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Buckling versus Crystal Expulsion Controlled by Deformation Rate of Particle-Coated Air Bubbles in Oil
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Buckling versus Crystal Expulsion Controlled by Deformation Rate of Particle-Coated Air Bubbles in Oil
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  • Saikat Saha
    Saikat Saha
    Department of Chemical Engineering, Delft University of Technology, 2629 HZ Delft, The Netherlands
    Department of Chemical Engineering, Imperial College London, London SW7 2AZ, United Kingdom
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  • Francis Pagaud
    Francis Pagaud
    Department of Chemical Engineering, Imperial College London, London SW7 2AZ, United Kingdom
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  • Bernard P. Binks
    Bernard P. Binks
    Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom
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    Orcidhttps://orcid.org/0000-0003-3639-8041
  • Valeria Garbin*
    Valeria Garbin
    Department of Chemical Engineering, Delft University of Technology, 2629 HZ Delft, The Netherlands
    Department of Chemical Engineering, Imperial College London, London SW7 2AZ, United Kingdom
    *Email: [email protected]
    More by Valeria Garbin
    Orcidhttps://orcid.org/0000-0002-0887-500X
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Langmuir

Cite this: Langmuir 2022, 38, 3, 1259–1265
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https://doi.org/10.1021/acs.langmuir.1c03171
Published January 13, 2022

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Abstract

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Oil foams stabilized by crystallizing agents exhibit outstanding stability and show promise for applications in consumer products. The stability and mechanics imparted by the interfacial layer of crystals underpin product shelf life, as well as optimal processing conditions and performance in applications. Shelf life is affected by the stability against bubble dissolution over a long time scale, which leads to slow compression of the interfacial layer. In processing flow conditions, the imposed deformation is characterized by much shorter time scales. In practical situations, the crystal layer is therefore subjected to deformation on extremely different time scales. Despite its importance, our understanding of the behavior of such interfacial layers at different time scales remains limited. To address this gap, here we investigate the dynamics of single, crystal-coated bubbles isolated from an oleofoam, at two extreme time scales: the diffusion-limited time scale characteristic of bubble dissolution, ∼104 s, and a fast time scale characteristic of processing flow conditions, ∼10–3 s. In our experiments, slow deformation is obtained by bubble dissolution, and fast deformation in controlled conditions with real-time imaging is obtained using ultrasound-induced bubble oscillations. The experiments reveal that the fate of the interfacial layer is dramatically affected by the dynamics of deformation: after complete bubble dissolution, a continuous solid layer remains; after fast, oscillatory deformation of the layer, small crystals are expelled from the layer. This observation shows promise toward developing stimuli-responsive systems, with sensitivity to deformation rate, in addition to the already known thermoresponsiveness and photoresponsiveness of oleofoams.

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Introduction

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Oil foams have diverse use in pharmaceutical, food, and cosmetic applications. (1,2) While air–oil interfaces are difficult to stabilize with molecular surfactants, (3) various crystallizing agents have been found to confer long-term stability to oil foams (2−8) and novel air-in-oil-in-water systems. (9) Crystals can nucleate and grow both in the bulk oil phase and directly on the oil–air interface, upon decreasing the temperature of a preheated solution. (10) Crystals formed in the bulk can then adsorb to the air–oil interfaces during mixing, while excess crystals can form a network in the oil phase (i.e., an oleogel). (5) In this case, foam stability is due to both the bulk and interfacial networks of crystals. (11,12) Much progress has been made in understanding the formation and stability of oil foams stabilized by crystallizing agents, yet our knowledge of the mechanical properties and dynamic behaviors of such systems remains limited, despite its importance for product shelf life, as well as optimal processing conditions and performance in applications. In particular, while there is significant literature available on the rheology of oleogels formed by the crystal network in the bulk oil phase, (1,2) and on the link between crystal formation and network properties, (13,14) less is understood about the microstructure and properties of the crystal-stabilized interfaces. (15)
The stability, dynamics, and mechanics of particle-stabilized (or “armored”) bubbles (16) have been studied extensively for model systems stabilized by micrometer-scale colloids; these studies provided important fundamental insights thanks to the ability to directly visualize the interface microstructure and its evolution. At high surface coverage, particle jamming at the interface supports anisotropic stresses, which oppose the shape-minimizing effects of surface tension and lead to stable, non-spherical bubbles. (16) If the particles have attractive interactions, the sufficient condition to arrest bubble dissolution depends on the surface viscoelasticity (17) rather than surface coverage. Such attractive interactions can be tuned by modifying the particle shape, size, roughness, or surface chemistry. (17) In addition to surface coverage and interparticle interactions, the morphology and deformation of the interfacial monolayer can also depend on the particle-to-bubble size ratio. (18−21) Upon forced dissolution of particle-stabilized bubbles, in both aqueous (22) and nonaqueous (23) media, either expulsion of particles or detachment of a continuous layer from the interface was observed, while for ultrafast deformation of colloid-coated bubbles in water forced by ultrasound, buckling of the monolayer and particle expulsion were observed. (24)
In this work, we explore the dynamic response to deformation of wax crystal stabilized bubbles isolated from the bulk crystal network of an oleofoam. We exploit bubble dissolution to apply slow interfacial compression on a diffusion-limited time scale, 103–104 s, important for product stability and shelf life. (25) Using ultrasound-driven bubble oscillations, we apply compression/expansion on a very fast time scale, 10–4 s, relevant in production processes (26) and performance in applications. The experiments reveal a striking difference in the fate of the interfacial wax crystal layer at these two extreme deformation time scales. These insights are also important for optimizing strategies for the design, synthesis, and application of stimuli-responsive systems stabilized by phase-changing materials. (2,27)

