, the coalescence of columns causes a phase inversion, leading to the formation of air columns or cylindrical holes trapped in the liq. matrix (air-in-liq. dispersion). Locally ordered aligned patterns are formed by imposing a spatial variation of the electrostatic field by using a topog. patterned electrode. For example, multiple rows/lines of liq. columns are formed near the edge of a steplike heterogeneity of the electrode and annular rings of ordered columns or concentric ripples are formed around a heterogeneous circular patch. Simulations predict that the electrode pattern is replicated in the film only when the pattern periodicity, Lp, exceeds the instability length scale on the basis of the min. interelectrode sepn. distance, Lp ≥ λm-dmin. Thus, the formation of secondary structures can be suppressed by employing an electrode with deep grooves and stronger field gradients, which produces almost ideal templating. The no. d. of the elec. field induced patterns can be altered by tuning the mean film thickness (or the vol. fraction of liq. in the gap), periodicity and depth (amplitude) of the grooves on the top electrode, and the applied voltage. The implications are in electrostatic lithog., pattern replication in soft materials, and the design and interpretation of thin film expts. involving elec. fields.
- 53Harkema, S. Capillary Instabilities in Thin Polymer Films. Ph.D. Thesis, University of Groningen, The Netherlands, 2006.Google ScholarThere is no corresponding record for this reference.
- 54Dwivedi, S.; Vivek; Mukherjee, R.; Atta, A. Formation and control of secondary nanostructures in electro-hydrodynamic patterning of ultra-thin films. Thin Solid Films 2017, 642, 241– 251, DOI: 10.1016/j.tsf.2017.09.029Google Scholar54Formation and control of secondary nanostructures in electro-hydrodynamic patterning of ultra-thin filmsDwivedi, Swarit; Vivek; Mukherjee, Rabibrata; Atta, ArnabThin Solid Films (2017), 642 (), 241-251CODEN: THSFAP; ISSN:0040-6090. (Elsevier B.V.)Based on a computational model involving finite element method, the authors report the morphol. evolution and pattern formation process in a thin polymer film subject to an externally imposed periodically varying heterogeneous DC elec. field, induced by topog. patterned top electrode. Quasi-steady state morphol. is obsd. to depend on remnant liq. layer after the evolution of primary structures. The dynamics of remnant layer de-wetting that essentially is dictated by electrode assembly, fluid properties, and initial film thickness, results in formation of intermediate secondary structures. The possibility of controlling these structures is explored by varying periodicity of the stamp/top electrode, applied potential and initial liq. vol. At a fixed operating condition, crit. periodicity is identified for stable secondary structure formation that may lead to ordered nano-patterns for diverse applications.
- 55Atta, A.; Crawford, D. G.; Koch, C. R.; Bhattacharjee, S. Influence of electrostatic and chemical heterogeneity on the electric-field-induced destabilization of thin liquid films. Langmuir 2011, 27, 12472– 12485, DOI: 10.1021/la202759jGoogle ScholarThere is no corresponding record for this reference.
- 56Dwivedi, S.; Mukherjee, R.; Atta, A. Re-entrant structural evolution using electrically heterogeneous patterned electrode. Comput.-Aided Chem. Eng. 2017, 40, 1213– 1218, DOI: 10.1016/b978-0-444-63965-3.50204-xGoogle ScholarThere is no corresponding record for this reference.
- 57Nazaripoor, H.; Koch, C. R.; Sadrzadeh, M. Ordered high aspect ratio nanopillar formation based on electrical and thermal reflowing of prepatterned thin films. J. Colloid Interface Sci. 2018, 530, 312, DOI: 10.1016/j.jcis.2018.06.080Google ScholarThere is no corresponding record for this reference.
- 58Hu, H.; Tian, H.; Shao, J.; Ding, Y.; Jiang, C.; Liu, H. Fabrication of bifocal microlens arrays based on controlled electrohydrodynamic reflowing of pre-patterned polymer. J. Micromech. Microeng. 2014, 24, 095027, DOI: 10.1088/0960-1317/24/9/095027Google Scholar58Fabrication of bifocal microlens arrays based on controlled electrohydrodynamic reflowing of pre-patterned polymerHu, Hong; Tian, Hongmiao; Shao, Jinyou; Ding, Yucheng; Jiang, Chengbao; Liu, HongzhongJournal of Micromechanics and Microengineering (2014), 24 (9), 095027/1-095027/8, 8 pp.CODEN: JMMIEZ; ISSN:1361-6439. (IOP Publishing Ltd.)An easy method based on electrohydrodynamic (EHD) reflowing of pre-patterned polymer is proposed in this study for the fabrication of bifocal microlens arrays (MLAs). The method comprises two sequential steps, i.e. hot embossing for generating a polymer-based micropillar array and EHD reflowing of the micropillars for the formation of a bifocal MLA with controllable surface shape and optical performance. The EHD reflowing process is achieved by applying a voltage across an electrode pair sandwiching an air gap and the pre-patterned polymer, and the EHD force induced on the air-polymer interface reshapes the pillar array into the MLA. The complex bifocal microlens can be achieved only when the elec. intensity is stronger than that required to produce a commonly known Taylor cone, which is formed when the EHD force exactly surpasses the surface tension. Finally, the light through MLA is imaged on a moving charge-coupled device (CCD) camera and leads to an observation of two focal planes.
- 59Williams, M. B.; Davis, S. H. Nonlinear theory of film rupture. J. Colloid Interface Sci. 1982, 90, 220– 228, DOI: 10.1016/0021-9797(82)90415-5Google ScholarThere is no corresponding record for this reference.
- 60Oron, A.; Davis, S. H.; Bankoff, S. G. Long-scale evolution of thin liquid films. Rev. Mod. Phys. 1997, 69, 931– 980, DOI: 10.1103/revmodphys.69.931Google Scholar60Long-scale evolution of thin liquid filmsOron, Alexander; Davis, Stephen H.; Bankoff, S. GeorgeReviews of Modern Physics (1997), 69 (3), 931-980CODEN: RMPHAT; ISSN:0034-6861. (American Physical Society)Macroscopic thin liq. films are entities that are important in biophysics, physics, and engineering, as well as in natural settings. They can be composed of common liqs. such as water or oil, rheol. complex materials such as polymers solns. or melts, or complex mixts. of phases or components. When the films are subjected to the action of various mech., thermal, or structural factors, they display interesting dynamic phenomena such as wave propagation, wave steepening, and development of chaotic responses. Such films can display rupture phenomena creating holes, spreading of fronts, and the development of fingers. In this review a unified math. theory is presented that takes advantage of the disparity of the length scales and is based on the asymptotic procedure of redn. of the full set of governing equations and boundary conditions to a simplified, highly nonlinear, evolution equation or to a set of equations. As a result of this long-wave theory, a math. system is obtained that does not have the math. complexity of the original free-boundary problem but does preserve many of the important features of its physics. The basics of the long-wave theory are explained. If, in addn., the Reynolds no. of the flow is not too large, the analogy with Reynolds theory of lubrication can be drawn. A general nonlinear evolution equation or equations are then derived and various particular cases are considered. Each case contains a discussion of the linear stability properties of the base-state solns. and of the nonlinear spatiotemporal evolution of the interface (and other scalar variables, such as temp. or solute concn.). The cases reducing to a single highly nonlinear evolution equation are first examd. These include: (a) films with const. interfacial shear stress and const. surface tension, (b) films with const. surface tension and gravity only, (c) films with van der Waals (long-range mol.) forces and const. surface tension only, (d) films with thermocapillarity, surface tension, and body force only, (e) films with temp.-dependent phys. properties, (f) evapg./condensing films, (g) films on a thick substrate, (h) films on a horizontal cylinder, and (i) films on a rotating disk. The dynamics of the films with a spatial dependence of the base-state soln. are then studied. These include the examples of nonuniform temp. or heat flux at liq.-solid boundaries. Problems which reduce to a set of nonlinear evolution equations are considered next. Those include (a) the dynamics of free liq. films, (b) bounded films with interfacial viscosity, and (c) dynamics of sol. and insol. surfactants in bounded and free films. The spreading of drops on a solid surface and moving contact lines, including effects of heat and mass transport and van der Waals attractions, are then addressed. Several related topics such as falling films and sheets and Hele-Shaw flows are also briefly discussed. The results discussed give motivation for the development of careful expts. which can be used to test the theories and exhibit new phenomena. Many refs.
- 61Li, H.; Yu, W.; Zhang, L.; Liu, Z.; Brown, K. E.; Abraham, E.; Cargill, S.; Tonry, C.; Patel, M. K.; Bailey, C.; Desmulliez, M. P. Y. Simulation and modelling of sub-30 nm polymeric channels fabricated by electrostatic induced lithography. RSC Adv. 2013, 3, 11839– 11845, DOI: 10.1039/c3ra40188jGoogle Scholar61Simulation and modeling of sub-30 nm polymeric channels fabricated by electrostatic induced lithographyLi, H.; Yu, W.; Zhang, L.; Liu, Z.; Brown, K. E.; Abraham, E.; Cargill, S.; Tonry, C.; Patel, M. K.; Bailey, C.; Desmulliez, M. P. Y.RSC Advances (2013), 3 (29), 11839-11845CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)This article demonstrates, through finite element anal., the possibility to manuf. sub-30 nm polymeric channels using electrostatic induced lithog. Channels with a width of 25 nm, a depth of 50 nm and an inter-channel wall of 28 nm can be obtained by this patterning process. The influence of operational parameters such as the filling factor, the aspect ratio of the master electrode, the applied voltage and the gap between the two electrodes and initial film thickness has been studied in detail to define the fabrication limits of this process in the case of periodic nanostructures. Conclusions for such nanostructures can be generalized to other shapes manufd. from polymers.
- 62COMSOL, COMSOL Multiphysics User’s Guide , 2015.Google ScholarThere is no corresponding record for this reference.
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Abstract
Figure 1
Figure 1. Schematic of electrode assembly with polymeric liquid film denoting the geometrical parameters and top electrode as (a) flat plate and (b) patterned stamp with square protrusions, and (c) 3D representation of patterned stamp with column protrusions electrode assembly.
