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Controlled Wrinkle Patterning on Thin Films to Improve Hydrophobicity
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Cite this: Langmuir 2024, 40, 25, 13017–13024
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https://doi.org/10.1021/acs.langmuir.4c00743
Published June 13, 2024

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Abstract

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Controlling surface morphology is one of the main strategies used to tune surface hydrophobic and icephobic properties. Taking advantage of coating growth by initiated chemical vapor deposition, random and ordered wrinkles were induced on a thin film of polyperfluorodecyl acrylate (pPFDA) deposited on polydimethylsiloxane (PDMS) to simultaneously modify surface chemistry and morphology. A range of wrinkles of different wavelengths were studied, and how the wrinkle characteristics change with varying coating thickness. Ordered wrinkles enhanced hydrophobicity more when compared to random wrinkles, with a noticeable effect for coating thickness on the order of hundreds of nanometers. An insight into the mechanism of surface wrinkling and its effect on freezing delay is also provided, and promising results were found on ordered wrinkles, where a freezing delay was observed.

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Introduction

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Over the last few decades, research has focused on studying wrinkling in both naturally occurring and artificially induced scenarios. Wrinkles can change the properties of the substrate on which they appear, for instance affecting the mechanical properties of a material or its conductivity. (1−3) In general, wrinkles enhance the surface area, which can be beneficial to enhance transport phenomena in a variety of applications: e.g., micropatterning was found to affect the dissolution rate in drug release. (4)
Within the context of surface wetting, wrinkles with the right geometrical characteristics can be used to induce the so-called Cassie–Baxter state. When the Cassie–Baxter state is induced, the hydrophobicity of the surface is enhanced. A surface is considered hydrophobic when the water contact angle measured for a water droplet deposited on it is above 90°. (5) Values above 150° for the water contact angle have been observed when inducing the Cassie–Baxter state, along with a hysteresis value ranging from 5 to 10°, thereby reaching the so-called superhydrophobic state. (6,7) The Cassie–Baxter state can be described as a state in which a drop has low contact with the substrate, touching the top of surface asperities and with air pockets limiting the contact between the surface and droplet. (8) This is why this state is also known as the fakir state. Although the Cassie–Baxter state is a metastable state, susceptible to transition to the Wenzel state, it can be obtained in special conditions, where the surface is patterned. An example of this is the induction of the Cassie–Baxter state through the presence of a micropattern distributed across the surface, such as in the case of micropillars. (9,10)
Wrinkles potentially represent a suitable configuration to obtain the Cassie–Baxter state (11) and so to enhance hydrophobicity. Until now, it has been widely accepted that wrinkles can appear in substrates of varying magnitudes, depending on the characteristics of the substrate involved, as discussed by Rodríguez-Hernández. (10) Wrinkles form when the top layer expands at a rate faster than that of the layer beneath it. In general, wrinkling can be understood as an out-of-plane surface bending that occurs due to instability under compression, induced by any parallel or perpendicular force above a certain stress threshold. (12) With these ideas in mind, we want to focus on the random and controlled wrinkling induced by depositing a thin polymer film via initiated chemical vapor deposition (iCVD). iCVD is a versatile method to deposit polymer thin films because it has the advantage of being a completely dry process and it can be tuned to a high degree. (13) Gleason et al. (14) managed to induce a controlled wrinkling on a thin polymeric film of two combined monomers, ethylene glycol diacrylate and 2-hydroxyethyl methacrylate, deposited on polydimethylsiloxane (PDMS) via iCVD. Random wrinkles were also obtained by a previous work of our group exploiting iCVD to form poly-N-vinyl Caprolactam thin films on Eudragit surface. (15) Wrinkling has been observed on Teflon layers deposited on various plastic substrates, resulting in a durable superhydrophobic state. (16) This finding provides further evidence of the influence of wrinkles on the Cassie–Baxter state. Moreover, patterning induced under an electric field on piezopolymers proved the possibility of controlling the wetting properties. (17)
In the present work, we have studied how different wrinkle sizes can enhance hydrophobicity and freezing delay of a surface. For this, we induced wrinkling on a thin film of polyperfluorodecyl acrylate (pPFDA), deposited on PDMS via iCVD. pPFDA was selected as a coating due to its well-known hydrophobic properties, which arise from the abundance of fluorine atoms present in its molecular chains. In general, very high values of water contact angles (>130°) can be measured on such a material, depending on the degree of crystallinity of the polymer. (18)

Experimental Section

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For PDMS preparation, silicon elastomer base and curing agent (SYLGARD 184, Sigma-Aldrich) in a ratio 10:1 were used. A homogeneous mixture of the elastomer and curing agent was obtained in a beaker after magnetic stirring for 30 min. To eliminate most bubbles in the mixture, an ultrasonic bath was used for 15 min. A Petri dish was used as a mold for the substrates, obtaining PDMS samples 1 mm thick. The molds were placed in the desiccator under vacuum for 1 h, then in the oven at 70 °C for 1 h, and additionally at 90 °C for 1 hour. Samples with area 2 × 2 cm were cut from the molds. The PDMS samples were then ready for iCVD deposition, without further treatment. Differently, reference silicon samples that were used to monitor coating growth were cleaned in a mixture of ethanol/acetone 2:5, heated up at 185 °C, and then dried with Ar/CO2.
The synthesis of the coating was performed in a custom-built iCVD reactor. (19) The monomer perfluorodecyl acrylate (PFDA, Aldrich 474487-25 ML) was heated to 85 °C and flown into the reactor at a rate of 0.200 ± 0.005 sccm. The initiator was kept at room temperature and entered the reactor with a flow rate of 1.0 ± 0.3 sccm. The working pressure was 400 mTorr. The hot wires were heated with a current of 1.15 A, reaching an average temperature of 180 °C, measured through a thermocouple. The reactor was heated to 60 °C, and the substrate holder was kept at 40 °C by a cooling system. A growth of 2.5 ± 0.1 nm/min was observed for pPFDA in these conditions. The PDMS samples were positioned in the stretcher and stretched up to 10% of their length (Figure 1a,b). After the deposition, the stretching was released, resulting in a sample appearance change from transparent to milky due to wrinkle formation. The coatings were grown up to thicknesses of 100, 150, 200, 300, and 600 nm. In addition, as a reference, some other PDMS samples were coated without applying any prior stretching, with thicknesses of 200 and 300 nm.

Figure 1

Figure 1. (a) Schematic representation of the stretching of the pPFDA-coated PDMS: (I) a 10% stretching is applied to the substrate during the coating process. The force is applied parallel to the substrate in a longitudinal direction. Both elastic moduli of polyperfluorodecyl acrylate (pPFDA) and polydimethylsiloxane (PDMS) are highlighted. (II) When the stretching is released, wrinkles appear. (b) The PDMS slice is placed in a self-built stretcher, put in contact with the cooling plate through a piece of copper. The stretcher is then surrounded by three silicon wafers, used as a reference to determine the thickness of the deposited coating via ellipsometry and to proceed to further characterization.

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Characterization Methods

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Fourier-Transformed InfraRed spectroscopy (FTIR, Bruker IFS 66v/S) was performed on silicon pPFDA-coated samples to verify the fluorinated functional group retention and successful polymerization. Spectra were recorded in transmission mode in the range 500 to 4000 cm–1 with a resolution of 4 cm–1 (100 scans) and baseline-corrected (Figure S1, Supporting Information).
The coating thickness on the silicon references was then determined through spectroscopic ellipsometry (J.A. Woollam, M-2000 V). The samples were scanned at angles from 45° to 65° with a 10° increase, with an acquisition time of 2 s. Each coated silicon wafer was analyzed with a model consisting of the Si substrate, a 1.5 nm native oxide layer, and the polymer layer. For the latter, a Cauchy function was used.
An optical microscope (Olympus BX51 connected to an Olympus Camedia C-5060 camera) was used in transmission mode on the coated PDMS samples to observe wrinkles at different magnifications (5×, 50×, and 100×). Sample morphology and roughness were also analyzed by using an Atomic Force Microscope (AFM, Nanosurf Easyscan 2). A cantilever (Tap190Al-G, BudgetSensors, Bulgaria) working in tapping mode scanned areas of 25 μm × 25 μm and 50 μm × 50 μm (0.05 μm resolution) at a speed of 1 s/line. A 450 mV amplitude was employed, and the set point was set to 50%. The images were then elaborated through Gwyddion. Using the 2D-Fourier Transform analysis, it was possible to obtain the wrinkle wavelengths, whereas their height was measured by splitting each scanned image into five areas and averaging the peaks found. The water contact angle was measured by using a KSV CAM 200 goniometer. Droplets of various volumes (3, 4, 5, 7, and 10 μL) were deposited on the samples, and a camera was used to record drop profiles and extract contact angles. Advancing and receding angles were measured with a sessile drop test and used to calculate the hysteresis values.
The second part of the characterization included frost formation observations and droplet freezing tests. To observe the substrate behavior at low temperatures, a Linkam stage (model T95-PE) was used, coupled first with an optical microscope and then with an IR camera. The samples were first observed via an optical microscope at −45, −60, and −80 °C, reaching the desired temperature at three different cooling rates, namely, 15, 20, and 25 °C/min. Each time, the temperature was increased cyclically to room temperature. The aim of these preliminary tests was to determine whether visible changes would be observed on the wrinkle pattern.
Subsequently, the Linkam Stage was coupled to an IR Camera (Optris PI, 80 Hz). Five droplets of 10 μL were deposited at the same time on each substrate. The substrate was then cooled to −25 °C at 30 °C/min to observe the freezing delay, at 30% relative humidity. The freezing delay was calculated from the instant in which −25 °C is reached until the droplet starts freezing. The freezing delays measured for each droplet were averaged, and standard deviations were calculated. Lastly, the coating robustness was tested qualitatively by scratching frozen droplets from the substrate surface. After the samples were left with some droplets of random sizes in the freezer at −18 °C for one night, they were scratched away using plastic tweezers. To quantify the coating abrasion, a profilometer (KLA Tencor D-500) was used on the areas from which the droplets were scratched from.

Results and Discussion

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The first goal of our study was to understand how the coating thickness affects the wrinkle shape. Specifically, we studied the effect of coating thickness on the wrinkle wavelength and height, as it is known from previous studies that these parameters are directly proportional to the coating thickness. (10) In all samples, wrinkles were oriented perpendicular to the direction of the applied stretching, indicating a consistent pattern (Figure 2a). Samples I and II exhibited a comparable shape, likely due to their relatively close wrinkle heights. For samples III–V, the distance between wrinkles increased with thickness. This also led to an increase of the wavelengths, as will be discussed later. In Figure 2a, the second row shows the AFM scans for each sample. Wrinkles in samples IV and V presented a double peak, so two wavelengths could be obtained through the 2D-FFT analysis. Differently, for samples I–III, only a single wavelength was found. Random wrinkles were observed on PDMS samples that were not prestretched. The shape of random wrinkles differs considerably from the ordered wrinkles on stretched PDMS, as visible in Figure 2b,c. The wavelength is rather small, ranging between 1.5 up to 2 μm, and their height closely corresponds to the thickness of the deposited coating. However, the measured roughness is lower than that found on samples with ordered wrinkles with the same coating thickness. The overall roughness on the surface of the 200 nm sample with random wrinkles measures 55 ± 5 nm.