Materials and Methods

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Preparation of Wax Crystal-Coated Bubbles

The wax used (Hydropel QB, Shamrock Technologies) is a blend of paraffin and synthetic waxes with a melting temperature range Tm = 50–105 °C, according to the manufacturer’s specifications. The wax particles were used as received. Consumer-grade sunflower oil (Tesco) was used as received. The density was measured to be ρoil = 0.8879 g cm–3 and the viscosity ηoil = 50 mPa s at T = 25 °C. The protocol for oleofoam preparation was provided previously. (11) Briefly, a vial containing 2.5 wt/vol % wax particles in sunflower oil was agitated at 3000 rpm for 2 min to form a well-mixed suspension. The suspension was then heated to T = 90 °C to melt the wax and immediately agitated for 2 min to incorporate air to form the oleofoam. During turbulent mixing, the sample cooled to T ≈ 45 °C. The vial was then left undisturbed to cool to room temperature. In this process, the excess wax crystals that are not incorporated on the interfaces of bubbles form a gel network in the oil phase (oleogel) which contributes to the stability of such systems. (11)
To prepare samples with wax-coated bubbles without the bulk gel network, the prepared oleofoam was diluted by placing a small sample (≈20 μL) on a microscope glass slide with a spacer of thickness 1.5 mm and adding sunflower oil to obtain isolated bubbles as schematically shown in Figure 1a. High-magnification optical micrographs suggest that the primary crystal size at the oil/air interface is in the range 1–10 μm (see the Supporting Information).

Figure 1

Figure 1. (a) Schematic of an air bubble stabilized by wax crystals in oil. (b) Wax-coated bubble viewed using bright-field microscopy. (c) The same bubble viewed under crossed polarizers. (d) Schematic of the experimental setup.

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The wax-coated bubbles, when extracted from the oleofoam and re-suspended in sunflower oil, still have their interfacial layers present, as confirmed by their textured interfaces and buckled shapes, visible in Figure 1b, recorded in bright field with an inverted microscope (IX71, Olympus) and digital camera (DCC1645C, Thorlabs). Using crossed polarizers, the layer of crystals is more clearly visible, as shown in Figure 1c. Wherever these crystals can be seen, the interface exhibits non-uniform curvature. Conversely, in regions devoid of crystals, the curvature is uniform. This is characteristic of a cohesive interfacial layer, which does not expand to fill the entire available area.

Bubble Dissolution Experiments

We exploit the phenomenon of bubble dissolution to access long time scales of interface deformation. In the absence of an interfacial layer, the characteristic time scale for bubble dissolution can be estimated (28) by integrating the equation governing the rate of change of radius from the Epstein and Plesset theory (29) to give
(1)
This long time scale is achieved by selecting bubbles with an initial radius R0 ≈ 100 μm. For such small bubbles, the Laplace pressure of the gas–liquid interface, 2σ/R0 with σ ≈ 30 mN/m for air–oil interfaces, (25) accelerates dissolution; this effect is described by the first term between parentheses in eq 1. The physicochemical properties affecting td are the gas diffusivity in the oil, D ∼ 10–10 m2/s for vegetable oils, (25) and the solubility of the gas in the oil, measured by Henry’s constant kH, which for N2 in nonpolar solvents (23) is on the order of kH ∼ 10–5 mol Pa–1 m–3. The other parameters are the gas density and molar mass, which for nitrogen are ρg = 1.2 kg/m3 and Mw = 28 × 10–3 kg/mol, respectively; the temperature T = 298 K; and the universal gas constant Rg = 8.31 J mol–1 K–1.
For wax-coated bubbles, after they are extracted from the bulk oleogel, the interfacial layer alone is not able to arrest bubble dissolution. (11) The stability imparted by the interfacial layer depends on the initial surface coverage and microstructure, and a large variability in dissolution time td of these crystal-coated bubbles is observed. Typically, dissolution occurs over a time scale in the same range as for uncoated bubbles, td ∼ 103–104 s.