Figure 2
Figure 2. Schematic of computational domain, boundaries, and points used in this work.
Figure 3
Figure 3. Initial electric field intensity distribution on a 30 nm polymeric film under protruded top electrode having w = λd1 = 214 nm, and p = 20 nm for 70 V, d2 = 100 nm, and Lp = 350 nm.
Figure 4
Figure 4. Temporal evolution of a 30 nm polymeric film under protruded top electrode having w = λd1 = 214 nm, and p = 20 nm for ψ = 70 V, d2 = 100 nm, and Lp = 350 nm at (a) 0, (b) 2, (c) 6, (d) 15, and (e) 100 μs.
Figure 5
Figure 5. Quasi-steady-state patterns obtained for ψ = 70 V, h0 = 30 nm, p = 20 nm, d2 = 100 nm, and Lp = 350 nm at (a) w < λd1, (b) w = λd1, (c) w = λ(d1+davg)/2, (d) w = λdavg, (e) w = λ(davg+d2)/2, and (f) w = λd2.
Figure 6
Figure 6. Initial electric-field-intensity distribution on a 30 nm polymeric film under protruded top electrode having w = λc, p = 20 nm for 70 V, d2 = 100 nm, and Lp = 350 nm.
Figure 7
Figure 7. Quasi-steady-state patterns of a 30 nm polymeric film under a protruded top electrode having w = 225 nm, and p = 20 nm at 70 V, and d2 = 100 nm at (a) Lp = 250 nm, (b) Lp = 300 nm, (c) Lp = 350 nm, and (d) Lp = 400 nm.
Figure 8
Figure 8. Initial electric-field-intensity distribution for different Lp on a 30 nm polymer film under protruded top electrode (w = 225 nm, p = 20 nm) at 70 V and d2 = 100 nm.
Figure 9
Figure 9. Quasi-steady-state structure obtained at ψ = 70 V under a protruded top electrode having w = λ(d1+davg)/2 nm, and p = 20 nm with d2 = 100 nm and filling factor = 0.6 for (a) h0 = 25 nm, (b) h0 = 30 nm, (c) h0 = 35 nm, and (d) h0 = 40 nm.
Figure 10
Figure 10. Quasi-steady-state structure obtained at ψ = 70 V under a protruded top electrode having w = 225 nm and p = 20 nm with d2 = 100 nm and Lp = 350 nm for (a) h0 = 25 nm, (b) h0 = 30 nm, (c) h0 = 35 nm, and (d) h0 = 40 nm.
Figure 11
Figure 11. Morphological phase diagram of single, nanogroove, and separate pillars for different electrode widths and initial film thicknesses.
Figure 12
Figure 12. Quasi-steady-state structure of a 30 nm polymeric film under a protruded top electrode having w = 225 nm and p = 20 nm with d2 = 100 nm and Lp = 350 nm at (a) ψ = 60 V, (b) ψ = 65 V, (c) ψ = 70 V, (d) ψ = 75 V, and (e) ψ = 80 V.
Figure 13
Figure 13. Quasi-steady-state structure of a 30 nm polymeric film at 70 V for different protrusion heights (p) (a) 10 nm, (b) 20 nm, (c) 30 nm, and (d) 40 nm with w = 214 nm, d2 = 100 nm, and Lp = 350 nm.
Figure 14
Figure 14. Quasi-steady-state structure of a 30 nm polymeric film at 70 V with w = 214 nm, p = 20 nm, and Lp = 350 nm for (a) d2 = 90 nm, (b) d2 = 100 nm, and (c) d2 = 110 nm.
References
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- 1Duncan, R. The dawning era of polymer therapeutics. Nat. Rev. Drug Discovery 2003, 2, 347– 360, DOI: 10.1038/nrd10881The dawning era of polymer therapeuticsDuncan, RuthNature Reviews Drug Discovery (2003), 2 (5), 347-360CODEN: NRDDAG; ISSN:1474-1776. (Nature Publishing Group)A review. As we enter the twenty-first century, research at the interface of polymer chem. and the biomedical sciences has given rise to the first nano-sized (5-100 nm) polymer-based pharmaceuticals, the 'polymer therapeutics'. Polymer therapeutics include rationally designed macromol. drugs, polymer-drug and polymer-protein conjugates, polymeric micelles contg. covalently bound drug, and polyplexes for DNA delivery. The successful clin. application of polymer-protein conjugates, and promising clin. results arising from trials with polymer-anticancer-drug conjugates, bode well for the future design and development of the ever more sophisticated bio-nanotechnologies that are needed to realize the full potential of the post-genomic age.
- 2Pais, A.; Banerjee, A.; Klotzkin, D.; Papautsky, I. High-sensitivity, disposable lab-on-a-chip with thin-film organic electronics for fluorescence detection. Lab Chip 2008, 8, 794– 800, DOI: 10.1039/b715143h2High-sensitivity, disposable lab-on-a-chip with thin-film organic electronics for fluorescence detectionPais, Andrea; Banerjee, Ansuman; Klotzkin, David; Papautsky, IanLab on a Chip (2008), 8 (5), 794-800CODEN: LCAHAM; ISSN:1473-0197. (Royal Society of Chemistry)The authors report a high-sensitivity, disposable lab-on-a-chip with a thin-film org. light-emitting diode (OLED) excitation source and an org. photodiode (OPD) detector for on-chip fluorescence anal. A NPB/Alq3 thin-film green OLED with an active area of 0.1 cm2 was used as the excitation source, while a CuPC/C60 thin-film OPD with 0.6 cm2 active area was used as a photodetector. A novel cost-effective, cross-polarization scheme was used to filter out excitation light from a fluorescent dye emission spectrum. The excitation light from the OLED was linearly polarized and used to illuminate a microfluidic device contg. a 1 μL vol. of dye dissolved in ethanol. The detector was shielded by a second polarizer, oriented orthogonally to the excitation light, thus reducing the photocurrent due to excitation light leakage on the detector by approx. 25 dB. The fluorescence emission light, which is randomly polarized, is only attenuated by ∼3 dB. Fluorescence signals from Rhodamine 6G (peak emission wavelength of 570 nm) and fluorescein (peak emission wavelength of 494 nm) dyes were measured in a diln. series in the microfluidic device with emission signals detected by the OPD. A limit-of-detection of 100 nM was demonstrated for Rhodamine 6G, and 10 μM for fluorescein. This suggests that an integrated microfluidic device, with an org. photodiode and LED excitation source and integrated polarizers, can be fabricated to realize a compact and economical lab-on-a-chip for point-of-care fluorescence assays.
- 3Stone, H. A.; Kim, S. Microfluidics: Basic issues, applications, and challenges. AIChE J. 2001, 47, 1250– 1254, DOI: 10.1002/aic.6904706023Microfluidics: basic issues, applications, and challengesStone, H. A.; Kim, S.AIChE Journal (2001), 47 (6), 1250-1254CODEN: AICEAC; ISSN:0001-1541. (American Institute of Chemical Engineers)A review with many refs. The objective of this article is to highlight some avenues of research and development in micro-fluidics.
- 4Wicks, Z. W., Jr.; Jones, F. N.; Pappas, S. P.; Wicks, D. A. Organic Coatings: Science and Technology; John Wiley & Sons, 2007.There is no corresponding record for this reference.
- 5Hoppe, H.; Niggemann, M.; Winder, C.; Kraut, J.; Hiesgen, R.; Hinsch, A.; Meissner, D.; Sariciftci, N. S. Nanoscale Morphology of Conjugated Polymer/Fullerene-Based Bulk- Heterojunction Solar Cells. Adv. Funct. Mater. 2004, 14, 1005– 1011, DOI: 10.1002/adfm.2003050265Nanoscale morphology of conjugated polymer/fullerene-based bulk-heterojunction solar cellsHoppe, Harald; Niggemann, Michael; Winder, Christoph; Kraut, Juergen; Hiesgen, Renate; Hinsch, Andreas; Meissner, Dieter; Sariciftci, Niyazi SerdarAdvanced Functional Materials (2004), 14 (10), 1005-1011CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)The relation between the nanoscale morphol. and device properties for conjugated polymer/fullerene bulk-heterojunction plastic solar cells, was studied. Complementary measurements on solid-state blends of poly[2-methoxy-5-(3,7-dimethyloctyloxy)]-1,4-phenylenevinylene (MDMO-PPV) and the sol. fullerene C60 deriv. 1-(3-methoxycarbonyl) propyl-1-Ph [6,6]C61 (PCBM), spin-cast from toluene or chlorobenzene solns., were made. The characterization of the nanomorphol. was carried out by SEM and AFM, while solar-cell devices were characterized by I-V and spectral photocurrent measurements. The morphol. was manipulated by annealing to increase the extent of phase sepn. in the thin-film blends and to identify the distribution of materials. Photoluminescence measurements confirm the demixing of the materials by the thermal treatment. The photoluminescence of PCBM clusters with sizes of up to a few hundred nanometers indicates a photocurrent loss in films of the coarser phase-sepd. blends cast from toluene. For toluene-cast films the scale of phase sepn. depends strongly on the ratio of MDMO-PPV to PCBM, as well as on the total concn. of the casting soln. Small beads of 20-30 nm diam. were obsd. in blend films cast from both toluene and chlorobenzene and these are attributed to MDMO-PPV.
- 6Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient planar heterojunction perovskite solar cells by vapour deposition. Nature 2013, 501, 395– 398, DOI: 10.1038/nature125096Efficient planar heterojunction perovskite solar cells by vapour depositionLiu, Mingzhen; Johnston, Michael B.; Snaith, Henry J.Nature (London, United Kingdom) (2013), 501 (7467), 395-398CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)Many different photovoltaic technologies are being developed for large-scale solar energy conversion. The wafer-based 1st-generation photovoltaic devices have been followed by thin-film solid semiconductor absorber layers sandwiched between 2 charge-selective contacts and nanostructured (or mesostructured) solar cells that rely on a distributed heterojunction to generate charge and to transport pos. and neg. charges in spatially sepd. phases. Although many materials have been used in nanostructured devices, the goal of attaining high-efficiency thin-film solar cells in such a way has yet to be achieved. Organometal halide perovskites have recently emerged as a promising material for high-efficiency nanostructured devices. Nanostructuring is not necessary to achieve high efficiencies with this material: a simple planar heterojunction solar cell incorporating vapor-deposited perovskite as the absorbing layer can have solar-to-elec. power conversion efficiencies of over 15 per cent (as measured under simulated full sunlight). Perovskite absorbers can function at the highest efficiencies in simplified device architectures, without the need for complex nanostructures.