Figure 2

Figure 2. (a) Optical microscopy images (first row) and the AFM scans (second row) resulting from the five different coated surfaces are here shown: (I) 100 nm, (II) 150 nm, (III) 200 nm, (IV) 300 nm, and (V) 600 nm. The scale bar for the optical microscope images is 20 μm, while for the AFM images, it is 10 μm. In the third row, the 3D scan is reported, showing the 3D shape of wrinkles and the different distributions on the surface due to the change in thickness. On the right angle, the highest value for the height of wrinkles is reported. (b) Optical microscopy image of random wrinkles on a 200 nm coated PDMS with pPFDA. The scale bar is 20 μm. (c) AFM micrographs of the same surface, the scale bar is 5 μm. (d) Wrinkle wavelength versus coating thickness; two different wavelengths were identified: a long wavelength, λ long, and a short wavelength, λ short. The long wavelength shows the strongest influence from the increase of thickness, while the short wavelength has mostly a constant behavior. Both wavelengths are in the range of μm. e. This plot shows the height of wrinkles versus coating thickness: the increase is again linear. The plotted heights were obtained by averaging the wrinkle heights measured by AFM.

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As shown in the plot in Figure 2d, the distance between the double peaks is described by a short wavelength (λshort), while the distance between two contiguous double peaks is a long wavelength (λlong). The short wavelength appeared to be weakly affected by the increase in thickness, as it remained approximately constant. On the contrary, the long wavelength was highly enhanced by the thickness, reaching a value of almost 16 μm on the 600 nm coated sample.
Between the wrinkles, the surface appears uneven, thus leading to a difference in roughness over the surface. Nevertheless, the overall roughness increased with the coating thickness, as already reported in the literature. (20,21) Defects and porosity are introduced into a pPFDA coating during synthesis via iCVD, (22,23) as the coating becomes thicker. This leads to an increase of surface roughness. (24)
In Figure 2e, the wrinkle height is shown as a function of the coating thickness. It appears that increasing the thickness also leads to an increase in the height of wrinkles, consequently influencing the pattern shape. This was also confirmed by the 3D scan of the surfaces, shown in Figure 2a, third line. Note that wrinkle height ranges around the coating thickness.

Hydrophobicity Characterization

To assess hydrophobicity and how this is affected by wrinkles, static water contact angle was measured. (25,26) The reference value of water contact angle for uncoated PDMS is 120 ± 2°, measured at a room temperature of 25 °C. Therefore, PDMS can already be considered hydrophobic. (27) The water contact angle measured on pPFDA deposited over a silicon wafer was 133 ± 2°. The fluorinated polymer is highly hydrophobic and highly crystalline when deposited on bare silicon. (26,27)
Figure 3a shows the water contact angles measured on the ordered and random wrinkles: both the thickness and patterning affected the water contact angle. (28−31) The ordered wrinkles resulted in higher static contact angles compared to random wrinkles but also slightly higher contact angle hysteresis, probably due to a pinning effect derived from the peculiar shape of surfaces. For the sample with a 100 nm coating and ordered wrinkles, the water contact angle was higher than that of the 300 nm coating with a random pattern. As the coating thickness increased from 100 to 300 nm, the water contact angle increased to 140°. However, the impact of the ordered pattern diminished when the thickness exceeded 300 nm. The 600 nm coated sample showed a comparable value to thinner coatings, indicating that the wavelength of wrinkles was too long to interact effectively with the water droplet. Higher water contact angle values were measured on hierarchical wrinkles on Teflon by Scarratt et al. (16)

Figure 3

Figure 3. (a) Water contact angles were measured on several samples with ordered and random wrinkles. The error bars are indicated for each data point in the plot, but sometimes, they are hidden by the symbols. An enhancement in the WCA could be seen already in the 100 nm coated sample with ordered wrinkles, in comparison to the random wrinkles. (b) Schematic representation of how the Cassie–Baxter theory, originally developed to study systems based on fibers, was adapted to a wrinkled system to calculate the areas of interface and the energy spent to originate a Cassie–Baxter state. The parameters used to describe the fibers (radius, r, and middle distance between fibers, d) were related to the parameters of wrinkling (long wavelength, λlong, and width, b). (c) In this plot, the total area values of solid–liquid, f1, and solid–air, f2, interfaces are represented. (d) The net energy (ED) expended in forming unit geometrical areas of interface is plotted with the long wavelength: the shape of wrinkles determines a clustering of the values.

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The Cassie–Baxter model is useful to calculate the nondimensional area of solid–liquid and liquid–air interfaces, f1 and f2, in the system and understand if the Cassie–Baxter state can be induced by the morphology of the wrinkles. From the total areas of the interface, it is possible to estimate the surface energy required to generate the Cassie–Baxter state. (6) The Cassie–Baxter theory was developed for fabrics, so the equations used to study the system refer to fabric parameters. In this work, the model was adapted to wrinkles as shown in detail in the Supporting Information, Section 2. In Figure 3b, we show the overlap of wrinkles and fibers to define the corresponding approximations. The wrinkles on the samples were characterized by two different wavelengths: a short one and long one. In the model, only the long wavelength was considered, assuming that only droplets of relatively small dimensions could sit in a Cassie–Baxter state over the double peak of a wrinkle. In the graph shown in Figure 3c, the total interface areas are shown with the thickness of the coating. The solid–liquid interface area, f1, decreases with increasing coating thickness. This area represents the area of interaction between the wrinkles and droplet. At higher thicknesses, the distance between wrinkles increases and therefore the contact between the liquid and the wrinkles decreases, leading to the presence of air pockets (Cassie–Baxter state) between two consecutive wrinkles and the droplet. Nevertheless, this effect depends also on the height of the features, a parameter not included in the model. However, for the sample with thickness <300 nm, the height-to-wavelength ratio is high enough to ensure that the droplets are not sinking into the area between wrinkles. In the case of the 600 nm sample, this is no more verified, since the droplets were observed mainly in the flat area between wrinkles. This scenario is not described by the model, which only considers the area of interaction between wrinkles and droplet. Since the wrinkles are largely apart, the area of interaction with the droplet is small, which should lead to a decrease in the WCA. In the measurements for the 600 nm thick sample, though, the droplet sinks between wrinkles, and the Cassie–Baxter theory is no more verified. Only a slight increase in the total liquid–air area of the interface, f2, is observed with the coating thickness.
Figure 3d shows the net expended energy, ED, required to generate or consume interface areas as a function of the long wavelength. What influences most the entity of ED is the wrinkle shape. Indeed, when the wrinkles showed higher wavelengths (i.e., for the 600 nm thick sample), less energy was needed to generate the interface area. This agrees with the plot of f1 and f2: when the wavelength was higher, the solid–liquid area of the interface, f1, was lower, and the liquid–air interface area, f2, was only slightly increasing. A lower area of the solid–liquid interface was found when there were fewer wrinkles on a specific area (e.g., greater wavelengths). Therefore, the net energy expended to form the interface area was lower.
Two forces act on the droplet to keep it suspended between two wrinkles: these are the Laplace pressure, which pushes the droplet down toward the surface of the sample, and opposite pressure related to the energy barrier to overcome when the Cassie–Baxter state transits to the Wenzel state. Figure 4a shows the Laplace and energy barrier pressures calculated from the model as a function of the coating thickness. First, both pressures are of the same order of magnitude, meaning that the pressure exerted by the surface could efficiently contrast liquid penetration. A decrease in the Laplace pressure is observed when the coating thickness is increased, because of higher wavelengths. The pressure is higher in shorter wavelengths. Instead in the 600 nm sample, the energy barrier pressure equals Laplace pressure. Also, when larger droplets were tested, a larger depth of penetration has been calculated (Figure S2, Supporting Information). Since the wavelength is strictly connected to the coating thickness, (10,16) the droplet penetration increases with coating thickness. Moreover, the activation energy related to the transition Cassie–Baxter to the wet state, for both wavelengths, was calculated and found to be correlated to the thickness. However, while the one related to short wavelength is directly proportional to it, the activation energy related to long wavelength is indirectly proportional to the thickness, as shown in Section 3, Supporting Information in detail.

Figure 4

Figure 4. (a) Values for the Laplace and energy barrier pressure were calculated for each sample, considering both wavelengths where possible. Here, they were plotted with the thickness of the coating. (b) The depinning force per unit length could be calculated for each sample; it is related to the hysteresis values, which are reported for each sample. Going further with the thickness of the coating results in a decrease of the depinning force, and the 600 nm coated sample shows characteristics similar to those of the thinner coated samples. This is because different roughnesses are observed in different areas of the sample. (c) Plot of the pinning force over the volume of the droplet used during the tests, respectively, 3, 4, 5, 7, and 10 μL. An increase in the calculated pinning force is observed with the thickness, finding the 600 nm coated sample with the highest value. (d) Plot of delay in freezing measured for a droplet at the enhancement of thickness. Results are shown for ordered (black) and random (red) wrinkles. Multiple droplets on the surface of 10 μL, T = −30 °C, 3 cycles were run and averaged. The large error bars are due to inhomogeneities on the PDMS thickness derived from its preparation. (e) Images taken during the cooling to −80 °C and the heating up to RT at a cooling rate of 15 °C/min. 50× magnification, optical microscope, Olympus BX51.