Oscillation of Bubbles Using Ultrasound

To access short time scales of interface deformation, we exploit ultrasound-driven bubble oscillations. The periodic oscillations of the pressure around the ambient value, p0, caused by an acoustic wave, force the gas core of a bubble to undergo periodic compression and expansion at the same frequency. (30) The driving frequency f was selected close to the natural frequency f0 of a bubble of radius R0 in an unbounded liquid. The relation between these quantities is given by the Minnaert frequency, (30) modified to include surface tension effects:
(2)
where κ is the polytropic exponent of the gas, taken to be 1.4 (assuming adiabatic behavior); σ is the surface tension; and ρl is the liquid density. For bubbles of radii R0 ≈ 100 μm, the driving frequencies were selected in the range f0 ∼ 10 kHz, leading to oscillatory deformation with a period
(3)
To transmit ultrasound waves into the sample and observe the dynamics of wax-coated bubbles, we used an ultrasound transducer (bolt-clamped Langevin transducer, Steminc; resonance frequency 25 kHz) glued onto a glass plate and driven by an arbitrary waveform generator (Agilent 33220A, Agilent Technologies, Inc.) connected to a linear power amplifier (AG 1021, T&C Power Conversion), as shown in Figure 1d. A sine wave of prescribed frequency f, acoustic pressure amplitude Δp, and number of cycles N was sent to the transducer by the waveform generator and amplifier. The ultrasound-driven bubble dynamics are recorded with a high-speed camera (FASTCAM SA5, Photron) connected to the inverted microscope. The frame rate used for the high-speed camera was at least 10 times the driving frequency. The amplitude of bubble oscillations, which is controlled both by the forcing frequency f and by the acoustic pressure amplitude Δp, is quantified from image analysis as x0 = ΔR/R0, with ΔR the maximum amplitude of bubble oscillations and R0 the equilibrium bubble radius.

Results and Discussion

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Experimental Observations

The dynamics of dissolution of a bubble in oil, stabilized by an interfacial layer of wax crystals, is shown in Figure 2 (Supporting Information (SI) Video 1). The schematic shows qualitatively the time evolution of the radius R(t), and the different stages of the process are shown in images i–vii, which were taken with crossed polarizers to highlight the layer of crystals (seen as bright in the images). Between t = 4430 s and t = 5360 s (Figure 2iii,iv), a sharp deformation of the interface is observed on the right side of the bubble, from where a cohesive layer of crystals detaches as the bubble dissolves. Between t = 5960 s and t = 6400 s (Figure 2v,vi), there is again a sharp distortion to the left side of the bubble, followed by detachment of the layer from that location. Thus, detachment appears to initiate from the points of maximum deformation. At t = 7090 s, after the bubble has completely dissolved (Figure 2vii), a cohesive layer of crystals, seen as a bright spiral-like structure in the image, is left behind. The layer of crystals was observed to remain intact over months. The layer appears to be composed of large crystal rafts with characteristic dimension l comparable to the initial bubble radius (R0 ≈ 100 μm).

Figure 2

Figure 2. Dissolution of wax crystal-coated bubble in oil (SI Video 1). Schematic of bubble dissolution, occurring over t ∼ 103–104 s, with points qualitatively marking the dissolution steps observed in the following panels (i–vii). The image sequence is taken using crossed polarizers.

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Figure 3 shows the effects of ultrasound-driven bubble oscillations of small and large amplitudes on the same wax-coated bubble. The frequency of oscillations is f = 25 kHz, and the bubble is successively excited by pulses of N cycles (two pulses of different amplitudes are shown in the schematic of Figure 3 by way of example). At moderate acoustic pressure, the amplitude of bubble oscillations is limited to x0 ≈ 3%, too small to be clearly visible in Figure 3a, and more clearly visible in the corresponding SI Video 2, which shows a pulse of N = 50 cycles. The interface buckles during compression (Figure 3a-iii) and, over a total of N = 260 cycles, undergoes a clear change from the initial non-spherical shape (Figure 3a-i) to almost spherical shape (Figure 3a-iv). For the same bubble at higher acoustic pressures, the amplitude of oscillations becomes x0 > 20%, which is clearly visible in the high-speed recording with crossed polarizers (SI Video 3) as well as in the corresponding image sequence in Figure 3b. The morphology of the crystal layer after large-amplitude oscillations at high frequency (N = 100 cycles) is profoundly different from the case of slow compression during bubble dissolution: crystals are expelled from the layer, which are much smaller than the initial bubble radius, l ≪ R0, seen in Figure 3b-viii. In contrast, the size of the segments forming the cohesive layer after bubble dissolution is l ≈ R0. Lastly, we examined the effect of ultrasound-driven oscillations on subsequent dissolution of wax-coated bubbles. Following a period of ultrasound-driven oscillations, the bubbles were left to dissolve completely, as shown in the schematic in Figure 4. Figure 4i shows a wax-coated bubble of initial radius R0 ≈ 102 μm before any deformation is applied. The bubble was first driven into large-amplitude oscillations (x0 > 20%) with ultrasound frequency f = 25 kHz, and for N = 480 cycles. After this number of cycles of oscillations, expulsion of a limited amount of crystals from the interface was obtained, without complete disruption of the interfacial layer, as seen in Figure 4ii. Forcing with ultrasound was stopped at this point, and the same bubble was then left to dissolve. An image sequence of the bubble dissolution steps is shown in Figure 4ii–vii (see also SI Video 3). The crystals and fragments of interfacial layer left behind after the bubble has completely dissolved (t = 1680 s; see Figure 4vii) appear larger compared to those that are expelled by ultrasound-driven oscillations; see Figure 3viii for comparison.