- 7Sperling, L. H. Introduction to Physical Polymer Science; Wiley-Blackwell, 2005; Chapter 13, pp 687– 756.There is no corresponding record for this reference.
- 8Wu, N.; Russel, W. B. Micro- and nano-patterns created via electrohydrodynamic instabilities. Nano Today 2009, 4, 180– 192, DOI: 10.1016/j.nantod.2009.02.0028Micro- and nano-patterns created via electrohydrodynamic instabilitiesWu, Ning; Russel, William B.Nano Today (2009), 4 (2), 180-192CODEN: NTAOCG; ISSN:1748-0132. (Elsevier Ltd.)A review. Intense interests on surfaces with micro- and nano-patterns originated from abundant applications in optics, electronics, tissue engineering, biomedical sensors, etc. Searching for ways to create small patterns in both org. and inorg. materials at reduced operation time and cost is one of the challenges in nanotechnol. This paper reviews recent advances in patterning of thin films by using electrohydrodynamic instabilities, a simple and cost-effective bottom-up method. Challenges and future directions are also discussed from the point of view of both fundamental understanding and technol. applications of the pattern formation induced by the elec. field.
- 9Mukherjee, R.; Sharma, A. Instability, self-organization and pattern formation in thin soft films. Soft Matter 2015, 11, 8717– 8740, DOI: 10.1039/c5sm01724f9Instability, self-organization and pattern formation in thin soft filmsMukherjee, Rabibrata; Sharma, AshutoshSoft Matter (2015), 11 (45), 8717-8740CODEN: SMOABF; ISSN:1744-683X. (Royal Society of Chemistry)The free surface of a thin soft polymer film is often found to become unstable and self-organizes into various meso-scale structures. In this article we classify the instability of a thin polymer film into three broad categories, which are: category 1: instability of an ultra-thin (<100 nm) viscous film engendered by amplification of thermally excited surface capillary waves due to interfacial dispersive van der Waals forces; category 2: instability arising from the attractive inter-surface interactions between the free surface of a soft film exhibiting room temp. elasticity and another rigid surface in its contact proximity; and category 3: instability caused by an externally applied field such as an elec. field or a thermal gradient, obsd. in both viscous and elastic films. We review the salient features of each instability class and highlight how characteristic length scales, feature morphologies, evolution pathways, etc. depend on initial properties such as film thickness, visco-elasticity (rheol.), residual stress, and film prepn. conditions. We emphasize various possible strategies for aligning and ordering of the otherwise isotropic structures by combining the essential concepts of bottom-up and top-down approaches. A perspective, including a possible future direction of research, novelty and limitations of the methods, particularly in comparison to the existing patterning techniques, is also presented for each setting.
- 10Bhandaru, N.; Das, A.; Mukherjee, R. Confinement induced ordering in dewetting of ultra-thin polymer bilayers on nanopatterned substrates. Nanoscale 2016, 8, 1073– 1087, DOI: 10.1039/c5nr06690e10Confinement induced ordering in dewetting of ultra-thin polymer bilayers on nanopatterned substratesBhandaru, Nandini; Das, Anuja; Mukherjee, RabibrataNanoscale (2016), 8 (2), 1073-1087CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)We report the dewetting of a thin bilayer of polystyrene (PS) and poly(methylmethacrylate) (PMMA) on a topog. patterned nonwettable substrate comprising an array of pillars, arranged in a square lattice. With a gradual increase in the concn. of the PMMA soln. (Cn-PMMA), the morphol. of the bottom layer changes to: (1) an aligned array of spin dewetted droplets arranged along substrate grooves at very low Cn-PMMA; (2) an interconnected network of threads surrounding each pillar at intermediate Cn-PMMA; and (3) a continuous bottom layer at higher Cn-PMMA. On the other hand the morphol. of the PS top layer depends largely on the nature of the pre-existing bottom layer, in addn. to Cn-PS. An ordered array of PMMA core-PS shell droplets forms right after spin coating when both Cn-PMMA and Cn-PS are very low. Bilayers with all other initial configurations evolve during thermal annealing, resulting in a variety of ordered structures. Unique morphologies realized include laterally coexisting structures of the two polymers confined within the substrate grooves due to initial rupture of the bottom layer on the substrate followed by a squeezing flow of the top layer; an array of core-shell and single polymer droplets arranged in an alternating order etc., to highlight a few. Such structures cannot be fabricated by any stand-alone lithog. technique.
- 11Swan, J. W. Stress and other effects produced in resin and in a viscid compound of resin and oil by electrification. Proc. R. Soc. London 1897, 62, 38– 46, DOI: 10.1098/rspl.1897.0077There is no corresponding record for this reference.
- 12Schäffer, E.; Thurn-Albrecht, T.; Russell, T. P.; Steiner, U. Electrically induced structure formation and pattern transfer. Nature 2000, 403, 874– 877, DOI: 10.1038/3500254012Electrically induced structure formation and pattern transferSchaffer, Erik; Thurn-Albrecht, Thomas; Russell, Thomas P.; Steiner, UllrichNature (London) (2000), 403 (6772), 874-877CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)The interaction of an elec. field with matter is well understood. Common expts. demonstrate the forces on dielec. materials in elec. fields, such as a dielec. liq. which is drawn into the air gap of a plate capacitor when a voltage is applied across the plates. As electrostatic interactions are relatively strong and long-range, they can be used to control structures on length scales which are difficult to manipulate in any other manner. One challenge in this context is the creation of lateral features below the diffraction limit of light. Various techniques for the manuf. of lithog. layers with submicrometre structures have been proposed, such as particle lithog., lithog. using scanning near-field microscopes, and printing or embossing structures onto surfaces. Here we describe a particularly simple addn. to this collection of submicrometre replication techniques: the electrostatic transfer of a master pattern into a polymer film. While elec. fields have been used before to orient the microphases in block copolymer melts, the work presented here focuses on the use of elec. fields to control and organize polymers laterally to replicate a master pattern.
- 13Morariu, M. D.; Voicu, N. E.; Schäffer, E.; Lin, Z.; Russell, T. P.; Steiner, U. Hierarchical structure formation and pattern replication induced by an electric field. Nat. Mater. 2003, 2, 48– 52, DOI: 10.1038/nmat78913Hierarchical structure formation and pattern replication induced by an electric fieldMorariu, Mihai D.; Voicu, Nicoleta E.; Schaeffer, Erik; Lin, Zhiqun; Russell, Thomas P.; Steiner, UllrichNature Materials (2003), 2 (1), 48-52CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)Several techniques based on soft lithog. have emerged to replicate micrometer-sized patterns. Similar to most other lithog. methods, these techniques structure a single layer of photoresist. For many applications, however, it is desirable to control the spatial arrangement of more than one component. With traditional methods, this requires an iterative, multistep procedure, making the replication process more complex and less reliable. Here, a replication process is described where multiple materials are processed simultaneously. Using a bilayer formed by two different polymers, electrohydrodynamic instabilities at both polymer surfaces produce a hierarchic lateral structure that exhibits two independent characteristic dimensions. A lateral modulation of the elec. field enables replication with a resoln. down to 100 nm. This approach might provide a simple strategy for large-area, sub-100-nm lithog.
- 14Xiang, H.; Lin, Y.; Russell, T. P. Electrically induced patterning in block copolymer films. Macromolecules 2004, 37, 5358– 5363, DOI: 10.1021/ma049888s14Electrically Induced Patterning in Block Copolymer FilmsXiang, Hongqi; Lin, Yao; Russell, Thomas P.Macromolecules (2004), 37 (14), 5358-5363CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)Thin films of block copolymers were manipulated on two different length scales simultaneously by use of an elec. field. The copolymers are asym. diblock copolymers of polystyrene-block-poly(2-vinylpyridine) (PS-b-PVP), polystyrene-block-polyisoprene (PS-b-PI), and polystyrene-block-polybutadiene (PS-b-PBD). Electrostatic pressure generated at the surface of a block copolymer film between two electrodes with an air gap sepg. the surface of the block copolymer film and the upper electrode produced, as in homopolymer films, an array of hexagonally ordered columns, tens of microns in size, that spanned between the two electrodes. Within each column the diblock copolymer microphase sepd. into hexagonally packed cylindrical microdomains, tens of nanometers in size. The orientation of these microdomains was controlled by the interfacial energy of each block with the surfaces of the electrode and the direction of the applied field. Microdomain alignment parallel to and normal to the applied field could be controlled by the strength of the interfacial interactions. Thus, block copolymers can be controlled on two different length scales simultaneously and the method is a new route toward fabricating functional nanoscopic structures in thin polymer films.
- 15Lee, S. H.; Kim, P.; Jeong, H. E.; Suh, K. Y. Electrically induced formation of uncapped, hollow polymeric microstructures. J. Micromech. Microeng. 2006, 16, 2292– 2297, DOI: 10.1088/0960-1317/16/11/00715Electrically induced formation of uncapped, hollow polymeric microstructuresLee, Sung Hun; Kim, Pilnam; Jeong, Hoon Eui; Suh, Kahp Y.Journal of Micromechanics and Microengineering (2006), 16 (11), 2292-2297CODEN: JMMIEZ; ISSN:0960-1317. (Institute of Physics Publishing)Uncapped, hollow polymeric microstructures were fabricated on a silicon substrate using elec. field induced stretching and detachment. Initially, square or cylinder microposts were generated using a solvent-assisted capillary molding technique, and a featureless electrode mask was positioned on the top of the microstructure with spacers maintaining an air gap (∼20 μm). Upon exposure to an external elec. field (1.0-3.0 V μm-1), the hollow microstructures were destabilized and stretched by the well-known electrohydrodynamic instability, resulting in contact of the top polymer surface with the mask. Subsequently, detachment of the capping layer occurred upon removal of the mask due to larger adhesion forces at the polymer/mask interface than cohesion forces of the polymer. These hollow microstructures were tested to capture the budding yeast, Saccharomyces cerevisiae, for shear protection.