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Finally, we evaluated the pinning force and force needed to remove a droplet from the surface; the description is brought on in detail in the Supporting Information, Section 3. The pinning force for the samples with ordered wrinkles is shown in Figure 4c, versus the volume of the droplets. The volume did not affect the entity of the pinning force, while when the thickness increases, the pinning force tends to increase as well. The depinning force (Figure 4b) needed to remove the droplet from the surface depends on the hysteresis. Quantifying this force gave information directly on the stickiness to the surface without considering the dimensions of wrinkling. The calculated depinning force was quite higher than the pinning force, as calculated. Although the 200 and 300 nm coated samples were the best performing with regard to hydrophobicity, they showed a greater hysteresis associated with a higher force needed to remove the droplet from the surface. The 600 nm coated sample proved once more to behave similarly to the thinnest samples. An explanation was found by measuring the roughness of these surfaces. As previously mentioned, the overall roughness increased with the thickness. On the 100 nm coated sample, the roughness was found to be 35 ± 2 and 37 ± 2 nm on the 150 nm coated sample. On the 600 nm coating, the overall roughness was 93 ± 11 nm. The high standard deviation derives from the inhomogeneity of the roughness over the surface. The measured roughness in the region between wrinkles was 42 ± 9 nm, and the coating appeared smoother already at first glance during the microscope analysis. This value is close to those found for the 100 and 150 nm coated samples. Differently, for the 200 and 300 nm coated samples, roughness of 63 ± 3 and 66 ± 4 nm were respectively measured. Moreover, the lower values of hysteresis found on the 600 nm coated sample suggested that the droplet has a favorite position, which is between two wrinkles on the smoother surface.
It is noteworthy to mention that the stickiness of these surfaces could also be characterized by measuring the roll-off-angle upon tilting the samples. For the case of surfaces with ordered wrinkles, it was noticed that the direction of tilting, either parallel or perpendicular to the wrinkle direction, caused differences in the roll-off-angle determination. Preliminary results on this are shown in Figure S4, Supporting Information.

Analysis of the Behavior toward Freezing and Icephobicity

The behavior of the substrates in freezing conditions was investigated to understand if wrinkle structures change at low temperatures and if water droplet freezing is affected by wrinkles. The goal was to assess the substrates icephobicity potential, and specifically assessing the ice nucleation delay. (32) First, we checked that wrinkles preserved their shape even after cooling down to −80 °C. In Figure 4e, the 600 nm thick coated sample is shown. No changes were observed in the wrinkle pattern, as the coating appeared unaffected by the cooling process. However, during the cooling, it was possible to observe the thermal contraction of PDMS. (33) By heating up the substrates to room temperature, the original shape was recovered. A simple observation of the surface through an optical microscope did not reveal any wrinkle pattern modification. Thus, it can also be inferred that there was no change in height as the wavelength remained constant. We concluded that the coating is unaffected by low temperatures, and changes in the shape of the substrate did not alter it.
With respect to water drop freezing, nucleation started at the air–liquid interface (34) (see the picture in Figure S5a, Supporting Information). The freezing front propagated toward the bottom of the droplet starting from the outer surface, and two phases could be distinguished. This was common among the pPFDA-coated samples: indeed, it was possible to remove the droplets from the surface of the substrates since the bottom part had a lower adhesion to the substrate surface. The experiment was repeated on a pPFDA-coated silicon sample, finding that the coating was damaged by the droplet (Figure S5, Supporting Information). Due to the coating stiffness (E = 2.2–2.4 GPa) (35) and the rigidity of silicon, the droplet ripped the coating during the freezing process.
Figure 4d shows the effect of wrinkles on the freezing of droplets. Random wrinkles can delay freezing respectively of 36 ± 15 s for the 200 nm and 48 ± 19 s for the 300 nm coated samples. Although just the presence of a pattern helped delaying the freezing, having an ordered pattern affects it even more. Observing the results for ordered wrinkles, the delay was above the threshold reached with random wrinkles already with the 100 nm coated sample. The 200 nm coated samples with ordered wrinkles showed the highest value of 199 ± 105 s.
The damage on the coating due to droplet removal can be observed in Figure S8, Supporting Information. The experiment was done at room temperature, and the substrate was previously at −18 °C. The droplets had a volume of 10 μL. Bare PDMS and silicon wafer were tested as well. The force needed to remove ice from the silicon surface with tweezers severely damaged the sample: this was expected since silicon is hydrophilic (36) and ice sticks to its surface. Instead, with bare PDMS ice could be more easily removed without damaging the substrate. The pPFDA coating significantly reduced ice adhesion: simple touching of the frozen droplets with tweezers was enough to promote ice detachment. Iced droplets left round transparent areas on every coated sample, meaning that damage occurred. The results from profilometry proved that a few nanometers of the coating were removed in those areas (Table S4, Supporting Information). The sample with the 300 nm coating was damaged the most, probably due to its higher hysteresis and pinning force.

Conclusions

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Our study focuses on iCVD and has been used for the enhancement of hydrophobic properties of PDMS by depositing via iCVD a fluorinated coating with a wrinkled pattern. The use of ordered wrinkles resulted in better performance than random wrinkling, and controlling the wavelength and height of wrinkles allowed for higher water contact angles at lower thicknesses of coating. A crucial finding in our study is the identification of a threshold value that enhances the hydrophobic properties of samples, thereby minimizing the required chemicals for the process. Moreover, iCVD proved to be a suitable instrument for highly tunable and controllable synthesis. A correlation between wrinkling parameters and thickness of the deposited layer was found, and it was also possible to calculate the energy involved in the system wrinkle-water droplet adapting the Cassie–Baxter theory. Through the analysis, it was proven that achieving thicknesses greater than 200 nm did not improve substrate properties, in terms of hydrophobicity and freezing delay. A decrease in the effect of wrinkling was observed when the coating thickness exceeded a certain value, as for the 600 nm coated sample, because the higher the film thickness, the more distant were the wrinkles, resulting in a faster loss of the Cassie–Baxter state. Wrinkles were stable at low temperatures, and freezing delay experiments showed an increase in the freezing time, with a maximum delay of 200 s observed for 300 nm thick coatings. Even though these are still preliminary results, they show that wrinkling holds promise for improvement also of the icephobic properties of PDMS.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.langmuir.4c00743.

  • FTIR analysis of the coating, the Cassie–Baxter theory adapted to wrinkles, surface characterization and energy calculations, and icing experiments (PDF)

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Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Authors
  • Authors
    • Margherita Aghito - Institute of Solid State Physics, Graz University of Technology, Petersgasse 16, Graz A-8010, Austria;  Department of Material Science, University of Milano-Bicocca, Via Roberto Cozzi 55, Milano 20125, Italy
    • Gabriel Hernandéz Rodríguez - Institute of Solid State Physics, Graz University of Technology, Petersgasse 16, Graz A-8010, Austria
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 956703 (SURFICE Smart surface design for efficient ice protection and control). We acknowledge Catalina Ospino for the fruitful scientific discussion.