Figure 3

Figure 3. Crystal expulsion after ultrasound-driven bubble oscillations. The schematic illustrates the protocol of oscillating a crystal-coated bubble with ultrasound pulses of different amplitudes. Images at selected times, labeled (i)–(viii), are shown below. (a) High-speed image sequence of a wax-coated bubble with initial radius R0 = 101 μm undergoing oscillations of moderate amplitude, with x0 = ΔR/R0 ≈ 3%, N = 50 cycles (SI Video 2). The frames show (i) the initial non-spherical shape of the bubble before oscillations, the (ii) maximum and (iii) minimum during oscillations at f = 25 kHz, and (iv) the final, almost spherical shape after a total of N = 260 cycles of oscillations of small amplitude. (b) High-speed image sequence with crossed polarizers, for large-amplitude oscillations, with x0 ≈ 20% (SI Video 3): (v) initial state before oscillations, (vi) maximum and (vii) minimum during oscillations at f = 25 kHz, and (viii) final state after N = 100 cycles of oscillations, showing expulsion of crystals around the bubble.

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Figure 4

Figure 4. Bubble dissolution after ultrasound-driven bubble oscillations. The schematic shows the protocol including large-amplitude radial oscillations, followed by unperturbed dissolution. The labels correspond to the image sequence shown in panels i–vii. The initial radius of the bubble before oscillations (i) is R0 = 102 μm. Oscillations are driven by ultrasound at f = 25 kHz for N = 480 cycles, with amplitude of oscillations x0 > 20% (shown in SI Video 4). (ii) t = 0 s represents the start time of recording as the bubble dissolves (ii–vii).

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Mechanism of Layer Detachment for Slow Compression

Observations of the dissolution of wax-coated bubbles (sample size n ≈ 10) indicate that, as the bubble decreases in size, the interfacial layer gradually detaches from the air–oil interface without significant changes in its morphology. In some instances (n = 3), the bubble remains fully covered with an interfacial layer during the entire process, until its complete dissolution (see Figure 2). These observations suggest that the energy penalty of deformation of the interfacial layer, measured by the compression modulus, exceeds the adhesive interaction energy between the wax layer and the air–oil interface. The compression modulus of the wax crystal layer on an air/oil interface was previously estimated (11) to be Kd ≈ 40–60 mJ/m2. As the interfacial layer detaches from the bubble interface, the inner surface of the (solid) wax crystal layer, initially in contact with air, becomes wetted by oil and a region of bare oil/air interface is formed. In this process, the change in interfacial energy per unit area can therefore be estimated to be in the range γos + γoa – γas ≈ 1–10 mJ/m2, with γoa ≈ 30 mJ/m2 the surface tension of the oil/air interface, γos ≈ 1–5 mJ/m2 the interfacial energy of the solid/oil interface, and γas ≈ 20–35 mJ/m2 the surface energy of the solid/air interface. It is therefore energetically favorable for the layer to detach from the gas interface during bubble dissolution, since the change in interfacial energy is small compared to the compression modulus. Comparing with the literature for clay-stabilized bubbles in alkanes (23) and latex-stabilized bubbles in water, (22) we identify two features common to all systems for when a continuous layer is left behind: the interface deformation is slow, occurring over a time scale td ∼ 103–104 s, and the size of the particles is much smaller than the bubble radius, leading to a continuum behavior. (21)

Mechanisms of Crystal Expulsion for Fast Deformation

During ultrasound-driven bubble oscillations, the wax crystal layer undergoes rapid compression and expansion, as well as out-of-plane deformation due to buckling, shown in Figure 3a-iii, which can break the initially cohesive layer. This hypothesis is supported by the observation in Figure 3a that, for small-amplitude oscillations, the shape of the bubble changes without expulsion of interfacial material. The bubble becomes less non-spherical, consistent with an interfacial microstructure that cannot support anisotropic stresses. Because compression occurs on a very short time scale, τ0 ∼ 10–4 s, compared to rearrangement time scales in densely packed colloid monolayers, there is not sufficient time for rearrangement of the crystals to form a cohesive network. When a bubble that has been pre-treated with ultrasound-driven oscillations is allowed to dissolve (Figure 4), the interface undergoes slow compression, allowing sufficient time for the crystals to rearrange and form again a cohesive, solid-like network that can support anisotropic stresses as evidenced by the non-spherical shape of the bubble (Figure 4iii–v).
For bubble oscillations of large amplitude, several concurrent mechanisms can contribute to the expulsion of crystals, similar to observations for colloid-coated bubbles in water, (24) including significant out-of-plane buckling of the layer (SI Video 5). In the case of large-amplitude oscillations, when the same bubble is allowed to dissolve, the fragments of interfacial layer left behind are larger than those produced by ultrasound oscillations alone. This suggests that the large-amplitude oscillations can break the original interfacial structure but, following this, as the interface is compressed slowly there is sufficient time for the remaining crystals to rearrange and form larger cohesive rafts.
Finally, a secondary, slow flow called “microstreaming” and caused by bubble oscillations in confined geometries (31) cannot be ruled out in our experiments, but is not expected to disrupt the interfacial layer. We do not see direct evidence of microstreaming flow, but its effect would be to re-distribute expelled crystals along characteristic, closed streamlines around the bubble.