- 16Zhou, S.; Zheng, H.; Li, G.; Liu, J.; Liu, S. Formation of controllable polymer micropatterns through liquid film electro-dewetting. Appl. Surf. Sci. 2018, 436, 839– 845, DOI: 10.1016/j.apsusc.2017.12.07016Formation of controllable polymer micropatterns through liquid film electro-dewettingZhou, Shangru; Zheng, Huai; Li, Guoliang; Liu, Jie; Liu, ShengApplied Surface Science (2018), 436 (), 839-845CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)Controllable polymer micropatterns, served as indispensable function structures, are extensively required in many micro/nano scientific areas and engineering applications. Exploring advanced methods of fabricating micropatterns is always a research hotspot. In this article, we introduce a novel method of patterning polymer by the electro-dewetting induced by corona discharge. For the first time, it is obsd. exptl. that liq. polymer on conductive/non-conductive patterned substrates, spontaneously converges from non-conductive areas to conductive areas under the action of ion wind. Taking advantage of such a flow phenomenon, controllable polymer micropatterns including microbump arrays and microwell arrays are fabricated successfully. Their sizes range from hundreds of microns to millimeters. Micropattern surfaces present an ultra-smooth characteristic, with roughness in the nanometer range.
- 17Rickard, J. J. S.; Farrer, I.; Oppenheimer, P. G. Tunable nanopatterning of conductive polymers via electrohydrodynamic lithography. ACS Nano 2016, 10, 3865– 3870, DOI: 10.1021/acsnano.6b0124617Tunable Nanopatterning of Conductive Polymers via Electrohydrodynamic LithographyRickard, Jonathan James Stanley; Farrer, Ian; Goldberg Oppenheimer, PolaACS Nano (2016), 10 (3), 3865-3870CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)The authors introduce advanced yet simple electrohydrodynamic lithog. (EHL) for patterning conductive polymers directly on a substrate with high fidelity. The generality of this method is demonstrated by structuring thin polypyrrole films via elec.-field-induced instabilities, yielding well-defined conductive structures with feature sizes ranging from tens of micrometers to hundreds of nanometers. Exploitation of a conductive polymer induces free charge suppression of the field in the polymer film, paving the way for accessing scale sizes in the low submicron range. The authors show the feasibility of the polypyrrole-based structures for field-effect transistor devices. Controlled EHL pattering of conductive polymer structures at the micro and nano scale demonstrated in this study combined with the possibility of effectively tuning the dimensions of the tailor-made architectures might herald a route toward various submicron device applications in supercapacitors, photovoltaics, sensors, and electronic displays.
- 18Goldberg-Oppenheimer, P.; Kabra, D.; Vignolini, S.; Hüttner, S.; Sommer, M.; Neumann, K.; Thelakkat, M.; Steiner, U. Hierarchical orientation of crystallinity by block-copolymer patterning and alignment in an electric field. Chem. Mater. 2013, 25, 1063– 1070, DOI: 10.1021/cm303807518Hierarchical Orientation of Crystallinity by Block-Copolymer Patterning and Alignment in an Electric FieldGoldberg-Oppenheimer, Pola; Kabra, Dinesh; Vignolini, Silvia; Huttner, Sven; Sommer, Michael; Neumann, Katharina; Thelakkat, Mukundan; Steiner, UllrichChemistry of Materials (2013), 25 (7), 1063-1070CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)Electron and hole conducting 10-nm-wide polymer morphologies hold great promise for org. electro-optical devices such as solar cells and light emitting diodes. The self-assembly of block-copolymers (BCPs) is often viewed as an efficient way to generate such materials. Here, a functional block copolymer that contains perylene bismide (PBI) side chains which can crystallize via π-π stacking to form an electron conducting microphase is patterned harnessing hierarchical electrohydrodynamic lithog. (HEHL). HEHL film destabilization creates a hierarchical structure with three distinct length scales: (1) micrometer-sized polymer pillars, contg. (2) a 10-nm BCP microphase morphol. that is aligned perpendicular to the substrate surface and (3) on a mol. length scale(coating process) (0.35-3 nm) PBI π-π-stacks traverse the HEHL-generated plugs in a continuous fashion. The good control over BCP and PBI alignment inside the generated vertical microstructures gives rise to liq.-crystal-like optical dichroism of the HEHL patterned films, and improves the electron cond. across the film by 3 orders of magnitude.
- 19Voicu, N. E.; Saifullah, M. S. M.; Subramanian, K. R. V.; Welland, M. E.; Steiner, U. TiO2 patterning using electro-hydrodynamic lithography. Soft Matter 2007, 3, 554– 557, DOI: 10.1039/b616538a19TiO2 patterning using electro-hydrodynamic lithographyVoicu, Nicoleta E.; Saifullah, M. S. M.; Subramanian, K. R. V.; Welland, Mark E.; Steiner, UllrichSoft Matter (2007), 3 (5), 554-557CODEN: SMOABF; ISSN:1744-683X. (Royal Society of Chemistry)An increasing no. of technologies require the fabrication of micro- and nanostructures over large areas. Soft lithog. methods are gaining in popularity for the manuf. of low-cost micrometer and sub-micrometer structures. Increasingly, these methods developed to structure org. resists can also be used to pattern inorg. materials. Here we introduce a simple lithog. technique that is able to pattern ceramic TiO2 micro- and nanostructures with high fidelity. Our method makes use of an electrohydrodynamic (EHD) film instability that is controlled by a laterally modulated elec. field. A spin-coated film of a stabilized metal alkoxide precursor material was patterned using EHD lithog. followed by a heat treatment at 400 °C to yield cryst. TiO2 micropatterns. Our technique is rather general and can be extended to a no. of single- and multicomponent oxide systems.
- 20Lv, G.; Liu, Y.; Shao, J.; Tian, H.; Yu, D. Facile fabrication of electrohydrodynamic micro-/nanostructures with high aspect ratio of a conducting polymer for large-scale superhydrophilic/superhydrophobic surfaces. Macromol. Mater. Eng. 2018, 303, 1700361, DOI: 10.1002/mame.201700361There is no corresponding record for this reference.
- 21Lv, G.; Zhang, S.; Shao, J.; Wang, G.; Tian, H.; Yu, D. Rapid fabrication of electrohydrodynamic micro-/nanostructures with high aspect ratio using a leaky dielectric photoresist. React. Funct. Polym. 2017, 118, 1– 9, DOI: 10.1016/j.reactfunctpolym.2017.06.01421Rapid fabrication of electrohydrodynamic micro-/nanostructures with high aspect ratio using a leaky dielectric photoresistLv, Guowei; Zhang, Shihu; Shao, Jinyou; Wang, Guolong; Tian, Hongmiao; Yu, DemeiReactive & Functional Polymers (2017), 118 (), 1-9CODEN: RFPOF6; ISSN:1381-5148. (Elsevier Ltd.)A leaky dielec. photoresist was designed and prepd. for rapid fabrication of high-aspect-ratio micro-/nanostructures via electrohydrodynamic patterning (EHDP). The rheol. behavior and elec. properties of the photoresists were systematically studied, since the structure formation in EHDP essentially originates from the flow and deformation of the polymeric film actuated by an applied elec. field. The photoresists exhibit the suitable rheol. behavior with a low viscosity of 2.4-157.7 mPa s, controllable elec. cond. of 5.0 × 10- 6 - 7.2 × 10- 4 S m- 1, as well as high homogeneity, minor surface tension of ∼30 mN m- 1, favorable wettability and film-forming property on substrate and an extremely large redn. in the contact angle (down to 1.64°) of electrowetting on dielec. (EWOD). The EHDP results showed that a higher elec. cond. of the photoresists can lead to a higher filling height, a smaller characteristic wavelength and a shorter patterning time, while a lower viscosity can also lead to a shorter patterning time, which is accordance with the theor. prediction. The patterning time of the photoresists cannot be too short because the following rapid ripening and coalescence of the formed microstructure will damage the high fidelity of the final pillar arrays.
- 22González, A.; Castellanos, A. Nonlinear electrohydrodynamic waves on films falling down an inclined plane. Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys. 1996, 53, 3573– 3578, DOI: 10.1103/physreve.53.3573There is no corresponding record for this reference.
- 23Wu, N.; Pease, L. F.; Russel, W. B. Electric-field-induced patterns in thin polymer films: weakly nonlinear and fully nonlinear evolution. Langmuir 2005, 21, 12290– 12302, DOI: 10.1021/la052099zThere is no corresponding record for this reference.
- 24Tseluiko, D.; Blyth, M. G.; Papageorgiou, D. T.; Vanden-Broeck, J.-M. Electrified falling-film flow over topography in the presence of a finite electrode. J. Eng. Math. 2010, 68, 339– 353, DOI: 10.1007/s10665-010-9377-9There is no corresponding record for this reference.
- 25Kim, H.; Bankoff, S. G.; Miksis, M. J. The effect of an electrostatic field on film flow down an inclined plane. Phys. Fluids A 1992, 4, 2117– 2130, DOI: 10.1063/1.85850825The effect of an electrostatic field on film flow down an inclined planeKim, Hyo; Bankoff, S. G.; Miksis, Michael J.Physics of Fluids A: Fluid Dynamics (1992), 4 (10), 2117-30CODEN: PFADEB; ISSN:0899-8213.A study of the interaction of an electrostatic field with a thin liq. film flowing under gravity down an inclined plane is presented. First, the effect of the elec. field on the stability of the film flow is examd. Next, several limits of the equations of motion are investigated anal., and then compared with an explicit numerical calcn. of the equations of motion. Also, applications of these calcns. to a proposed electrostatic liq. film space radiator are discussed.