References

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    Thin-film electrets have been patterned with trapped charge with submicrometer resoln. using a flexible, elec. conductive electrode. A poly(dimethylsiloxane) stamp, patterned in bas-relief and supporting an 80-nm-thick gold film, is brought into contact with an 80-nm-thick film of poly(methylmethacrylate) supported on n-doped silicon. A voltage pulse between the gold film and the silicon transfers charge at the contact areas between the gold and the polymer electret. Areas as large as 1 cm2 were patterned with trapped charges at a resoln. better than 150 nm in less than 20 s. This process provides a new method for patterning; it suggests possible methods for high-d., charge-based data storage and for high-resoln. charge-based printing.
  3. 3
    Zu, M.; Li, Q.; Wang, G.; Byun, J.-H.; Chou, T.-W. Carbon Nanotube Fiber Based Stretchable Conductor. Adv. Funct. Mater. 2013, 23, 789– 793,  DOI: 10.1002/adfm.201202174
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    Carbon Nanotube Fiber Based Stretchable Conductor
    Zu, Mei; Li, Qingwen; Wang, Guojian; Byun, Joon-Hyung; Chou, Tsu-Wei
    Advanced Functional Materials (2013), 23 (7), 789-793CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Carbon nanotube (CNT) based continuous fiber, a CNT assembly that could potentially retain the superb properties of individual CNTs on a macroscopic scale, belongs to a fascinating new class of electronic materials with potential applications in electronics, sensing, and conducting wires. Here, the fabrication of CNT fiber based stretchable conductors by a simple prestraining-then-buckling approach is reported. To enhance the interfacial bonding between the fibers and the poly(dimethylsiloxane) (PDMS) substrate and thus facilitate the buckling formation, CNT fibers are first coated with a thin layer of liq. PDMS before being transferred to the prestrained substrate. The CNT fibers are deformed into massive buckles, resulting from the compressive force generated upon releasing the fiber/substrate assembly from prestrain. This buckling shape is quite different from the sinusoidal shape obsd. previously in otherwise analogous systems. Similar expts. performed on carbon fiber/PDMS composite film, result in extensive fiber fracture due to the higher fiber flexural modulus. Furthermore, the CNT fiber/PDMS composite film shows little variation in resistance (≈1%) under multiple stretching-and-releasing cycles up to a prestrain level of 40%, indicating the outstanding stability and repeatability in performance as stretchable conductors.
  4. 4
    Bauleth-Ramos, T. Recent approaches for enhancing the performance of dissolving microneedles in drug delivery applications. Mater. Today 2023, 63, 239– 287,  DOI: 10.1016/j.mattod.2022.12.007
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    Law, K.-Y. Definitions for Hydrophilicity, Hydrophobicity, and Superhydrophobicity: Getting the Basics Right. J. Phys. Chem. Lett. 2014, 5, 686– 688,  DOI: 10.1021/jz402762h
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    Definitions for Hydrophilicity, Hydrophobicity, and Superhydrophobicity: Getting the Basics Right
    Law, Kock-Yee
    Journal of Physical Chemistry Letters (2014), 5 (4), 686-688CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    There is no expanded citation for this reference.
  6. 6
    B D Cassie, B. A.; Baxter, S. OF POROUS SURFACES.
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    Karaman, M.; Çabuk, N.; Özyurt, D.; Köysüren, Ö. Self-supporting superhydrophobic thin polymer sheets that mimic the nature’s petal effect. Appl. Surf. Sci. 2012, 259, 542– 546,  DOI: 10.1016/j.apsusc.2012.07.079
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    Self-supporting superhydrophobic thin polymer sheets that mimic the nature's petal effect
    Karaman, Mustafa; Cabuk, Nihat; Ozyurt, Demet; Koysuren, Ozcan
    Applied Surface Science (2012), 259 (), 542-546CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    The high adhesive force between the red rose petal and the water droplet on its surface is termed as the 'petal effect', which is caused by the hierarchical array of micro papilla on the surfaces together with the nano-folds existing on top of each papilla. Because of that special surface topog., the surface is superhydrophobic, but at the same time highly adherent to the water droplet such that the droplet cannot move even if the surface is turned upside down. In this work, we produced a thin (thickness below 1 μm) self-supporting polymer sheet that mimics the surface of a red rose petal. The product is a two-layer polymer sheet made from poly(glycidyl methacrylate) (PGMA) as the supporting layer and a hydrophobic poly(1H,1H,2H,2H-perfluorodecyl acrylate) (PPFDA) on top of it as the functional layer, both of which were deposited by initiated chem. vapor deposition (iCVD) process. The integration of conformal and solvent-free iCVD process into the classical two-step molding procedure allowed exact transfer of surface topog. of the petal surface, which was verified by SEM anal. The static contact angle of water droplet on the surface of the polymer replica was found to be 152 ± 3°, and the water droplet did not roll-off even when the polymer sheet is tilted or turned upside down.
  8. 8
    Murakami, D.; Jinnai, H.; Takahara, A. Wetting transition from the cassie-baxter state to the wenzel state on textured polymer surfaces. Langmuir 2014, 30, 2061– 2067,  DOI: 10.1021/la4049067
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    Wetting Transition from the Cassie-Baxter State to the Wenzel State on Textured Polymer Surfaces
    Murakami, Daiki; Jinnai, Hiroshi; Takahara, Atsushi
    Langmuir (2014), 30 (8), 2061-2067CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    The wetting transition from the Cassie-Baxter state to the Wenzel state on textured surfaces was investigated. Nano- to microscale hexagonal pillared lattices were prepd. by nanoimprint lithog. on fluorinated cycloolefin polymer substrates. The transition was clearly obsd. for water and some ionic liqs. through contact angle measurements and optical microscopy. A simple model clearly demonstrated that the energy barrier in the wetting transition from the Cassie-Baxter state to the Wenzel state was dominated by the competition between the energy barrier and external forces, particularly the Laplace pressure in the present case.
  9. 9
    Chen, A.-F.; Huang, H.-X. Rapid Fabrication of T-Shaped Micropillars on Polypropylene Surfaces with Robust Cassie–Baxter State for Quantitative Droplet Collection. J. Phys. Chem. C 2016, 120, 1556– 1561,  DOI: 10.1021/acs.jpcc.5b10079
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    9
    Rapid Fabrication of T-Shaped Micropillars on Polypropylene Surfaces with Robust Cassie-Baxter State for Quantitative Droplet Collection
    Chen, An-Fu; Huang, Han-Xiong
    Journal of Physical Chemistry C (2016), 120 (3), 1556-1561CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Applying microinjection compression molding technol., a fast and flexible method is first proposed for the successive and mass replication of polypropylene surfaces with T-shaped micropillars in the present work. The water droplet on the titled surfaces grows in size to roll off due to gravitation effect. Interestingly, the roll-off angles on such surfaces are as a quadratic function of specified water droplet vol., meaning quant. droplet collection and lossless transfer can be performed on the replicated surfaces. Meanwhile, self-cleaning behavior is preserved on the surfaces. Moreover, the robust Cassie-Baxter state on the replicated surfaces against the external pressure is demonstrated with droplet compression and immersion expts. Specifically, a droplet sitting on the replicated surface can recover its spherical shape after squeezed to a water film as thin as 0.37 mm, and the replica is remained fully spotless after it is submerged into dyed water. The proposed method for fast replication of microstructured surfaces can be an excellent candidate for the development of microfluidics and droplet manipulation.
  10. 10
    Rodríguez-Hernández, J. Wrinkled interfaces: Taking advantage of surface instabilities to pattern polymer surfaces. Prog. Polym. Sci. 2015, 42, 1– 41,  DOI: 10.1016/j.progpolymsci.2014.07.008
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    Wrinkled interfaces: Taking advantage of surface instabilities to pattern polymer surfaces
    Rodriguez-Hernandez, Juan
    Progress in Polymer Science (2015), 42 (), 1-41CODEN: PRPSB8; ISSN:0079-6700. (Elsevier Ltd.)
    A review. The generation of nano-microstructured polymer film surfaces has been a challenge during the last decades. Advances in the fabrication of structured polymer surfaces to obtain micro and nano patterns have been accomplished following two different approaches, i.e., by adapting techniques, such as molding (embossing) or nano/microimprinting or by developing novel techniques including laser ablation, soft lithog. or laser scanning among others. Thus, higher resoln. capabilities are directly related with technol. advances. In contrast to the use of highly sophisticated tools required by the above mentioned techniques, surface instabilities produced by different mechanisms take advantage of the inherent properties of polymers to induce particular surface patterns. Some of the surface instabilities are well known since decades but novel and old known instability mechanisms have been only recently extended their use to pattern polymer surfaces. This recent interest relies on the rich and complex patterns obtained as a result of self-organizing processes that are rather difficult if not impossible to fabricate by using traditional patterning techniques. Among the approaches to obtain patterned interfaces by means of surface instabilities the formation of wrinkles is the most explored method and will be the center of this review. The fabrication approaches employed to induce wrinkle formation and the possibilities to fine tune the amplitude and period of the wrinkles, the functionality and their final morphol. are thoroughly described. Finally, an overview about the main applications in which buckled interfaces have been already employed or may have an impact in the near future is provided. Their use as templates, as flexible electronics, as supports with controlled wettability and/or adhesion or for biorelated applications are few of the fields in which the unique characteristics of wrinkled interfaces play distinguishing role.
  11. 11
    Aktas, O. C.; Schröder, S.; Veziroglu, S.; Ghori, M. Z.; Haidar, A.; Polonskyi, O.; Strunskus, T.; Gleason, K.; Faupel, F. Superhydrophobic 3D Porous PTFE/TiO 2 Hybrid Structures. Adv. Mater. Interfaces 2019, 6, 1801967,  DOI: 10.1002/admi.201801967
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    Basu, S. K.; Scriven, L. E.; Francis, L. F.; McCormick, A. V. Mechanism of wrinkle formation in curing coatings. Prog. Org. Coat. 2005, 53, 1– 16,  DOI: 10.1016/j.porgcoat.2004.08.007
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    12
    Mechanism of wrinkle formation in curing coatings
    Basu, Soumendra K.; Scriven, L. E.; Francis, L. F.; McCormick, A. V.
    Progress in Organic Coatings (2005), 53 (1), 1-16CODEN: POGCAT; ISSN:0300-9440. (Elsevier B.V.)
    Five wrinkling coating systems - ranging from liq.-applied, thermally cured acrylic-melamine and alkyd systems to powder-applied, thermally cured epoxy and polyester systems to a liq.-applied, UV-cured acrylate system - were investigated by optical microscopy and mech. profilometry. Each system had multi-functional reactants and cross-linkers, and produced a highly cross-linked coating. Upon curing, each appeared to produce a depth-wise gradient in degree of solidification and thereby developed a mech. skin. The presence of a mech. skin was demonstrated by phys. probing the top surface of the semi-cured coating. Under externally applied compressive stresses, the skin showed wrinkled patterns. It is hypothesized that during cure compressive elastic stress developed in the skin when unreacted low-mol. wt. oligomer below the skin diffuses up into the oligomer-depleted crosslinking skin and tends to swell it. This compressive stress, once above a crit. value, could be relieved by out-of-plane deformation or buckling that wrinkles the skin. Exptl. support for the hypothesis is gathered by wrinkling a homogeneous skin solely by absorption of unreacted material, and by noting the similarity of patterns between curing wrinkled coatings and those produced by compressed elastic films on elastic or viscous sub-layers. Moreover, expts. agree with buckling anal. of a skin-sublayer system, which predicts a linear relationship between the crit. wrinkle wavelength and skin thickness.
  13. 13
    Baxamusa, S. H.; Im, S. G.; Gleason, K. K. Initiated and oxidative chemical vapor deposition: a scalable method for conformal and functional polymer films on real substrates. Phys. Chem. Chem. Phys. 2009, 11, 5227,  DOI: 10.1039/b900455f
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    Initiated and oxidative chemical vapor deposition: a scalable method for conformal and functional polymer films on real substrates
    Baxamusa, Salmaan H.; Im, Sung Gap; Gleason, Karen K.
    Physical Chemistry Chemical Physics (2009), 11 (26), 5227-5240CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    A review on chem. vapor deposition (CVD) for the prepn. of conformal and defect-free inorg. thin films with systematically tunable properties. Polymers are a desirable class of materials for surface modification because of their low cost, wide array of chem. and phys. functionality and mech. flexibility. Initiated and oxidative chem. vapor deposition (iCVD and oCVD) are polymer CVD methods that combine the benefits of CVD processing with the possibilities of polymeric materials. Using these technologies, our lab. has synthesized a no. of functional, biocompatible and elec. conducting polymers as thin films on micro- and nano-structured surfaces. This Perspective will review recent advances in these areas and highlight devices and applications that utilize iCVD and oCVD polymers.
  14. 14
    Yin, J.; Yagüe, J. L.; Eggenspieler, D.; Gleason, K. K.; Boyce, M. C. Deterministic order in surface micro-topologies through sequential wrinkling. Adv. Mater. 2012, 24, 5441– 5446,  DOI: 10.1002/adma.201201937
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    14
    Deterministic Order in Surface Micro-Topologies through Sequential Wrinkling
    Yin, Jie; Yaguee, Jose Luis; Eggenspieler, Damien; Gleason, Karen K.; Boyce, Mary C.
    Advanced Materials (Weinheim, Germany) (2012), 24 (40), 5441-5446CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The authors explore the deterministic design of ordered wrinkled topologies through a sequential wrinkling strategy. The success of the proposed process is demonstrated in example systems of thin polymeric films synthesized from monomers including ethylene glycol diacrylate (EGDA) and 2-hydroxyethyl methacrylate (HEMA) on PDMS substrates. The initiated chem. vapor deposition (iCVD) technique is used for the 1st time for the deposition of thin polymeric coatings without use of solvents to obtain wrinkles. ICVD yields a conformal thin coating on virtually any substrate, giving a controllable thickness and tunable structural, mech., thermal, wetting, and swelling properties. Here, using the iCVD technique, a variety of ordered deterministic herringbone patterns are created through the wrinkling of polymeric coatings on PDMS substrates and also, the sequential buckling mechanisms underpinning the ordered patterns are revealed. Also, a simplified theor. model is developed to predict the geometry of the ordered herringbone pattern. In addn. to providing the ability to deterministically yield ordered surface topologies, the method also provides a tool to measure the elastic modulus of the thin film.