Conclusions

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We investigated the response to deformation of an interfacial layer formed by a crystallizing agent on the surface of bubbles in an oil phase, at two extreme time scales that are, on one hand, relevant to the shelf life of such systems and, on the other hand, relevant to their processing and application. We found that the morphology of the interfacial layer is modified, depending on the interplay between the time scales of deformation and crystal rearrangement. For slow deformation, the crystals form a cohesive network, whereas for fast deformation the layer is fragmented. Conversely, the fate of the monolayer can be controlled by carefully combining these deformation protocols on the same layer. Also, depending on the requirement, the bubble shapes can be altered. These effects can find utility in modifying the morphology, rheology, and texture of foams in cosmetic and food applications. Furthermore, by tuning the timing and duration of ultrasound-induced deformation, protocols can be devised for the use of crystal-coated microbubbles for controlled release, useful for delivery of active pharmaceutical ingredients. Our findings pave the way for the design of stimuli-responsive systems that can be triggered to behave uniquely depending on the time scale of deformation, in addition to the already-known temperature dependence of the phase-changing stabilizing agent.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.langmuir.1c03171.

  • High-magnification optical micrographs and characterization of crystal shape and size in bulk oleogel network and at oil/air interface (PDF)

  • Dissolution of a wax-coated bubble in oil, imaged with crossed polarizers (MP4)

  • Small-amplitude oscillations (driven by ultrasound at 25 kHz) of a wax-coated bubble in oil (MP4)

  • Large-amplitude oscillations (driven by ultrasound at 25 kHz) of a wax-coated bubble in oil, imaged with crossed polarizers (AVI)

  • Dissolution of a wax-coated bubble after pre-treatment with large-amplitude ultrasound-driven oscillations, imaged with crossed polarizers (MP4)

  • Large-amplitude oscillations (driven by ultrasound at 25 kHz) of a wax-coated bubble showing buckling (AVI)

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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 Author
    • Valeria Garbin - Department of Chemical Engineering, Delft University of Technology, 2629 HZ Delft, The Netherlands;  Department of Chemical Engineering, Imperial College London, London SW7 2AZ, United Kingdom;  Orcidhttps://orcid.org/0000-0002-0887-500X;  Email: [email protected]
  • Authors
    • Saikat Saha - Department of Chemical Engineering, Delft University of Technology, 2629 HZ Delft, The Netherlands;  Department of Chemical Engineering, Imperial College London, London SW7 2AZ, United Kingdom
    • Francis Pagaud - Department of Chemical Engineering, Imperial College London, London SW7 2AZ, United Kingdom;  Present Address: Université de Lyon, École Normale Supérieure de Lyon, CNRS, Laboratoire de Physique, F-69342 Lyon, France
    • Bernard P. Binks - Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom;  Orcidhttps://orcid.org/0000-0003-3639-8041
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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The authors thank P. Luckham, J. Seddon, B. Dollet, and E. Wagner for helpful discussions. This work is supported by European Research Council Starting Grant No. 639221 (V.G.).