- 26Uma, B.; Usha, R. A thin conducting viscous film on an inclined plane in the presence of a uniform normal electric field: Bifurcation scenarios. Phys. Fluids 2008, 20, 032102, DOI: 10.1063/1.289630026A thin conducting viscous film on an inclined plane in the presence of a uniform normal electric field: Bifurcation scenariosUma, B.; Usha, R.Physics of Fluids (2008), 20 (3), 032102/1-032102/17CODEN: PHFLE6; ISSN:1070-6631. (American Institute of Physics)A theory for two dimensional long and stationary waves of finite amplitude on a thin viscous liq. film down an inclined plane in the presence of uniform elec. field at infinity is investigated. A set of exact averaged equations for the film flow system is described and linearized stability anal. of the uniform flow is performed using normal-mode formulation and the crit. condition for linear instability is obtained. The linearized instability for the permanent wave equation, consistent to the second order in ε, is examd. and the eigenvalue properties of the fixed points are classified in various parametric regimes. Numerical integration of the permanent wave equation as a third-order dynamical system is carried out. Different bifurcation scenarios leading to multiple-hump solitary waves or leading to chaos are exhibited in the parametric space. (c) 2008 American Institute of Physics.
- 27Tseluiko, D.; Blyth, M. G.; Papageorgiou, D. T.; Vanden-Broeck, J.-M. Electrified viscous thin film flow over topography. J. Fluid Mech. 2008, 597, 449– 475, DOI: 10.1017/s002211200700986xThere is no corresponding record for this reference.
- 28Tseluiko, D.; Blyth, M. G.; Papageorgiou, D. T.; Vanden-Broeck, J.-M. Effect of an electric field on film flow down a corrugated wall at zero Reynolds number. Phys. Fluids 2008, 20, 042103, DOI: 10.1063/1.290966028Effect of an electric field on film flow down a corrugated wall at zero Reynolds numberTseluiko, D.; Blyth, M. G.; Papageorgiou, D. T.; Vanden-Broeck, J.-M.Physics of Fluids (2008), 20 (4), 042103/1-042103/19CODEN: PHFLE6; ISSN:1070-6631. (American Institute of Physics)The effect of an elec. field on a liq. layer flowing down an inclined, corrugated wall at zero Reynolds no. is investigated. The layer is taken to be either a perfect conductor or a perfect dielec. The region above the layer is assumed to be a perfect dielec. Steady flow down a wall with small-amplitude sinusoidal corrugations is considered, and it is shown how the elec. field can be used to control the amplitude of the free-surface deflection and the phase shift between the free surface and the wall profile. Steady flow over walls with large amplitude sinusoidal corrugations or other-shaped indentations is studied by using the boundary-element method. Results for flow into a wide rectangular trench are compared to previous model predictions based on the lubrication approxn. For a perfect-conductor film, the results confirm that the height of the capillary ridge, which appears above a downward step, monotonically decreases as the elec. field strength increases. Solns. for a perfect-dielec. film with relative permittivity larger than unity are similar to those for a perfect-conductor film, although the height of the capillary ridge nonmonotonically varies with the elec. field strength. The behavior of the solns. for a perfect-dielec. film with relative permittivity less than unity is qual. different. The height of the capillary ridge monotonically increases as the elec. field strength increases. Flows into narrow trenches and over narrow mounds are also computed. (c) 2008 American Institute of Physics.
- 29Lv, G.; Zhang, S.; Shao, J.; Tian, H.; Wang, G.; Yu, D. Preparation, properties, and efficient electrically induced structure formation of a leaky dielectric photoresist. RSC Adv. 2016, 6, 82450– 82458, DOI: 10.1039/c6ra17957f29Preparation, properties, and efficient electrically induced structure formation of a leaky dielectric photoresistLv, Guowei; Zhang, Shihu; Shao, Jinyou; Tian, Hongmiao; Wang, Guolong; Yu, DemeiRSC Advances (2016), 6 (85), 82450-82458CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)A leaky dielec. photoresist was designed and prepd. by doping a sol. conductive polypyrrole into a low-viscosity photocurable resin (perfect dielec.) to achieve efficient elec. induced structure formation (EISF). The comprehensive properties of both the leaky dielec. photoresist and its cured film were systematically investigated. It was found that the leaky dielec. photoresist is homogeneous and stable in both the liq. state and solid state after curing, as a result of the low mol. wt. and the bulky side groups of polypyrrole. The leaky dielec. photoresist retains the Newtonian nature of the photocurable resin with low viscosity, displays a significant increase in the elec. cond. with an increase in the polypyrrole loading, and shows favorable wettability on a silicon substrate. Meanwhile, the cured film is still transparent, thermally stable and featureless, following the increase in the polypyrrole loading. It is worth noting that by using a low-viscosity leaky dielec. photoresist, pillar arrays can be rapidly fabricated over large areas at ambient temp. via EISF onto a featureless template. The resulting patterned film is hydrophobic with an apparent contact angle of 109°, even though the cured film is hydrophilic with an intrinsic contact angle of 64°.
- 30Wu, D.; Sun, Y.; Teh, K. S.; Zhu, Y.; Luo, Y.; Deng, L.; Zhao, L.; Luo, G.; Zhao, Y.; Wang, L.; Sun, D. Investigation of electrohydrodynamic behaviors from open planar solution under rod-induced electrospinning. J. Phys. D: Appl. Phys. 2017, 50, 455602, DOI: 10.1088/1361-6463/aa8ddb30Investigation of electrohydrodynamic behaviors from open planar solution under rod-induced electrospinningWu, Dezhi; Sun, Yu; Teh, Kwok Siong; Zhu, Yuchao; Luo, Yihui; Deng, Lei; Zhao, Libo; Luo, Guoxi; Zhao, Yang; Wang, Lingyun; Sun, DaohengJournal of Physics D: Applied Physics (2017), 50 (45), 455602/1-455602/10CODEN: JPAPBE; ISSN:0022-3727. (IOP Publishing Ltd.)Understanding and controlling the electrohydrodynamic (EHD) behaviors of polymer solns. are of great importance to the design and fabrication of micro/nano devices and sensors. In this work, rod-induced electrospinning (RIES) was used as a versatile technique to study and characterize the EHD behaviors of polyethylene oxide soln. during the electrospinning process. In RIES, a grounded insulated rod moves back and forth several millimeters above an open bath of polymer soln. subjected to high voltage to induce the formation of Taylor cones that lead to subsequent polymer jet emissions from the soln. surface. Using a phase field method, we performed finite element anal. to explore the EHD behaviors of air-polymer interface in RIES process to better understand fundamentals of this process. Pertinent theories such as phase field theories and elec. field theories were briefly discussed to provide relevant background to the techniques used in this work. We investigated the RIES process and the effect of the diam. of the induction rod, applied voltage and soln. cond. on the formation of the jets. The numerical results of the formation and merging process of the jets based on different exptl. parameters are in good agreement with exptl. observations and provide a better understanding of the RIES process to improve the throughput of nanofibers.
- 31Dickey, M. D.; Gupta, S.; Leach, K. A.; Collister, E.; Willson, C. G.; Russell, T. P. Novel 3-D structures in polymer films by coupling external and internal fields. Langmuir 2006, 22, 4315– 4318, DOI: 10.1021/la052954e31Novel 3-D Structures in Polymer Films by Coupling External and Internal FieldsDickey, Michael D.; Gupta, Suresh; Leach, K. Amanda; Collister, Elizabeth; Willson, C. Grant; Russell, Thomas P.Langmuir (2006), 22 (9), 4315-4318CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)A route to produce novel three-dimensional structures in thin films is demonstrated. Such structures are most difficult to produce in a simple manner without the use of multiple fabrication steps. Here, we show the generation of 3-D cage-type structures using a combination of electrohydrodynamic instabilities and dewetting in a polymer/polymer/air trilayer. Removal of one of the components by use of a selective solvent or by degrdn. of one of the components reveals the formation of a three-dimensional structure, where one polymer is encased in the other. Thus, by coupling an external field with a surface field inherent to the polymers, a novel fabrication strategy is shown that has clear applications in microfluidics and microelectromech. systems with extensions to patterned surfaces and structured fluids, like block copolymers.
- 32Lyutakov, O.; Hüttel, I.; Prajzler, V.; Jeřábek, V.; Jančárek, A.; Hnatowicz, V.; Švorčík, V. Pattern formation in PMMA film induced by electric field. J. Polym. Sci., Part B: Polym. Phys. 2009, 47, 1131– 1135, DOI: 10.1002/polb.2171832Pattern formation in PMMA film induced by electric fieldLyutakov, Oleksiy; Huettel, Ivan; Prajzler, Vaclav; Jerabek, Vitezslav; Jancarek, Alexander; Hnatowicz, Vladimir; Svorcik, VaclavJournal of Polymer Science, Part B: Polymer Physics (2009), 47 (12), 1131-1135CODEN: JPBPEM; ISSN:0887-6266. (John Wiley & Sons, Inc.)Micro-sized patterns were created on thin poly(Me methacrylate) (PMMA) films by the effect of external field, perpendicular to the film surface. The PMMA film, prepd. by spin-coating onto Si wafer, was heated to the fluid temp. (275 °C) and a linear pattern was created by the effect of elec. field produced by a strip electrode. In another expt., a round pattern was created as a result of local laser heating of the PMMA film under homogeneous elec. field. The created patterns were analyzed by optical microscopy and profile meter. The dependence of the form and size of the created patterns on the intensity of the elec. field, exposure time, and initial film thickness was examd. Wave guiding property of a linear pattern, produced by the above technique, was examd. in a simple expt. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1131-1135, 2009.