  15. 15
    Muralter, F.; Coclite, A. M.; Werzer, O. Wrinkling of an Enteric Coating Induced by Vapor-Deposited Stimuli-Responsive Hydrogel Thin Films. J. Phys. Chem. C 2019, 123, 24165– 24171,  DOI: 10.1021/acs.jpcc.9b07340
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    15
    Wrinkling of an Enteric Coating Induced by Vapor-Deposited Stimuli-Responsive Hydrogel Thin Films
    Muralter, Fabian; Coclite, Anna Maria; Werzer, Oliver
    Journal of Physical Chemistry C (2019), 123 (39), 24165-24171CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    In this contribution, we report on the thin-film synthesis of a thermoresponsive polymer onto another polymer used as an enteric coating in drug applications. In particular, we deposit cross-linked poly(N-vinylcaprolactam) (pNVCL) thin films by initiated chem. vapor deposition (iCVD) onto spin-coated Eudragit (EUD) layers. Already upon iCVD synthesis, the layered structure starts to form wrinkles at a min. iCVD thickness of 30 nm. By changing the EUD layer thickness and the amt. of crosslinking used during iCVD, the morphol. of the wrinkles is demonstrated to be readily tunable. Laterally, the double-layer structures vary in morphol. from being ultrasmooth to exhibiting up to a 3.5μm wrinkle wavelength. The surface roughness and, thus, the wrinkles' height can be tailored from below 1 nm up to 100 nm. From the resulting wavelength of wrinkles, an estn. of the elastic modulus of pNVCL proves its tunability over a wide range of values thanks to the iCVD process. This study elucidates an uncomplicated way to tune the wrinkles' morphol. and, thus, the surface area of a system that can be employed in drug delivery applications. Hence, an enteric coating of EUD together with an iCVD-synthesized thermoresponsive thin film is proposed as a promising composite encapsulation layer to outperform established systems in terms of tunability of the response to multiple external stimuli.
  16. 16
    Scarratt, L. R. J.; Hoatson, B. S.; Wood, E. S.; Hawkett, B. S.; Neto, C. Durable Superhydrophobic Surfaces via Spontaneous Wrinkling of Teflon AF. ACS Appl. Mater. Interfaces 2016, 8, 6743– 6750,  DOI: 10.1021/acsami.5b12165
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    16
    Durable Superhydrophobic Surfaces via Spontaneous Wrinkling of Teflon AF
    Scarratt, Liam R. J.; Hoatson, Ben S.; Wood, Elliot S.; Hawkett, Brian S.; Neto, Chiara
    ACS Applied Materials & Interfaces (2016), 8 (10), 6743-6750CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Fabrication of both single-scale and hierarchical superhydrophobic surfaces, created by exploiting the spontaneous wrinkling of a rigid Teflon AF film on two types of shrinkable plastic substrates, is reported. Sub-100 nm to micrometric wrinkles were reproducibly generated by this simple process, with remarkable control over the size and hierarchy. Hierarchical Teflon AF wrinkled surfaces showed extremely high water repellence (contact angle 172°) and very low contact angle hysteresis (2°), resulting in droplets rolling off the surface at tilt angles lower than 5°. The wrinkling process intimately binds the Teflon AF layer with its substrate, making these surfaces mech. robust, as revealed by macroscale and nanoscale wear tests: hardness values were close to that of com. optical lenses and aluminum films, resistance to scratch was comparable to com. hydrophobic coatings, and damage by extensive sonication did not significantly affect water repellence. By this fabrication method the size of the wrinkles can be reproducibly tuned from the nanoscale to the microscale, across the whole surface in one step; the fabrication procedure is extremely rapid, requiring only 2 min of thermal annealing to produce the desired topog., and uses inexpensive materials. The very low roll-off angles achieved in the hierarchical surfaces offer a potentially up-scalable alternative as self-cleaning and drag-reducing coatings.
  17. 17
    Guselnikova, O.; Svanda, J.; Postnikov, P.; Kalachyova, Y.; Svorcik, V.; Lyutakov, O. Fast and Reproducible Wettability Switching on Functionalized PVDF/PMMA Surface Controlled by External Electric Field. Adv. Mater. Interfaces 2017, 4, 1600886,  DOI: 10.1002/admi.201600886
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    Perrotta, A.; Christian, P.; Jones, A. O. F.; Muralter, F.; Coclite, A. M. Growth Regimes of Poly(perfluorodecyl acrylate) Thin Films by Initiated Chemical Vapor Deposition. Macromolecules 2018, 51, 5694– 5703,  DOI: 10.1021/acs.macromol.8b00904
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    18
    Growth Regimes of Poly(perfluorodecyl acrylate) Thin Films by Initiated Chemical Vapor Deposition
    Perrotta, Alberto; Christian, Paul; Jones, Andrew O. F.; Muralter, Fabian; Coclite, Anna Maria
    Macromolecules (Washington, DC, United States) (2018), 51 (15), 5694-5703CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
    Control over thin film growth (e.g., crystallog. orientation and morphol.) is of high technol. interest as it affects several physicochem. material properties, such as chem. affinity, mech. stability, and surface morphol. The effect of process parameters on the mol. organization of perfluorinated polymers deposited via initiated chem. vapor deposition (iCVD) has been previously reported. We showed that the tendency of poly(1H,1H,2H,2H-perfluorodecyl acrylate) (pPFDA) to organize in an ordered lamellar structure is a function of the filament and substrate temps. adopted during the iCVD process. In this contribution, a more thorough investigation of the effect of such parameters is presented, using synchrotron radiation grazing incidence and specular X-ray diffraction (GIXD and XRD) and at. force microscopy (AFM). The parameters influencing the amorphization, mosaicity, and preferential orientation are addressed. Different growth regimes were witnessed, characterized by a different surface structuring and by the presence of particular crystallog. textures. The combination of morphol. and crystallog. analyses allowed the identification of pPFDA growth possibilities between island or columnar growth.
  19. 19
    Ranacher, C. Layered Nanostructures in Proton Conductive Polymers Obtained by Initiated Chemical Vapor Deposition. Macromolecules 2015, 48, 6177– 6185,  DOI: 10.1021/acs.macromol.5b01145
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    19
    Layered Nanostructures in Proton Conductive Polymers Obtained by Initiated Chemical Vapor Deposition
    Ranacher, Christian; Resel, Roland; Moni, Priya; Cermenek, Bernd; Hacker, Viktor; Coclite, Anna Maria
    Macromolecules (Washington, DC, United States) (2015), 48 (17), 6177-6185CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
    Proton conductive copolymers of 1H,1H,2H,2H-perfluorodecyl acrylate (PFDA) and methacrylic acid (MAA) have been synthesized by initiated chem. vapor deposition (iCVD). Detailed insights into the copolymers' mol. organization were gained through an X-ray-based investigation to serve as a starting point for systematic studies on the relation among proton cond. and polymer structure. The method of copolymn., iCVD, facilitated the tuning of the ratio between acidic -COOH groups, coming from MAA, and the hydrophobic matrix from the PFDA components. It was demonstrated that the copolymers crystallize into a bilayer structure, formed by the perfluorinated pendant chains of PFDA, perpendicular to the substrate surface. The MAA mols. form COOH-enriched regions among the bilayers-parallel to the substrate surface-which can act as ionic channels for proton conduction when the acid groups are deprotonated. The interplanar distance between the bilayer lamellar structures increases by the presence of MAA units from 3.19 to 3.56 nm for the MAA-PFDA copolymer with 41% MAA, therefore yielding to 0.4 nm wide channels. Proton conductivities as high as 55 mS/cm have been achieved for copolymers with 41% MAA fraction. Such ordered, layered nanostructures were never shown before for copolymers deposited from the vapor phase, and their anisotropy can be of inspiration for many applications beyond proton conduction. Moreover, the one-step copolymn. process has the potential to manuf. inexpensive, high quality membranes for proton exchange membrane fuel cells.
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    Xin, Z.; Xiao-Hui, S.; Dian-Lin, Z. Thickness dependence of grain size and surface roughness for dc magnetron sputtered Au films. Chin. Phys. B 2010, 19, 086802  DOI: 10.1088/1674-1056/19/8/086802
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    Bandeira, R. M.; van Drunen, J.; Garcia, A. C.; Tremiliosi-Filho, G. Influence of the thickness and roughness of polyaniline coatings on corrosion protection of AA7075 aluminum alloy. Electrochim. Acta 2017, 240, 215– 224,  DOI: 10.1016/j.electacta.2017.04.083
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    Influence of the thickness and roughness of polyaniline coatings on corrosion protection of AA7075 aluminum alloy
    Bandeira, Rafael Marinho; van Drunen, Julia; Garcia, Amanda Cristina; Tremiliosi-Filho, Germano
    Electrochimica Acta (2017), 240 (), 215-224CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
    Polyaniline coatings of various controlled thicknesses are applied to 7075-T6 Al alloy substrates and evaluated for their corrosion protection ability in a corrosive saline environment using electrochem. techniques. The coating thicknesses are in the range of ∼5 to 35 μm and they increase linearly with increasing quantities of PANI soln. deposited on the substrate. The relation between the coating thickness, roughness, and hydrophilicity was examd. Coatings ranging from 12 to 23 μm in thickness have the lowest roughness values and highest contact angles with H2O. The PANI-coated 7075 alloy electrodes with low roughness and high contact angles show better corrosion protection properties in saline soln. (3.5% NaCl), as detd. by electrochem. impedance spectroscopy (EIS) and voltammetric polarization measurements. Scanning electronic microscopy (SEM) images demonstrate the homogeneous surface morphol. of the PANI coatings on the micrometer scale. Even after prolonged exposure to corrosive media, no cracks were found.
  22. 22
    Yang, R.; Gleason, K. K. Ultrathin Antifouling Coatings with Stable Surface Zwitterionic Functionality by Initiated Chemical Vapor Deposition (iCVD). Langmuir 2012, 28, 12266– 12274,  DOI: 10.1021/la302059s
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    Ultrathin Antifouling Coatings with Stable Surface Zwitterionic Functionality by Initiated Chemical Vapor Deposition (iCVD)
    Yang, Rong; Gleason, Karen K.
    Langmuir (2012), 28 (33), 12266-12274CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    Antifouling thin films of poly[N,N-dimethyl-N-methacryloxyethyl-N-(3-sulfopropyl)-co-2-(dimethylamino)ethyl methacrylate-co-ethylene glycol dimethacrylate] (PDDE) were synthesized via a substrate-independent and all-dry-initiated chem. vapor deposition (iCVD) technique followed by a diffusion-limited vapor-phase reaction with 1,3-propane sultone. Coated surfaces exhibited very low absorption of various foulants including bovine serum albumin (BSA), humic acid (HA), and sodium alginate (SA), as measured with the quartz crystal microbalance with dissipation monitoring (QCM-D). The fouling by humic acid was dependent on the presence of divalent cations such as Ca2+. Both depth profiling and angle-resolved XPS measurements indicated that the zwitterionic groups were highly concd. in the top ∼3 nm of the film. The contact angle measurements revealed a limited degree of surface chain reorganization upon contacting water. The dynamic contact angles remained unchanged after 100 days of storage in air, indicating the stability of the interface. The coating was substrate-independent, and the film was conformal on surface nanostructures including trenches, reverse osmosis membranes, and electrospun nanofiber mats.
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    Ozaydin-Ince, G.; Matin, A.; Khan, Z.; Zaidi, S. M. J.; Gleason, K. K. Surface modification of reverse osmosis desalination membranes by thin-film coatings deposited by initiated chemical vapor deposition. Thin Solid Films 2013, 539, 181– 187,  DOI: 10.1016/j.tsf.2013.04.133
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    23
    Surface modification of reverse osmosis desalination membranes by thin-film coatings deposited by initiated chemical vapor deposition
    Ozaydin-Ince, Gozde; Matin, Asif; Khan, Zafarullah; Zaidi, S. M. Javaid; Gleason, Karen K.
    Thin Solid Films (2013), 539 (), 181-187CODEN: THSFAP; ISSN:0040-6090. (Elsevier B.V.)
    Thin-film polymeric reverse osmosis membranes, due to their high permeation rates and good salt rejection capabilities, are widely used for seawater desalination. However, these membranes are prone to biofouling, which affects their performance and efficiency. In this work, we report a method to modify the membrane surface without damaging the active layer or significantly affecting the performance of the membrane. Amphiphilic copolymer films of hydrophilic hydroxyethylmethacrylate and hydrophobic perfluorodecylacrylate (PFA) were synthesized and deposited on com. RO membranes using an initiated chem. vapor deposition technique which is a polymer deposition technique that involves free-radical polymn. initiated by gas-phase radicals. Relevant surface characteristics such as hydrophilicity and roughness could be systematically controlled by varying the polymer chem. Increasing the hydrophobic PFA content in the films leads to an increase in the surface roughness and hydrophobicity. Furthermore, the surface morphol. studies performed using the at. force microscopy show that as the thickness of the coating increases av. surface roughness increases. Using this knowledge, the coating thickness and chem. were optimized to achieve high permeate flux and to reduce cell attachment. Results of the static bacterial adhesion tests show that the attachment of bacterial cells is significantly reduced on the coated membranes.
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    Wang, F. Surface Wrinkling with Memory for Programming Adhesion and Wettability. ACS Appl. Nano Mater. 2023, 6, 4097– 4104,  DOI: 10.1021/acsanm.2c05410
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    Huhtamäki, T.; Tian, X.; Korhonen, J. T.; Ras, R. H. A. Surface-wetting characterization using contact-angle measurements. Nat. Protoc. 2018, 13, 1521– 1538,  DOI: 10.1038/s41596-018-0003-z
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    Surface-wetting characterization using contact-angle measurements
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  1. Kwang‐Won Park, Karen K. Gleason, Rong Yang. Advanced Morphological Control of Polymeric Surfaces Using Initiated Chemical Vapor Deposition (iCVD). Advanced Functional Materials 2024, 36 https://doi.org/10.1002/adfm.202417620
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  • Abstract