References

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    Mishima, Shoko; Suzuki, Atsushi; Sato, Kiyotaka; Ueno, Satoru
    Journal of the American Oil Chemists' Society (2016), 93 (11), 1453-1466CODEN: JAOCA7; ISSN:0003-021X. (Springer)
    This paper reports the exptl. results of processes used for the formation of whipped oils composed of vegetable oils (salad oil) and high-melting fat crystals [fully hydrogenated rapeseed oil rich in behenic acid (FHR-B)]. No emulsifier was added to form this whipped oil. Microprobe FT-IR spectroscopy, synchrotron radiation microbeam X-ray diffraction (SR-μ-XRD), polarized optical microscopy, and differential scanning calorimetry (DSC) were employed to observe fine fat crystal particles of the most stable polymorph of β (β-fat crystal), FHR-B, and their adsorption at the air-oil surface before, during, and after the formation of the whipped oil. The results obtained revealed the following: (1) The prepn. of an organogel composed of salad oil and small fibrous β-fat crystals using a special tempering procedure was a prerequisite for forming whipped oil. (2) The β-fat crystals were adsorbed at the air-oil surface to encapsulate the air bubbles during the formation process of whipped oil. (3) The values of overrun of the whipped oil reached >200 % after an aeration time of 30 min at 20 °C. (4) The SR-μ-XRD expts. demonstrated that the lamellar planes of the β-fat crystals near the air-oil surface were arranged almost parallel to the air-oil surface plane. The present study provides the first evidence that tiny fat crystal particles may cause aeration in liq. oils without the addn. of other whip-assisting substances such as emulsifier crystals.
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  5. 5
    Gunes, D. Z.; Murith, M.; Godefroid, J.; Pelloux, C.; Deyber, H.; Schafer, O.; Breton, O. Oleofoams: Properties of crystal-coated bubbles from whipped oleogels–Evidence for Pickering stabilization. Langmuir 2017, 33, 1563– 1575,  DOI: 10.1021/acs.langmuir.6b04141
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    5
    Oleofoams: Properties of Crystal-Coated Bubbles from Whipped Oleogels-Evidence for Pickering Stabilization
    Gunes, D. Z.; Murith, M.; Godefroid, J.; Pelloux, C.; Deyber, H.; Schafer, O.; Breton, O.
    Langmuir (2017), 33 (6), 1563-1575CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    The authors report evidence that lipidic crystals made of a high fraction of fully sol. monoglyceride (MG) in oil do not only adsorb at the oil-air interface but also can easily form a jammed, closely packed layer of crystals around the bubbles of a foam produced by whipping (Pickering effect). Very fine bubbles, soft textures, or firmer ones such as for shaving foams could be obtained, with a high air fraction (up to 75%), which is unprecedented. A thin, jammed layer of crystals on bubbles can cause bubbles to retain nonspherical shapes in the absence of bulk effects for times much longer than the characteristic capillary relaxation time for bare bubbles, which is actual evidence for Pickering-type interfacial stabilization. By comparing to foams obtained by depressurization, we show that whipping is necessary for bubble wrapping with a layer of crystals. The origin of high stability against Ostwald ripening at long times is also discussed. These Pickering whipped foams have rheol. properties dominated by interfacial or film contributions, which is of high interest for food and cosmetics applications because of their high moduli. This system can be considered to be a model of the crystn. behavior of MG in oil, which is similar to that in many fats.
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  6. 6
    Heymans, R.; Tavernier, I.; Danthine, S.; Rimaux, T.; Van der Meeren, P.; Dewettinck, K. Food-grade monoglyceride oil foams: the effect of tempering on foamability, foam stability and rheological properties. Food Funct. 2018, 9, 3143– 3154,  DOI: 10.1039/C8FO00536B
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    6
    Food-grade monoglyceride oil foams: the effect of tempering on foamability, foam stability and rheological properties
    Heymans, Robbe; Tavernier, Iris; Danthine, Sabine; Rimaux, Tom; Van der Meeren, Paul; Dewettinck, Koen
    Food & Function (2018), 9 (6), 3143-3154CODEN: FFOUAI; ISSN:2042-6496. (Royal Society of Chemistry)
    Foams with a continuous oil phase may be stabilized using cryst. particles. Those systems are compelling because of their potential in edible oil structuring, modifying sensorial properties and creating healthier food products. This study aimed to relate oleogel (unwhipped state) properties to oil foam (whipped state) properties using a monoglyceride-sunflower oil model system. The properties of crystal-oil mixts. were influenced by time and temp. during prepn. and storage. Therefore, oleogels were prepd. using different tempering protocols and their resulting microstructure was investigated with rheol., differential scanning calorimetry and X-ray diffraction. The corresponding oil foams were characterized in terms of foamability and foam stability. The properties of both systems were studied immediately after prepn. as well as after 4 wk of storage. We demonstrated that there is a large influence of the time-temp. history on the foam properties. Partially crystd. mixts. were shown to form weaker structures which capture more air because of their lower viscosity and as crystn. would preferentially take place at the interface. They were characterized by larger bubbles and were less stable and firm. It is proposed that their rheol. properties are mainly dominated by interfacial contributions. Fully crystd. and stored monoglyceride-oil mixts. were seen to form stronger gel networks which included less air, contained smaller air bubbles and were stable during storage. It is hypothesized that these samples also included an important bulk gelation contribution.