- 33Wu, N.; Kavousanakis, M. E.; Russel, W. B. Coarsening in the electrohydrodynamic patterning of thin polymer films. Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys. 2010, 81, 026306, DOI: 10.1103/physreve.81.02630633Coarsening in the electrohydrodynamic patterning of thin polymer filmsWu, Ning; Kavousanakis, Michail E.; Russel, William B.Physical Review E: Statistical, Nonlinear, and Soft Matter Physics (2010), 81 (2-2), 026306/1-026306/13CODEN: PRESCM; ISSN:1539-3755. (American Physical Society)Periodic pillarlike microstructures can be created from initially flat polymer films via the electrohydrodynamic instabilities. Those patterns, however, are metastable. Our exptl. observations show that the av. pillar size increases slowly after linear growth. Major coarsening events then take place over times several orders of magnitude longer than the linear growth time. For all fill ratios, a logarithmic time dependence of the av. pillar size can be identified, i.e., 〈S〉.varies.ln t. Thicker films, however, have faster coarsening rates than thinner films. Linear stability anal. of the pseudosteady states reveals two major coarsening mechanisms, collision and Ostwald ripening, which can also be identified from exptl. images. We then reduce the original partial differential equation (PDE) into a pair of ODEs, which govern the interaction between pillars due to the above two coarsening mechanisms. From this, a logarithm scaling law is obtained for both low and high fill ratios and the coarsening rate is slower for lower fill ratios, consistent with exptl. observations. We also find that arrays with more uniform sizes tend to start coarsening later, but they coarsen faster than more "disperse" arrays, which could be possibly utilized in expts. for controlling the onset and speed of coarsening. The logarithm scaling in the electrohydrodynamic coarsening phenomenon, which differs from coarsening in spinodal decompn. and dewetting of thin liq. films, is due to the significant nonlinear effect of Maxwell stresses and geometric confinement on the disjoining pressure at both top and bottom electrodes.
- 34Lau, C. Y.; Russel, W. B. Fundamental limitations on ordered electrohydrodynamic patterning. Macromolecules 2011, 44, 7746– 7751, DOI: 10.1021/ma200952uThere is no corresponding record for this reference.
- 35Schäffer, E.; Thurn-Albrecht, T.; Russell, T. P.; Steiner, U. Electrohydrodynamic instabilities in polymer films. Europhys. Lett. 2001, 53, 518– 524, DOI: 10.1209/epl/i2001-00183-235Electrohydrodynamic instabilities in polymer filmsSchaffer, E.; Thurn-Albrecht, T.; Russell, T. P.; Steiner, U.Europhysics Letters (2001), 53 (4), 518-524CODEN: EULEEJ; ISSN:0295-5075. (EDP Sciences)We have studied the influence of elec. fields on highly viscous polymer films. An electrohydrodynamic (EHD) instability causes a wave pattern with a characteristic wavelength λ, leading to an array of polymer columns which span the gap of a capacitor device. When represented as a master curve, the data is quant. described by an EHD model, without any adjustable parameters. Our results suggest that EHD expts. using polymer films are well suited to study non-equil. pattern formation in quasi-two-dimensional systems.
- 36Gambhire, P.; Thaokar, R. Electrokinetic model for electric-field-induced interfacial instabilities. Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys. 2014, 89, 032409, DOI: 10.1103/physreve.89.03240936Electrokinetic model for electric-field-induced interfacial instabilitiesGambhire, Priya; Thaokar, RochishPhysical Review E: Statistical, Nonlinear, and Soft Matter Physics (2014), 89 (3-A), 032409/1-032409/9CODEN: PRESCM; ISSN:1539-3755. (American Physical Society)Technol. based on elec.-field-induced instabilities on thin polymer film surfaces has emerged as a promising candidate for soft lithog. Typically, the instability is modeled using the perfect dielec. (PD) or the leaky dielec. (LD) model. These assume the elec. diffuse layer to be infinitesimally large or small, resp. In the present work we conduct stability anal. assuming a PD-electrolyte soln. interface. The concn. of ions and, hence, the diffuse layer thickness is in general assumed to be of the same order as the electrolyte film thickness. The PD-LD models are then realized as limiting cases of the ratio of the double layer thickness to the film thickness.
- 37Lu, W.; Kim, D. Thin-film structures induced by electrostatic field and substrate kinetic constraint. Appl. Phys. Lett. 2006, 88, 153116, DOI: 10.1063/1.2195095There is no corresponding record for this reference.
- 38Pease, L. F., III; Russel, W. B. Linear stability analysis of thin leaky dielectric films subjected to electric fields. J. Non-Newtonian Fluid Mech. 2002, 102, 233– 250, DOI: 10.1016/s0377-0257(01)00180-xThere is no corresponding record for this reference.
- 39Wu, N.; Russel, W. B. Electrohydrodynamic instability of dielectric bilayers: Kinetics and thermodynamics. Ind. Eng. Chem. Res. 2006, 45, 5455– 5465, DOI: 10.1021/ie051087639Electrohydrodynamic Instability of Dielectric Bilayers: Kinetics and ThermodynamicsWu, Ning; Russel, William B.Industrial & Engineering Chemistry Research (2006), 45 (16), 5455-5465CODEN: IECRED; ISSN:0888-5885. (American Chemical Society)The kinetic and thermodn. properties are studied of a bilayer patterning process induced by an electrohydrodynamic instability. A parametric map is constructed depending on the dielec. contrast and ratio of two film thicknesses that describes the conditions under which hexagonally ordered pillars or holes can form when the viscosity of the upper layer is negligible. The distinct formation of arrays of pillars and holes results from the nonlinear interactions among different modes and, hence, is governed by the kinetics. The dynamic structures of pillars or holes continue to evolve to decrease the system's free energy. During this evolution, individual pillars or holes coalesce in a coarsening process until a thermodynamically stable state is reached in the form of a localized pillar, hole, or a roll structure. The selection of the pillar or hole at the final steady state represents a thermodn. preference that can be predicted qual. without solving the fully nonlinear partial differential equation.
- 40Yeoh, H. K.; Xu, Q.; Basaran, O. A. Equilibrium shapes and stability of a liquid film subjected to a nonuniform electric field. Phys. Fluids 2007, 19, 114111, DOI: 10.1063/1.279880640Equilibrium shapes and stability of a liquid film subjected to a nonuniform electric fieldYeoh, Hak Koon; Xu, Qi; Basaran, Osman A.Physics of Fluids (2007), 19 (11), 114111/1-114111/22CODEN: PHFLE6; ISSN:1070-6631. (American Institute of Physics)Stresses induced by a spatially nonuniform elec. field acting on an initially flat fluid-fluid interface can be exploited beneficially to pattern polymer microstructures without the use of resists, exposure, development, and etching, but cause undesirable nonuniformity in film thickness in precision coating processes. The equil. shape of an interface sepg. a liq. film from an ambient fluid subjected to a uniform elec. field is flat so long as the field strength is below a crit. value. A nonuniform elec. field, however, results in the deformation of the interface no matter how small its strength, an important difference which previous theor. studies have not addressed satisfactorily. Hence, whereas under a uniform field loss of stability occurs via a bifurcation from the flat film soln., under a nonuniform field destabilization may occur at a turning point at which the film profile already exhibits a finite-amplitude deformation. This deficiency in understanding is remedied here by analyzing a model problem in which a gas overlying a perfect dielec. liq. film is sandwiched between two electrodes wherein the top electrode is grounded and the elec. potential of the bottom electrode varies sinusoidally with distance measured along it. The equil. shapes and stability of the liq.-gas interface are detd. directly in the present work by simultaneously solving the augmented Young-Laplace equation governing the shape of the free surface and the Laplace equation governing elec. potentials theor. by domain perturbation anal. and numerically by finite element anal. For small nonuniformities in the elec. field, anal. solns. are reported for the profile of the free surface. The computational predictions are shown to be in excellent accord with these small-deformation results. Moreover, computations are used to extend the investigations into the nonlinear regime where nonuniformities in the elec. field and deformations of the free surface are large, and loss of stability may occur. The variation of the equil. shapes and the limits of stability with the governing dimensionless groups are investigated thoroughly. It is shown that the rich response exhibited by the system can be rationalized by interrogating the computed solns. and scrutinizing the balance of stresses due to the normal component of the elec. field, which are destabilizing, and those due to its tangential component, which are stabilizing.
- 41Sarkar, J.; Sharma, A.; Shenoy, V. B. Electric-field induced instabilities and morphological phase transitions in soft elastic films. Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys. 2008, 77, 031604, DOI: 10.1103/physreve.77.03160441Electric-field induced instabilities and morphological phase transitions in soft elastic filmsSarkar, Jayati; Sharma, Ashutosh; Shenoy, Vijay B.Physical Review E: Statistical, Nonlinear, and Soft Matter Physics (2008), 77 (3-1), 031604/1-031604/10CODEN: PRESCM; ISSN:1539-3755. (American Physical Society)We investigate the morphol. transitions of surface patterns induced in a soft elastic film in the presence of an applied elec. field by the linear stability anal. and simulations. The surface patterns emerge beyond a crit. strength of the elec. field and the pattern length scale is always nearly three times the film thickness, regardless of the precise pattern morphol. and other phys. parameters. Interestingly, the simulations show that the precise pattern morphol. depends strongly on the film and the field parameters and can be classified into three broad morphol. phases: columns, stripes, and cavities. By tuning the elec. field and the gap distance, we show that transitions from one morphol. phase to another can be induced as described by a morphol. phase diagram for this phenomenon. We also study the conditions under which the transitions can be "glassy" or "hysteretic.". In addn. to uncovering the rich physics underlying these nearly two-dimensional morphol. phase transitions, our simulations also suggest expts. and applications of this phenomenon in mesopatterning. In particular, simulations demonstrate the possibility of controlling the pattern morphol. and alignment by using the elec.-field induced instability in soft elastic films. Unlike the short range adhesive interactions which cannot be varied, the use of an elec. field allows far greater flexibility in modulation and control of the pattern morphol. and its height.