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    Figure 1

    Figure 1. (a) Schematic representation of the stretching of the pPFDA-coated PDMS: (I) a 10% stretching is applied to the substrate during the coating process. The force is applied parallel to the substrate in a longitudinal direction. Both elastic moduli of polyperfluorodecyl acrylate (pPFDA) and polydimethylsiloxane (PDMS) are highlighted. (II) When the stretching is released, wrinkles appear. (b) The PDMS slice is placed in a self-built stretcher, put in contact with the cooling plate through a piece of copper. The stretcher is then surrounded by three silicon wafers, used as a reference to determine the thickness of the deposited coating via ellipsometry and to proceed to further characterization.

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    Figure 2

    Figure 2. (a) Optical microscopy images (first row) and the AFM scans (second row) resulting from the five different coated surfaces are here shown: (I) 100 nm, (II) 150 nm, (III) 200 nm, (IV) 300 nm, and (V) 600 nm. The scale bar for the optical microscope images is 20 μm, while for the AFM images, it is 10 μm. In the third row, the 3D scan is reported, showing the 3D shape of wrinkles and the different distributions on the surface due to the change in thickness. On the right angle, the highest value for the height of wrinkles is reported. (b) Optical microscopy image of random wrinkles on a 200 nm coated PDMS with pPFDA. The scale bar is 20 μm. (c) AFM micrographs of the same surface, the scale bar is 5 μm. (d) Wrinkle wavelength versus coating thickness; two different wavelengths were identified: a long wavelength, λ long, and a short wavelength, λ short. The long wavelength shows the strongest influence from the increase of thickness, while the short wavelength has mostly a constant behavior. Both wavelengths are in the range of μm. e. This plot shows the height of wrinkles versus coating thickness: the increase is again linear. The plotted heights were obtained by averaging the wrinkle heights measured by AFM.

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    Figure 3

    Figure 3. (a) Water contact angles were measured on several samples with ordered and random wrinkles. The error bars are indicated for each data point in the plot, but sometimes, they are hidden by the symbols. An enhancement in the WCA could be seen already in the 100 nm coated sample with ordered wrinkles, in comparison to the random wrinkles. (b) Schematic representation of how the Cassie–Baxter theory, originally developed to study systems based on fibers, was adapted to a wrinkled system to calculate the areas of interface and the energy spent to originate a Cassie–Baxter state. The parameters used to describe the fibers (radius, r, and middle distance between fibers, d) were related to the parameters of wrinkling (long wavelength, λlong, and width, b). (c) In this plot, the total area values of solid–liquid, f1, and solid–air, f2, interfaces are represented. (d) The net energy (ED) expended in forming unit geometrical areas of interface is plotted with the long wavelength: the shape of wrinkles determines a clustering of the values.

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    Figure 4

    Figure 4. (a) Values for the Laplace and energy barrier pressure were calculated for each sample, considering both wavelengths where possible. Here, they were plotted with the thickness of the coating. (b) The depinning force per unit length could be calculated for each sample; it is related to the hysteresis values, which are reported for each sample. Going further with the thickness of the coating results in a decrease of the depinning force, and the 600 nm coated sample shows characteristics similar to those of the thinner coated samples. This is because different roughnesses are observed in different areas of the sample. (c) Plot of the pinning force over the volume of the droplet used during the tests, respectively, 3, 4, 5, 7, and 10 μL. An increase in the calculated pinning force is observed with the thickness, finding the 600 nm coated sample with the highest value. (d) Plot of delay in freezing measured for a droplet at the enhancement of thickness. Results are shown for ordered (black) and random (red) wrinkles. Multiple droplets on the surface of 10 μL, T = −30 °C, 3 cycles were run and averaged. The large error bars are due to inhomogeneities on the PDMS thickness derived from its preparation. (e) Images taken during the cooling to −80 °C and the heating up to RT at a cooling rate of 15 °C/min. 50× magnification, optical microscope, Olympus BX51.