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  7. 7
    Liu, Y.; Binks, B. P. A novel strategy to fabricate stable oil foams with sucrose ester surfactant. J. Colloid Interface Sci. 2021, 594, 204– 216,  DOI: 10.1016/j.jcis.2021.03.021
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    7
    A novel strategy to fabricate stable oil foams with sucrose ester surfactant
    Liu, Yu; Binks, Bernard P.
    Journal of Colloid and Interface Science (2021), 594 (), 204-216CODEN: JCISA5; ISSN:0021-9797. (Elsevier B.V.)
    Can a mixt. of sucrose ester surfactant in vegetable oil be aerated to yield stable oleofoams. Is foaming achievable from one-phase mol. solns. and/or two-phase crystal dispersions. Does cooling a foam after formation induce surfactant crystn. and enhance foam stability. Concg. on extra virgin olive oil, we first study the effect of aeration temp. and surfactant concn. on foamability and foam stability of mixts. cooled from a one-phase oil soln. Based on this, we introduce a strategy to increase foam stability by rapidly cooling foam prepd. at high temp. which induces surfactant crystn. in situ. Differential scanning calorimetry, X-ray diffraction, infra-red spectroscopy, surface tension and rheol. are used to elucidate the mechanisms. Unlike previous reports, both foamability and foam stability decrease upon decreasing the aeration temp. into the two-phase region contg. surfactant crystals. At high temp. in the one-phase region, substantial foaming is achieved (over-run 170%) within minutes of whipping but foams ultimately collapse within a week. We show that surfactant mols. are surface-active at high temp. and that hydrogen bonds form between surfactant and oil mols. The generic nature of our findings is demonstrated for a range of vegetable oil foams with a max. over-run of 330% and the absence of drainage, coalescence and disproportionation being achievable.
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  8. 8
    Binks, B. P.; Vishal, B. Particle-stabilized oil foams. Adv. Colloid Interface Sci. 2021, 291, 102404,  DOI: 10.1016/j.cis.2021.102404
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    8
    Particle-stabilized oil foams
    Binks, Bernard P.; Vishal, Badri
    Advances in Colloid and Interface Science (2021), 291 (), 102404CODEN: ACISB9; ISSN:0001-8686. (Elsevier B.V.)
    A review. The area of oil foams although important industrially has received little academic attention until the last decade. The early work using mol. surfactants for stabilization was limited and as such it is difficult to obtain general rules of thumb. Recently however, interest has grown in the area partly fuelled by the understanding gained in the general area of colloidal particles at fluid interfaces. We review the use of solid particles as foaming agents for oil foams in cases where particles (inorg. or polymer) are prepd. ex situ and in cases where crystals of surfactant or fat are prepd. in situ. There is considerable activity in the latter area which is particularly relevant to the food industry. Discussion of crude oil/lubricating oil foams is excluded from this review.
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  9. 9
    Goibier, L.; Pillement, C.; Monteil, J.; Faure, C.; Leal-Calderon, F. Emulsification of non-aqueous foams stabilized by fat crystals: Towards novel air-in-oil-in-water food colloids. Food Chem. 2019, 293, 49– 56,  DOI: 10.1016/j.foodchem.2019.04.080
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    9
    Emulsification of non-aqueous foams stabilized by fat crystals: Towards novel air-in-oil-in-water food colloids
    Goibier, Lucie; Pillement, Christophe; Monteil, Julien; Faure, Chrystel; Leal-Calderon, Fernando
    Food Chemistry (2019), 293 (), 49-56CODEN: FOCHDJ; ISSN:0308-8146. (Elsevier Ltd.)
    We designed Air-in-Oil-in-Water (A/O/W) emulsions. First, Air-in-Oil foams were fabricated by whipping anhyd. milk fat. The max. overrun was obtained at 20 °C. The foams contained 30-35 vol% air and were stabilized solely by fat crystals. To refine the bubble size, foams were further sheared in a Couette's cell. The av. bubble size reached a value as small as 6.5 μm at a shear rate of 5250 s-1. The nonaq. foams were then dispersed in a viscous aq. phase contg. sodium caseinate to obtain A/O/W emulsions. The shear rate was varied from 1000 to 7500 s-1, allowing to obtain Air-in-Oil globules whose av. diam. ranged from 15 to 60 μm. To avoid globule creaming, the aq. phase was gelled by incorporating hydroxyethyl cellulose. Homogeneous emulsions were obtained with fat globules contg. around 22 vol% of residual air. The systems were kinetically stable for at least 3 wk at 4 °C.
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  10. 10
    Saha, S. Micromechanics of Particle-Coated Bubbles: Deformation from Quasistatic to Millisecond Timescales. Ph.D. Thesis, Imperial College London, 2020.
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    There is no corresponding record for this reference.
  11. 11
    Saha, S.; Saint-Michel, B.; Leynes, V.; Binks, B. P.; Garbin, V. Stability of bubbles in wax-based oleofoams: decoupling the effects of bulk Oleogel rheology and interfacial rheology. Rheol. Acta 2020, 59, 255– 266,  DOI: 10.1007/s00397-020-01192-x
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    11
    Stability of bubbles in wax-based oleofoams: decoupling the effects of bulk oleogel rheology and interfacial rheology
    Saha, S.; Saint-Michel, B.; Leynes, V.; Binks, B. P.; Garbin, V.
    Rheologica Acta (2020), 59 (4), 255-266CODEN: RHEAAK; ISSN:0035-4511. (Springer)