- 42Li, B.; Li, Y.; Xu, G.-K.; Feng, X.-Q. Surface patterning of soft polymer film-coated cylinders via an electric field. J. Phys.: Condens. Matter 2009, 21, 445006, DOI: 10.1088/0953-8984/21/44/44500642Surface patterning of soft polymer film-coated cylinders via an electric fieldLi, Bo; Li, Yue; Xu, Guang-Kui; Feng, Xi-QiaoJournal of Physics: Condensed Matter (2009), 21 (44), 445006/1-445006/8CODEN: JCOMEL; ISSN:0953-8984. (Institute of Physics Publishing)Using the linear stability anal. method, we investigate the surface wrinkling of a thin polymer coating on a cylinder in an externally applied elec. field. It is demonstrated that energy competition between surface energy, van der Waals interactive potential energy and electrostatic interaction energy may lead to ordered patterns on the film surface. The anal. solns. are derived for the crit. conditions of both longitudinal and circumferential instabilities. The wavelengths of the generated surface patterns can be mediated by changing the magnitude of the elec. field. Our anal. shows that the surface morphol. is sensitive to the curvature radius of the fiber, esp. in the micrometer and nanometer length scales. Furthermore, we suggest a potential approach for fabricating hierarchical patterns on curved surfaces.
- 43Roberts, S. A.; Kumar, S. Electrohydrodynamic instabilities in thin liquid trilayer films. Phys. Fluids 2010, 22, 122102, DOI: 10.1063/1.352013443Electrohydrodynamic instabilities in thin liquid trilayer filmsRoberts, Scott A.; Kumar, SatishPhysics of Fluids (2010), 22 (12), 122102/1-122102/15CODEN: PHFLE6; ISSN:1070-6631. (American Institute of Physics)Expts. by Dickey et al. and Leach et al. show that novel pillar shapes can be generated from electrohydrodynamic instabilities at the interfaces of thin polymer/polymer/air trilayer films. In this paper, we use linear stability anal. to investigate the effect of free charge and ac elec. fields on the stability of trilayer systems. Our work is also motivated by our recent theor. study which demonstrates how ac elec. fields can be used to increase control over the pillar formation process in thin liq. bilayer films. For perfect dielec. films, the effect of an ac elec. field can be understood by considering an equiv. dc field. Leaky dielec. films yield pillar configurations that are drastically different from perfect dielec. films, and ac fields can be used to control the location of free charge within the trilayer system. This can alter the pillar instability modes and generate smaller diam. pillars when conductivities are mismatched. The results presented here may be of interest for the creation of complex topog. patterns on polymer coatings and in microelectronics. (c) 2010 American Institute of Physics.
- 44Bandyopadhyay, D.; Sharma, A.; Thiele, U.; Reddy, P. D. S. Electric-field-induced interfacial instabilities and morphologies of thin viscous and elastic bilayers. Langmuir 2009, 25, 9108– 9118, DOI: 10.1021/la900635fThere is no corresponding record for this reference.
- 45Gambhire, P.; Thaokar, R. M. Electrohydrodynamic instabilities at interfaces subjected to alternating electric field. Phys. Fluids 2010, 22, 064103, DOI: 10.1063/1.343104345Electrohydrodynamic instabilities at interfaces subjected to alternating electric fieldGambhire, P.; Thaokar, R. M.Physics of Fluids (2010), 22 (6), 064103/1-064103/16CODEN: PHFLE6; ISSN:1070-6631. (American Institute of Physics)Instabilities at the interface of two immiscible fluids, either perfect or leaky dielecs., subjected to alternating elec. fields, is studied using a linear stability anal. in the limit of the electrode spacing being large compared to the wavelength of the perturbation. The Floquet anal. of the stability of this system indicates a significant effect of the frequency on the value of smax, the growth rate of the fastest growing instabilities and ETaylor, the min. field required to excite an instability. It is seen that alternating fields act to damp the system instabilities compared to the d.c. (dc) case. Moreover, the growth rate of the instabilities can be tuned from that of leaky dielec. fluids subjected to dc fields, in the low frequency limit, to that of perfect dielecs. in the high frequency limit. It is also obsd. that for a leaky dielec.-leaky dielec. interface, the a.c. (ac) fields can induce instabilities in a system which is stable at zero frequency, by increasing the frequency of the applied voltage. (c) 2010 American Institute of Physics.
- 46Reddy, P. D. S.; Bandyopadhyay, D.; Sharma, A. Self-Organized Ordered Arrays of Core–Shell Columns in Viscous Bilayers Formed by Spatially Varying Electric Fields. J. Phys. Chem. C 2010, 114, 21020– 21028, DOI: 10.1021/jp106253kThere is no corresponding record for this reference.
- 47Srivastava, S.; Reddy, P. D. S.; Wang, C.; Bandyopadhyay, D.; Sharma, A. Electric field induced microstructures in thin films on physicochemically heterogeneous and patterned substrates. J. Chem. Phys. 2010, 132, 174703, DOI: 10.1063/1.3400653There is no corresponding record for this reference.
- 48Ghosh, A.; Bandyopadhyay, D.; Sharma, A. Electric field mediated elastic contact lithography of thin viscoelastic films for miniaturized and multiscale patterns. Soft Matter 2018, 14, 3963– 3977, DOI: 10.1039/c8sm00428eThere is no corresponding record for this reference.
- 49Tian, H.; Shao, J.; Hu, H.; Wang, L.; Ding, Y. Role of space charges inside a dielectric polymer in the electrohydrodynamic structure formation on a prepatterned polymer (ESF-PP). RSC Adv. 2016, 6, 77275– 77283, DOI: 10.1039/c6ra14479a49Role of space charges inside a dielectric polymer in the electrohydrodynamic structure formation on a prepatterned polymer (ESF-PP)Tian, Hongmiao; Shao, Jinyou; Hu, Hong; Wang, Li; Ding, YuchengRSC Advances (2016), 6 (81), 77275-77283CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)Electrohydrodynamic structure formation on a prepatterned polymer (ESF-PP) can duplicate structures identical to the initial geometry, but with a higher aspect ratio, under the influence of a spatially modulated elec. field. In this process, a voltage is applied between a flat template and a flat substrate, sandwiching a prepatterned polymer and an air gap so as to generate an electrohydrodynamic (EHD) force at the air-polymer interface. Subsequently, the prepatterned polymer can be non-uniformly pulled upwards, causing deformation in its micro/nano-structure. Until now, most of the research into ESF-PP has explored various dielec. polymers, which are all considered to be the perfect dielecs. because of their low elec. cond. However, the assumption of a perfect dielec. typically creates discrepancies between theor. anal. and exptl. results in terms of the polymer motion and the final morphol. This phenomenon can be attributed to ignoring the action of the small no. of free space charges within dielec. polymer motion (although the elec. cond. of the dielec. polymer may be even lower than that of deionized water), which emphasizes the importance of the influence of space charges inside the dielec. polymer on deformation. This paper explored the role of free space charges by making a comparison between the perfect dielec. polymer and the leaky dielec. polymer on the progressive development, the surface topog. and the aspect ratio from exptl. tests and numerical simulations, and a discussion of the effect of the different elec. conductivities. Results showed that the free charges inside the dielec. polymer can lead to a larger EHD force because of the addnl. Coulomb force, even at a low cond. of 10-7 S m-1, thus demonstrating the ability to duplicate a mushroom-like structure with a high aspect ratio, which has wide applications in superhydrophobicity, dry adhesion, nanogenerators, etc.
- 50Tian, H.; Shao, J.; Ding, Y.; Li, X.; Hu, H. Electrohydrodynamic micro-/nanostructuring processes based on prepatterned polymer and prepatterned template. Macromolecules 2014, 47, 1433– 1438, DOI: 10.1021/ma402456uThere is no corresponding record for this reference.
- 51Tian, H.; Shao, J.; Ding, Y.; Li, X.; Liu, H. Simulation of polymer rheology in an electrically induced micro- or nano-structuring process based on electrohydrodynamics and conservative level set method. RSC Adv. 2014, 4, 21672– 21680, DOI: 10.1039/c4ra00553h51Simulation of polymer rheology in an electrically induced micro- or nano-structuring process based on electrohydrodynamics and conservative level set methodTian, Hongmiao; Shao, Jinyou; Ding, Yucheng; Li, Xiangming; Liu, HongzhongRSC Advances (2014), 4 (42), 21672-21680CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)An elec. induced structuring process, such as an electrohydrodynamic (EHD) approach for fabricating polymeric micro-/nano-structures in various micro-/nano-devices, was performed by applying a voltage to an electrode pair consisting of a planar or structured template and a polymer-coated substrate sandwiching an air gap, resulting in either periodic columns or template-modulated structures. Anal. approaches were explored to characterize this micro-/nano-structuring process, based on a linear thermodn. instability of the polymer film combining capillary waves and electrostatic forces, leading to a definition of "most unstable wavelength" in relation to various process variables such as external voltage, polymer film thickness, and so on. For math. simplicity, the linear stability anal. was only carried out to demonstrate an initiation of the polymer structuring under elec. induction by an infinite planar template, and cannot numerically visualize the evolution of the polymer structure which grows from an initially flat film upwards to the template underside. Therefore, a numerical modeling of such a process, which is capable of demonstrating a full-cycle evolution of the polymer structuring, is desirable to provide an in-depth insight into this elec. induced structuring technique. This paper presents a detailed numerical formulation for simulating the rheol. behavior for this structuring process based on EHD equations and a conservative level set approach. Firstly, a numerical simulation is performed to demonstrate the dynamic evolution of periodic structures for a planar (non-structured) template and compared with the linear instability anal. to validate the effectiveness of the proposed numerical modeling. Then simulations are performed to numerically visualize the evolution of a polymer structure induced by a structured template, with a subsequent discussion about the influences of some crit. process variables, such as voltage, air gap, polymer thickness, depth of template patterns, and so on, on the electrohydrodynamic rheol. of the polymer.