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      Baxamusa, Salmaan H.; Im, Sung Gap; Gleason, Karen K.
      Physical Chemistry Chemical Physics (2009), 11 (26), 5227-5240CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      A review on chem. vapor deposition (CVD) for the prepn. of conformal and defect-free inorg. thin films with systematically tunable properties. Polymers are a desirable class of materials for surface modification because of their low cost, wide array of chem. and phys. functionality and mech. flexibility. Initiated and oxidative chem. vapor deposition (iCVD and oCVD) are polymer CVD methods that combine the benefits of CVD processing with the possibilities of polymeric materials. Using these technologies, our lab. has synthesized a no. of functional, biocompatible and elec. conducting polymers as thin films on micro- and nano-structured surfaces. This Perspective will review recent advances in these areas and highlight devices and applications that utilize iCVD and oCVD polymers.
    14. 14
      Yin, J.; Yagüe, J. L.; Eggenspieler, D.; Gleason, K. K.; Boyce, M. C. Deterministic order in surface micro-topologies through sequential wrinkling. Adv. Mater. 2012, 24, 5441– 5446,  DOI: 10.1002/adma.201201937
      14
      Deterministic Order in Surface Micro-Topologies through Sequential Wrinkling
      Yin, Jie; Yaguee, Jose Luis; Eggenspieler, Damien; Gleason, Karen K.; Boyce, Mary C.
      Advanced Materials (Weinheim, Germany) (2012), 24 (40), 5441-5446CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The authors explore the deterministic design of ordered wrinkled topologies through a sequential wrinkling strategy. The success of the proposed process is demonstrated in example systems of thin polymeric films synthesized from monomers including ethylene glycol diacrylate (EGDA) and 2-hydroxyethyl methacrylate (HEMA) on PDMS substrates. The initiated chem. vapor deposition (iCVD) technique is used for the 1st time for the deposition of thin polymeric coatings without use of solvents to obtain wrinkles. ICVD yields a conformal thin coating on virtually any substrate, giving a controllable thickness and tunable structural, mech., thermal, wetting, and swelling properties. Here, using the iCVD technique, a variety of ordered deterministic herringbone patterns are created through the wrinkling of polymeric coatings on PDMS substrates and also, the sequential buckling mechanisms underpinning the ordered patterns are revealed. Also, a simplified theor. model is developed to predict the geometry of the ordered herringbone pattern. In addn. to providing the ability to deterministically yield ordered surface topologies, the method also provides a tool to measure the elastic modulus of the thin film.
    15. 15
      Muralter, F.; Coclite, A. M.; Werzer, O. Wrinkling of an Enteric Coating Induced by Vapor-Deposited Stimuli-Responsive Hydrogel Thin Films. J. Phys. Chem. C 2019, 123, 24165– 24171,  DOI: 10.1021/acs.jpcc.9b07340
      15
      Wrinkling of an Enteric Coating Induced by Vapor-Deposited Stimuli-Responsive Hydrogel Thin Films
      Muralter, Fabian; Coclite, Anna Maria; Werzer, Oliver
      Journal of Physical Chemistry C (2019), 123 (39), 24165-24171CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      In this contribution, we report on the thin-film synthesis of a thermoresponsive polymer onto another polymer used as an enteric coating in drug applications. In particular, we deposit cross-linked poly(N-vinylcaprolactam) (pNVCL) thin films by initiated chem. vapor deposition (iCVD) onto spin-coated Eudragit (EUD) layers. Already upon iCVD synthesis, the layered structure starts to form wrinkles at a min. iCVD thickness of 30 nm. By changing the EUD layer thickness and the amt. of crosslinking used during iCVD, the morphol. of the wrinkles is demonstrated to be readily tunable. Laterally, the double-layer structures vary in morphol. from being ultrasmooth to exhibiting up to a 3.5μm wrinkle wavelength. The surface roughness and, thus, the wrinkles' height can be tailored from below 1 nm up to 100 nm. From the resulting wavelength of wrinkles, an estn. of the elastic modulus of pNVCL proves its tunability over a wide range of values thanks to the iCVD process. This study elucidates an uncomplicated way to tune the wrinkles' morphol. and, thus, the surface area of a system that can be employed in drug delivery applications. Hence, an enteric coating of EUD together with an iCVD-synthesized thermoresponsive thin film is proposed as a promising composite encapsulation layer to outperform established systems in terms of tunability of the response to multiple external stimuli.
    16. 16
      Scarratt, L. R. J.; Hoatson, B. S.; Wood, E. S.; Hawkett, B. S.; Neto, C. Durable Superhydrophobic Surfaces via Spontaneous Wrinkling of Teflon AF. ACS Appl. Mater. Interfaces 2016, 8, 6743– 6750,  DOI: 10.1021/acsami.5b12165
      16
      Durable Superhydrophobic Surfaces via Spontaneous Wrinkling of Teflon AF
      Scarratt, Liam R. J.; Hoatson, Ben S.; Wood, Elliot S.; Hawkett, Brian S.; Neto, Chiara
      ACS Applied Materials & Interfaces (2016), 8 (10), 6743-6750CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Fabrication of both single-scale and hierarchical superhydrophobic surfaces, created by exploiting the spontaneous wrinkling of a rigid Teflon AF film on two types of shrinkable plastic substrates, is reported. Sub-100 nm to micrometric wrinkles were reproducibly generated by this simple process, with remarkable control over the size and hierarchy. Hierarchical Teflon AF wrinkled surfaces showed extremely high water repellence (contact angle 172°) and very low contact angle hysteresis (2°), resulting in droplets rolling off the surface at tilt angles lower than 5°. The wrinkling process intimately binds the Teflon AF layer with its substrate, making these surfaces mech. robust, as revealed by macroscale and nanoscale wear tests: hardness values were close to that of com. optical lenses and aluminum films, resistance to scratch was comparable to com. hydrophobic coatings, and damage by extensive sonication did not significantly affect water repellence. By this fabrication method the size of the wrinkles can be reproducibly tuned from the nanoscale to the microscale, across the whole surface in one step; the fabrication procedure is extremely rapid, requiring only 2 min of thermal annealing to produce the desired topog., and uses inexpensive materials. The very low roll-off angles achieved in the hierarchical surfaces offer a potentially up-scalable alternative as self-cleaning and drag-reducing coatings.
    17. 17
      Guselnikova, O.; Svanda, J.; Postnikov, P.; Kalachyova, Y.; Svorcik, V.; Lyutakov, O. Fast and Reproducible Wettability Switching on Functionalized PVDF/PMMA Surface Controlled by External Electric Field. Adv. Mater. Interfaces 2017, 4, 1600886,  DOI: 10.1002/admi.201600886
      There is no corresponding record for this reference.
    18. 18
      Perrotta, A.; Christian, P.; Jones, A. O. F.; Muralter, F.; Coclite, A. M. Growth Regimes of Poly(perfluorodecyl acrylate) Thin Films by Initiated Chemical Vapor Deposition. Macromolecules 2018, 51, 5694– 5703,  DOI: 10.1021/acs.macromol.8b00904
      18
      Growth Regimes of Poly(perfluorodecyl acrylate) Thin Films by Initiated Chemical Vapor Deposition
      Perrotta, Alberto; Christian, Paul; Jones, Andrew O. F.; Muralter, Fabian; Coclite, Anna Maria
      Macromolecules (Washington, DC, United States) (2018), 51 (15), 5694-5703CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
      Control over thin film growth (e.g., crystallog. orientation and morphol.) is of high technol. interest as it affects several physicochem. material properties, such as chem. affinity, mech. stability, and surface morphol. The effect of process parameters on the mol. organization of perfluorinated polymers deposited via initiated chem. vapor deposition (iCVD) has been previously reported. We showed that the tendency of poly(1H,1H,2H,2H-perfluorodecyl acrylate) (pPFDA) to organize in an ordered lamellar structure is a function of the filament and substrate temps. adopted during the iCVD process. In this contribution, a more thorough investigation of the effect of such parameters is presented, using synchrotron radiation grazing incidence and specular X-ray diffraction (GIXD and XRD) and at. force microscopy (AFM). The parameters influencing the amorphization, mosaicity, and preferential orientation are addressed. Different growth regimes were witnessed, characterized by a different surface structuring and by the presence of particular crystallog. textures. The combination of morphol. and crystallog. analyses allowed the identification of pPFDA growth possibilities between island or columnar growth.
    19. 19
      Ranacher, C. Layered Nanostructures in Proton Conductive Polymers Obtained by Initiated Chemical Vapor Deposition. Macromolecules 2015, 48, 6177– 6185,  DOI: 10.1021/acs.macromol.5b01145
      19
      Layered Nanostructures in Proton Conductive Polymers Obtained by Initiated Chemical Vapor Deposition
      Ranacher, Christian; Resel, Roland; Moni, Priya; Cermenek, Bernd; Hacker, Viktor; Coclite, Anna Maria
      Macromolecules (Washington, DC, United States) (2015), 48 (17), 6177-6185CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
      Proton conductive copolymers of 1H,1H,2H,2H-perfluorodecyl acrylate (PFDA) and methacrylic acid (MAA) have been synthesized by initiated chem. vapor deposition (iCVD). Detailed insights into the copolymers' mol. organization were gained through an X-ray-based investigation to serve as a starting point for systematic studies on the relation among proton cond. and polymer structure. The method of copolymn., iCVD, facilitated the tuning of the ratio between acidic -COOH groups, coming from MAA, and the hydrophobic matrix from the PFDA components. It was demonstrated that the copolymers crystallize into a bilayer structure, formed by the perfluorinated pendant chains of PFDA, perpendicular to the substrate surface. The MAA mols. form COOH-enriched regions among the bilayers-parallel to the substrate surface-which can act as ionic channels for proton conduction when the acid groups are deprotonated. The interplanar distance between the bilayer lamellar structures increases by the presence of MAA units from 3.19 to 3.56 nm for the MAA-PFDA copolymer with 41% MAA, therefore yielding to 0.4 nm wide channels. Proton conductivities as high as 55 mS/cm have been achieved for copolymers with 41% MAA fraction. Such ordered, layered nanostructures were never shown before for copolymers deposited from the vapor phase, and their anisotropy can be of inspiration for many applications beyond proton conduction. Moreover, the one-step copolymn. process has the potential to manuf. inexpensive, high quality membranes for proton exchange membrane fuel cells.
    20. 20
      Xin, Z.; Xiao-Hui, S.; Dian-Lin, Z. Thickness dependence of grain size and surface roughness for dc magnetron sputtered Au films. Chin. Phys. B 2010, 19, 086802  DOI: 10.1088/1674-1056/19/8/086802
      There is no corresponding record for this reference.
    21. 21
      Bandeira, R. M.; van Drunen, J.; Garcia, A. C.; Tremiliosi-Filho, G. Influence of the thickness and roughness of polyaniline coatings on corrosion protection of AA7075 aluminum alloy. Electrochim. Acta 2017, 240, 215– 224,  DOI: 10.1016/j.electacta.2017.04.083
      21
      Influence of the thickness and roughness of polyaniline coatings on corrosion protection of AA7075 aluminum alloy
      Bandeira, Rafael Marinho; van Drunen, Julia; Garcia, Amanda Cristina; Tremiliosi-Filho, Germano
      Electrochimica Acta (2017), 240 (), 215-224CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
      Polyaniline coatings of various controlled thicknesses are applied to 7075-T6 Al alloy substrates and evaluated for their corrosion protection ability in a corrosive saline environment using electrochem. techniques. The coating thicknesses are in the range of ∼5 to 35 μm and they increase linearly with increasing quantities of PANI soln. deposited on the substrate. The relation between the coating thickness, roughness, and hydrophilicity was examd. Coatings ranging from 12 to 23 μm in thickness have the lowest roughness values and highest contact angles with H2O. The PANI-coated 7075 alloy electrodes with low roughness and high contact angles show better corrosion protection properties in saline soln. (3.5% NaCl), as detd. by electrochem. impedance spectroscopy (EIS) and voltammetric polarization measurements. Scanning electronic microscopy (SEM) images demonstrate the homogeneous surface morphol. of the PANI coatings on the micrometer scale. Even after prolonged exposure to corrosive media, no cracks were found.
    22. 22
      Yang, R.; Gleason, K. K. Ultrathin Antifouling Coatings with Stable Surface Zwitterionic Functionality by Initiated Chemical Vapor Deposition (iCVD). Langmuir 2012, 28, 12266– 12274,  DOI: 10.1021/la302059s
      22
      Ultrathin Antifouling Coatings with Stable Surface Zwitterionic Functionality by Initiated Chemical Vapor Deposition (iCVD)
      Yang, Rong; Gleason, Karen K.
      Langmuir (2012), 28 (33), 12266-12274CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      Antifouling thin films of poly[N,N-dimethyl-N-methacryloxyethyl-N-(3-sulfopropyl)-co-2-(dimethylamino)ethyl methacrylate-co-ethylene glycol dimethacrylate] (PDDE) were synthesized via a substrate-independent and all-dry-initiated chem. vapor deposition (iCVD) technique followed by a diffusion-limited vapor-phase reaction with 1,3-propane sultone. Coated surfaces exhibited very low absorption of various foulants including bovine serum albumin (BSA), humic acid (HA), and sodium alginate (SA), as measured with the quartz crystal microbalance with dissipation monitoring (QCM-D). The fouling by humic acid was dependent on the presence of divalent cations such as Ca2+. Both depth profiling and angle-resolved XPS measurements indicated that the zwitterionic groups were highly concd. in the top ∼3 nm of the film. The contact angle measurements revealed a limited degree of surface chain reorganization upon contacting water. The dynamic contact angles remained unchanged after 100 days of storage in air, indicating the stability of the interface. The coating was substrate-independent, and the film was conformal on surface nanostructures including trenches, reverse osmosis membranes, and electrospun nanofiber mats.
    23. 23
      Ozaydin-Ince, G.; Matin, A.; Khan, Z.; Zaidi, S. M. J.; Gleason, K. K. Surface modification of reverse osmosis desalination membranes by thin-film coatings deposited by initiated chemical vapor deposition. Thin Solid Films 2013, 539, 181– 187,  DOI: 10.1016/j.tsf.2013.04.133
      23
      Surface modification of reverse osmosis desalination membranes by thin-film coatings deposited by initiated chemical vapor deposition
      Ozaydin-Ince, Gozde; Matin, Asif; Khan, Zafarullah; Zaidi, S. M. Javaid; Gleason, Karen K.
      Thin Solid Films (2013), 539 (), 181-187CODEN: THSFAP; ISSN:0040-6090. (Elsevier B.V.)
      Thin-film polymeric reverse osmosis membranes, due to their high permeation rates and good salt rejection capabilities, are widely used for seawater desalination. However, these membranes are prone to biofouling, which affects their performance and efficiency. In this work, we report a method to modify the membrane surface without damaging the active layer or significantly affecting the performance of the membrane. Amphiphilic copolymer films of hydrophilic hydroxyethylmethacrylate and hydrophobic perfluorodecylacrylate (PFA) were synthesized and deposited on com. RO membranes using an initiated chem. vapor deposition technique which is a polymer deposition technique that involves free-radical polymn. initiated by gas-phase radicals. Relevant surface characteristics such as hydrophilicity and roughness could be systematically controlled by varying the polymer chem. Increasing the hydrophobic PFA content in the films leads to an increase in the surface roughness and hydrophobicity. Furthermore, the surface morphol. studies performed using the at. force microscopy show that as the thickness of the coating increases av. surface roughness increases. Using this knowledge, the coating thickness and chem. were optimized to achieve high permeate flux and to reduce cell attachment. Results of the static bacterial adhesion tests show that the attachment of bacterial cells is significantly reduced on the coated membranes.
    24. 24
      Wang, F. Surface Wrinkling with Memory for Programming Adhesion and Wettability. ACS Appl. Nano Mater. 2023, 6, 4097– 4104,  DOI: 10.1021/acsanm.2c05410
      There is no corresponding record for this reference.
    25. 25
      Huhtamäki, T.; Tian, X.; Korhonen, J. T.; Ras, R. H. A. Surface-wetting characterization using contact-angle measurements. Nat. Protoc. 2018, 13, 1521– 1538,  DOI: 10.1038/s41596-018-0003-z
      25
      Surface-wetting characterization using contact-angle measurements
      Huhtamaki, Tommi; Tian, Xuelin; Korhonen, Juuso T.; Ras, Robin H. A.
      Nature Protocols (2018), 13 (7), 1521-1538CODEN: NPARDW; ISSN:1750-2799. (Nature Research)
      Wetting, the process of water interacting with a surface, is crit. in our everyday lives and in many biol. and technol. systems. The contact angle is the angle at the interface where water, air and solid meet, and its value is a measure of how likely the surface is to be wetted by the water. Low contact-angle values demonstrate a tendency of the water to spread and adhere to the surface, whereas high contact-angle values show the surface's tendency to repel water. The most common method for surface-wetting characterization is sessile-drop goniometry, due to its simplicity. The method dets. the contact angle from the shape of the droplet and can be applied to a wide variety of materials, from biol. surfaces to polymers, metals, ceramics, minerals and so on. The apparent simplicity of the method is misleading, however, and obtaining meaningful results requires minimization of random and systematic errors. This article provides a protocol for performing reliable and reproducible measurements of the advancing contact angle (ACA) and the receding contact angle (RCA) by slowly increasing and reducing the vol. of a probe drop, resp. One pair of ACA and RCA measurements takes ~ 15-20 min to complete, whereas the whole protocol with repeat measurements may take ~ 1-2 h. This protocol focuses on using water as a probe liq., and advice is given on how it can be modified for the use of other probe liqs.
    26. 26
      Hebbar, R. S.; Isloor, A. M.; Ismail, A. F. Contact Angle Meeasurements. in Membrane Characterization; Elsevier, 2017, 219– 255 DOI: 10.1016/B978-0-444-63776-5.00012-7 .
      There is no corresponding record for this reference.
    27. 27
      Kim, D. S. Fabrication of PDMS micro/nano hybrid surface for increasing hydrophobicity. Microelectron. Eng. 2009, 86, 1375– 1378,  DOI: 10.1016/j.mee.2009.02.017
      27
      Fabrication of PDMS micro/nano hybrid surface for increasing hydrophobicity
      Kim, Dong Sung; Lee, Bong-Kee; Yeo, Jihoon; Choi, Min Jin; Yang, Wonseok; Kwon, Tai Hun
      Microelectronic Engineering (2009), 86 (4-6), 1375-1378CODEN: MIENEF; ISSN:0167-9317. (Elsevier B.V.)
      In this paper, we present a simple and low-cost fabrication method of PDMS (polydimethylsiloxane) micro/nano hybrid surfaces for the purpose of increasing hydrophobicity of a solid surface based on the PDMS replica molding using a photolithog. microstructured nanodimpled aluminum (MNA) master. To est. the effect of micro/nano hybrid surfaces on the surface wettability, we have designed three geometry model surfaces consisting of: (1) a nanolens array, (2) a circular micropillar array, and (3) a micro/nano hybrid structure array (the nanolens array on top of the circular micropillar array). The MNA master was fabricated by combining the chem. oxidization of an aluminum substrate and UV-photolithog., thereby having a periodic microporous photoresist pattern on top of the nanodimpled aluminum surface. The micro/nano hybrid PDMS surface shows a higher contact angle compared with those of flat, nanopatterned and micropatterned PDMS surfaces. From the theor. and exptl. results, it was found that the nanolens array having a low aspect ratio of an intrinsically hydrophobic material enhances the hydrophobicity of the solid surface through increasing surface roughness within the Wenzel wetting mode.
    28. 28
      Chen, Z.; Lau, K. K. S. Suppressing Crystallinity by Nanoconfining Polymers Using Initiated Chemical Vapor Deposition. Macromolecules 2019, 52, 5183– 5191,  DOI: 10.1021/acs.macromol.9b00496
      28
      Suppressing Crystallinity by Nanoconfining Polymers Using Initiated Chemical Vapor Deposition
      Chen, Zhengtao; Lau, Kenneth K. S.
      Macromolecules (Washington, DC, United States) (2019), 52 (14), 5183-5191CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
      Poly(1H,1H,2H,2H-perfluorodecyl acrylate) (PPFDA) is incorporated within a sintered mesoporous network of TiO2 nanoparticles via initiated chem. vapor deposition (iCVD). iCVD PPFDA grows uniformly on the pore surface of the mesoporous TiO2 (25 nm pore diam., 2-4 μm thick), resulting in up to 91% of the pore vol. being filled. This leads to a high TiO2 loading (58 vol %, 67 wt %) in the PPFDA-TiO2 nanocomposite and a large decrease in the PPFDA crystallinity by >93% compared to the bulk polymer film. The significantly suppressed crystallinity is attributed to the nanoconfinement of polymer chains in the tortuous, interconnected nanopore channels that frustrates chain alignment, and from the large interfacial energy difference between PPFDA fluoropolymer and hydroxylated TiO2 that impedes heterogeneous nucleation for crystal growth.
    29. 29
      Marchetto, D. Hydrophobic effect of surface patterning on Si surface. Wear 2010, 268, 488– 492,  DOI: 10.1016/j.wear.2009.09.005
      There is no corresponding record for this reference.
    30. 30
      Lafuma, A.; Quéré, D. Superhydrophobic states. Nat. Mater. 2003, 2, 457– 460,  DOI: 10.1038/nmat924
      30
      Superhydrophobic states
      Lafuma, Aurelie; Quere, David
      Nature Materials (2003), 2 (7), 457-460CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
      It is well known that the roughness of a hydrophobic solid enhances its hydrophobicity. The contact angle of water on such flat solids is typically of the order of 100 to 120°, but reaches values as high as 160 to 175° if they are rough or microtextured. This result is remarkable because such behavior cannot be generated by surface chem. alone. Two distinct hypotheses are classically proposed to explain this effect. On one hand, roughness increases the surface area of the solid, which geometrically enhances hydrophobicity (Wenzel model). On the other hand, air can remain trapped below the drop, which also leads to a superhydrophobic behavior, because the drop sits partially on air (Cassie model). However, it is shown here that both situations are very different from their adhesive properties, because Wenzel drops are found to be highly pinned. In addn., irreversible transitions can be induced between Cassie and Wenzel states, with a loss of the anti-adhesive properties generally assocd. with superhydrophobicity.
    31. 31
      Hejazi, V.; Moghadam, A. D.; Rohatgi, P.; Nosonovsky, M. Beyond Wenzel and Cassie–Baxter: Second-Order Effects on the Wetting of Rough Surfaces. Langmuir 2014, 30, 9423– 9429,  DOI: 10.1021/la502143v
      31
      Beyond Wenzel and Cassie-Baxter: Second-order effects on wetting of rough surfaces
      Hejazi, Vahid; Moghadam, Afsaneh Dorri; Rohatgi, Pradeep; Nosonovsky, Michael
      Langmuir (2014), 30 (31), 9423-9429CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      The Wenzel and Cassie-Baxter models are almost exclusively used to explain the contact angle dependence of the structure of rough and patterned solid surfaces. However, these two classical models do not always accurately predict the wetting properties of surfaces since they fail to capture the effect of many interactions occurring during wetting, including, for example, the effect of the disjoining pressure and of crystal microstructure, grains, and defects. We call such effects the second-order effects and present here a model showing how the disjoining pressure isotherm can affect wettability due to the formation of thin liq. films. We measure water contact angles on pairs of metallic surfaces with nominally the same Wenzel roughness obtained by abrasion and by chem. etching. These two methods of surface roughening result in different rough surface structure, thus leading to different values of the contact angle, which cannot be captured by the Wenzel- and Cassie-type models. The chem. and phys. changes that occur on the stainless steel and aluminum alloy surfaces as a result of intergranular corrosion, along with selective intermetallic dissoln., lead to a surface roughness generated on the nano- and microscales.
    32. 32
      Tagliaro, I., Cerpelloni, A., Nikiforidis, V.-M., Pillai, R., Antonini, C. On the Development of Icephobic Surfaces: Bridging Experiments and Simulations. in The Surface Wettability Effect on Phase Change (eds. Marengo, M.; De Coninck, J.) 235– 272(Springer International Publishing, Cham, 2022). doi: DOI: 10.1007/978-3-030-82992-6_8 .
      There is no corresponding record for this reference.
    33. 33
      Wang, Z., Volinsky, A. A.; Gallant, N. D. Crosslinking effect on polydimethylsiloxane elastic modulus measured by custom-built compression instrument. J. Appl. Polym. Sci. (2014) 131,.
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      Meng, Z.; Zhang, P. Freezing dynamics of supercooled micro-sized water droplets. Int. J. Heat Mass Transfer 2022, 193, 122955  DOI: 10.1016/j.ijheatmasstransfer.2022.122955
      There is no corresponding record for this reference.
    35. 35
      Gleason, K. K. Fluoropolymers by initiated chemical vapor deposition (iCVD). in Opportunities for Fluoropolymers 113– 135 (Elsevier, 2020). doi: DOI: 10.1016/b978-0-12-821966-9.00005-5 .
      There is no corresponding record for this reference.
    36. 36
      Kissinger, G.; Kissinger, W. Hydrophilicity of Silicon Wafers for Direct Bonding. Phys. Status Solidi A 1991, 123, 185– 192,  DOI: 10.1002/pssa.2211230117
      There is no corresponding record for this reference.

  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.langmuir.4c00743.

    • FTIR analysis of the coating, the Cassie–Baxter theory adapted to wrinkles, surface characterization and energy calculations, and icing experiments (PDF)


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