- 52Verma, R.; Sharma, A.; Kargupta, K.; Bhaumik, J. Electric field induced instability and pattern formation in thin liquid films. Langmuir 2005, 21, 3710– 3721, DOI: 10.1021/la047210052Electric Field Induced Instability and Pattern Formation in Thin Liquid FilmsVerma, Ruhi; Sharma, Ashutosh; Kargupta, Kajari; Bhaumik, JaitaLangmuir (2005), 21 (8), 3710-3721CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)Electrostatic field induced instability, morphol., and patterning of a thin liq. film confined between two electrodes with an air gap are studied on the basis of nonlinear 3D simulations, both for spatially homogeneous and heterogeneous fields. In addn. to the spinodal flow resulting from the variation of field because of local thickness changes, a heterogeneous imposed field also moves the liq. from the regions of low field to high field, thus allowing a more precise control of pattern. Hexagonal packing of liq. columns is obsd. for a spatially homogeneous elec. field, which is in accord with the e-field expts. on thin polymer films (Schaffer et al. Nature 2000, 403, 874). For a large liq. vol. fraction in the gap, φ ≥ 0.75, the coalescence of columns causes a phase inversion, leading to the formation of air columns or cylindrical holes trapped in the liq. matrix (air-in-liq. dispersion). Locally ordered aligned patterns are formed by imposing a spatial variation of the electrostatic field by using a topog. patterned electrode. For example, multiple rows/lines of liq. columns are formed near the edge of a steplike heterogeneity of the electrode and annular rings of ordered columns or concentric ripples are formed around a heterogeneous circular patch. Simulations predict that the electrode pattern is replicated in the film only when the pattern periodicity, Lp, exceeds the instability length scale on the basis of the min. interelectrode sepn. distance, Lp ≥ λm-dmin. Thus, the formation of secondary structures can be suppressed by employing an electrode with deep grooves and stronger field gradients, which produces almost ideal templating. The no. d. of the elec. field induced patterns can be altered by tuning the mean film thickness (or the vol. fraction of liq. in the gap), periodicity and depth (amplitude) of the grooves on the top electrode, and the applied voltage. The implications are in electrostatic lithog., pattern replication in soft materials, and the design and interpretation of thin film expts. involving elec. fields.
- 53Harkema, S. Capillary Instabilities in Thin Polymer Films. Ph.D. Thesis, University of Groningen, The Netherlands, 2006.There is no corresponding record for this reference.
- 54Dwivedi, S.; Vivek; Mukherjee, R.; Atta, A. Formation and control of secondary nanostructures in electro-hydrodynamic patterning of ultra-thin films. Thin Solid Films 2017, 642, 241– 251, DOI: 10.1016/j.tsf.2017.09.02954Formation and control of secondary nanostructures in electro-hydrodynamic patterning of ultra-thin filmsDwivedi, Swarit; Vivek; Mukherjee, Rabibrata; Atta, ArnabThin Solid Films (2017), 642 (), 241-251CODEN: THSFAP; ISSN:0040-6090. (Elsevier B.V.)Based on a computational model involving finite element method, the authors report the morphol. evolution and pattern formation process in a thin polymer film subject to an externally imposed periodically varying heterogeneous DC elec. field, induced by topog. patterned top electrode. Quasi-steady state morphol. is obsd. to depend on remnant liq. layer after the evolution of primary structures. The dynamics of remnant layer de-wetting that essentially is dictated by electrode assembly, fluid properties, and initial film thickness, results in formation of intermediate secondary structures. The possibility of controlling these structures is explored by varying periodicity of the stamp/top electrode, applied potential and initial liq. vol. At a fixed operating condition, crit. periodicity is identified for stable secondary structure formation that may lead to ordered nano-patterns for diverse applications.
- 55Atta, A.; Crawford, D. G.; Koch, C. R.; Bhattacharjee, S. Influence of electrostatic and chemical heterogeneity on the electric-field-induced destabilization of thin liquid films. Langmuir 2011, 27, 12472– 12485, DOI: 10.1021/la202759jThere is no corresponding record for this reference.
- 56Dwivedi, S.; Mukherjee, R.; Atta, A. Re-entrant structural evolution using electrically heterogeneous patterned electrode. Comput.-Aided Chem. Eng. 2017, 40, 1213– 1218, DOI: 10.1016/b978-0-444-63965-3.50204-xThere is no corresponding record for this reference.
- 57Nazaripoor, H.; Koch, C. R.; Sadrzadeh, M. Ordered high aspect ratio nanopillar formation based on electrical and thermal reflowing of prepatterned thin films. J. Colloid Interface Sci. 2018, 530, 312, DOI: 10.1016/j.jcis.2018.06.080There is no corresponding record for this reference.
- 58Hu, H.; Tian, H.; Shao, J.; Ding, Y.; Jiang, C.; Liu, H. Fabrication of bifocal microlens arrays based on controlled electrohydrodynamic reflowing of pre-patterned polymer. J. Micromech. Microeng. 2014, 24, 095027, DOI: 10.1088/0960-1317/24/9/09502758Fabrication of bifocal microlens arrays based on controlled electrohydrodynamic reflowing of pre-patterned polymerHu, Hong; Tian, Hongmiao; Shao, Jinyou; Ding, Yucheng; Jiang, Chengbao; Liu, HongzhongJournal of Micromechanics and Microengineering (2014), 24 (9), 095027/1-095027/8, 8 pp.CODEN: JMMIEZ; ISSN:1361-6439. (IOP Publishing Ltd.)An easy method based on electrohydrodynamic (EHD) reflowing of pre-patterned polymer is proposed in this study for the fabrication of bifocal microlens arrays (MLAs). The method comprises two sequential steps, i.e. hot embossing for generating a polymer-based micropillar array and EHD reflowing of the micropillars for the formation of a bifocal MLA with controllable surface shape and optical performance. The EHD reflowing process is achieved by applying a voltage across an electrode pair sandwiching an air gap and the pre-patterned polymer, and the EHD force induced on the air-polymer interface reshapes the pillar array into the MLA. The complex bifocal microlens can be achieved only when the elec. intensity is stronger than that required to produce a commonly known Taylor cone, which is formed when the EHD force exactly surpasses the surface tension. Finally, the light through MLA is imaged on a moving charge-coupled device (CCD) camera and leads to an observation of two focal planes.
- 59Williams, M. B.; Davis, S. H. Nonlinear theory of film rupture. J. Colloid Interface Sci. 1982, 90, 220– 228, DOI: 10.1016/0021-9797(82)90415-5There is no corresponding record for this reference.
- 60Oron, A.; Davis, S. H.; Bankoff, S. G. Long-scale evolution of thin liquid films. Rev. Mod. Phys. 1997, 69, 931– 980, DOI: 10.1103/revmodphys.69.93160Long-scale evolution of thin liquid filmsOron, Alexander; Davis, Stephen H.; Bankoff, S. GeorgeReviews of Modern Physics (1997), 69 (3), 931-980CODEN: RMPHAT; ISSN:0034-6861. (American Physical Society)Macroscopic thin liq. films are entities that are important in biophysics, physics, and engineering, as well as in natural settings. They can be composed of common liqs. such as water or oil, rheol. complex materials such as polymers solns. or melts, or complex mixts. of phases or components. When the films are subjected to the action of various mech., thermal, or structural factors, they display interesting dynamic phenomena such as wave propagation, wave steepening, and development of chaotic responses. Such films can display rupture phenomena creating holes, spreading of fronts, and the development of fingers. In this review a unified math. theory is presented that takes advantage of the disparity of the length scales and is based on the asymptotic procedure of redn. of the full set of governing equations and boundary conditions to a simplified, highly nonlinear, evolution equation or to a set of equations. As a result of this long-wave theory, a math. system is obtained that does not have the math. complexity of the original free-boundary problem but does preserve many of the important features of its physics. The basics of the long-wave theory are explained. If, in addn., the Reynolds no. of the flow is not too large, the analogy with Reynolds theory of lubrication can be drawn. A general nonlinear evolution equation or equations are then derived and various particular cases are considered. Each case contains a discussion of the linear stability properties of the base-state solns. and of the nonlinear spatiotemporal evolution of the interface (and other scalar variables, such as temp. or solute concn.). The cases reducing to a single highly nonlinear evolution equation are first examd. These include: (a) films with const. interfacial shear stress and const. surface tension, (b) films with const. surface tension and gravity only, (c) films with van der Waals (long-range mol.) forces and const. surface tension only, (d) films with thermocapillarity, surface tension, and body force only, (e) films with temp.-dependent phys. properties, (f) evapg./condensing films, (g) films on a thick substrate, (h) films on a horizontal cylinder, and (i) films on a rotating disk. The dynamics of the films with a spatial dependence of the base-state soln. are then studied. These include the examples of nonuniform temp. or heat flux at liq.-solid boundaries. Problems which reduce to a set of nonlinear evolution equations are considered next. Those include (a) the dynamics of free liq. films, (b) bounded films with interfacial viscosity, and (c) dynamics of sol. and insol. surfactants in bounded and free films. The spreading of drops on a solid surface and moving contact lines, including effects of heat and mass transport and van der Waals attractions, are then addressed. Several related topics such as falling films and sheets and Hele-Shaw flows are also briefly discussed. The results discussed give motivation for the development of careful expts. which can be used to test the theories and exhibit new phenomena. Many refs.
- 61Li, H.; Yu, W.; Zhang, L.; Liu, Z.; Brown, K. E.; Abraham, E.; Cargill, S.; Tonry, C.; Patel, M. K.; Bailey, C.; Desmulliez, M. P. Y. Simulation and modelling of sub-30 nm polymeric channels fabricated by electrostatic induced lithography. RSC Adv. 2013, 3, 11839– 11845, DOI: 10.1039/c3ra40188j61Simulation and modeling of sub-30 nm polymeric channels fabricated by electrostatic induced lithographyLi, H.; Yu, W.; Zhang, L.; Liu, Z.; Brown, K. E.; Abraham, E.; Cargill, S.; Tonry, C.; Patel, M. K.; Bailey, C.; Desmulliez, M. P. Y.RSC Advances (2013), 3 (29), 11839-11845CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)This article demonstrates, through finite element anal., the possibility to manuf. sub-30 nm polymeric channels using electrostatic induced lithog. Channels with a width of 25 nm, a depth of 50 nm and an inter-channel wall of 28 nm can be obtained by this patterning process. The influence of operational parameters such as the filling factor, the aspect ratio of the master electrode, the applied voltage and the gap between the two electrodes and initial film thickness has been studied in detail to define the fabrication limits of this process in the case of periodic nanostructures. Conclusions for such nanostructures can be generalized to other shapes manufd. from polymers.
- 62COMSOL, COMSOL Multiphysics User’s Guide , 2015.There is no corresponding record for this reference.