    Oleofoams are dispersions of gas bubbles in a continuous oil phase and can be stabilized by crystals of fatty acids or waxes adsorbing at the oil-air interface. Because excess crystals in the continuous phase form an oleogel, an effect of the bulk rheol. of the continuous phase is also expected. Here, we evaluate the contributions of bulk and interfacial rheol. below and above the m.p. of a wax forming an oleogel in sunflower oil. We study the dissoln. behavior of single bubbles using microscopy on a temp.-controlled stage. We compare the behavior of a bubble embedded in an oleofoam, which owes its stability to both bulk and interfacial rheol., to that of a bubble extd. from the oleofoam and resuspended in oil, for which the interfacial dilatational rheol. alone provides stability. We find that below the m.p. of the wax, bubbles in the oleofoam are stable whereas bubbles that are only coated with wax crystals dissolve. Both systems dissolve when heated above the m.p. of the wax. These findings are rationalized through independent bulk rheol. measurements of the oleogel at different temps., as well as measurements of the dilatational rheol. properties of a wax-coated oil-air interface.
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  12. 12
    Liascukiene, I.; Amselem, G.; Landoulsi, J.; Gunes, D. Z.; Baroud, C. N. Intermittent dynamics of bubble dissolution due to interfacial growth of fat crystals. Soft Matter 2021, 17, 10042– 10052,  DOI: 10.1039/D1SM00902H
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    12
    Intermittent dynamics of bubble dissolution due to interfacial growth of fat crystals
    Liascukiene, Irma; Amselem, Gabriel; Landoulsi, Jessem; Gunes, Deniz Z.; Baroud, Charles N.
    Soft Matter (2021), 17 (44), 10042-10052CODEN: SMOABF; ISSN:1744-6848. (Royal Society of Chemistry)
    Foams are inherently unstable objects, that age and disappear over time. The main cause of foam aging is Ostwald ripening: smaller air bubbles within the foam empty their gas content into larger ones. One strategy to counter Ostwald ripening consists in creating armored bubbles, where solid particles adsorbed at the air/liq. interface prevent bubbles from shrinking below a given size. Here, we study the efficiency of coating air bubbles with fat crystals to prevent bubble dissoln. A monoglyceride, monostearin, is directly crystd. at the air/oil interface. Expts. on single bubbles in a microfluidic device show that the presence of monostearin fat crystals slows down dissoln., with an efficiency that depends on the crystal size. Bubble ripening in the presence of crystals exhibits intermittent dissoln. dynamics, with phases of arrest, when crystals jam at the interface, followed by phases of dissoln., when monostearin crystals are ejected from the interface. In the end, crystals do not confer enough mech. strength to the bubbles to prevent them from fully dissolving.
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  13. 13
    Tavernier, I.; Moens, K.; Heyman, B.; Danthine, S.; Dewettinck, K. Relating crystallization behavior of monoacylglycerols-diacylglycerol mixtures to the strength of their crystalline network in oil. Food Res. Int. 2019, 120, 504– 513,  DOI: 10.1016/j.foodres.2018.10.092
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    13
    Relating crystallization behavior of monoacylglycerols-diacylglycerol mixtures to the strength of their crystalline network in oil
    Tavernier, Iris; Moens, Kim; Heyman, Bart; Danthine, Sabine; Dewettinck, Koen
    Food Research International (2019), 120 (), 504-513CODEN: FORIEU; ISSN:0963-9969. (Elsevier B.V.)
    Diacylglycerols (DAGs) are interesting oil structuring mols. as they are structurally similar to triacylglycerols (TAGs), but are metabolized differently which results in wt. loss and improved blood cholesterol levels upon dietary replacement of TAGs with DAGs. Many com. products consist of a mixt. of monoacylglycerols (MAGs) and DAGs, yet the effect of MAGs on the crystn. behavior of DAGs is still to be unraveled. Two types of com. MAGs, one originating from hydrogenated palm stearin and one of hydrogenated rapeseed oil, were added in concns. 1, 2 and 4% to 20% DAGs derived from hydrogenated soybean oil. Using differential scanning calorimetry, it was shown that the presence of MAGs delayed the onset of DAG crystn. Rheol. anal. revealed that MAGs also hindered crystal network development. Synchrotron X-ray diffraction anal. demonstrated that the addn. of MAGs suppressed the formation of the β form and stimulated the development of the β' form. Likely, MAGs mainly hindered the crystn. of 1,3-DAGs, which are responsible for the development of the β form, and stimulated the crystn. of the 1,2-DAGs, which can crystallize in the α and β' forms. The presence of two polymorphic forms resulted in a decrease of the crystal network strength, as was derived from oscillatory rheol. measurements. This research implies a different effect of monoacylglycerols on both the nucleation and crystal growth of 1,2- and 1,3-DAG isomers. This insight is not only relevant for oleogelation research, but also for emulsifying agents which often contain blends of MAGs, 1,2-DAGs and 1,3-DAGs.
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  14. 14
    Metilli, L.; Lazidis, A.; Francis, M.; Marty-Terrade, S.; Ray, J.; Simone, E. The effect of crystallization conditions on the structural properties of oleofoams made of cocoa butter crystals and high oleic sunflower oil. Cryst. Growth Des. 2021, 21, 1562– 1575,  DOI: 10.1021/acs.cgd.0c01361
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    14
    The effect of crystallization conditions on the structural properties of oleofoams made of cocoa butter crystals and high oleic sunflower oil
    Metilli, Lorenzo; Lazidis, Aris; Francis, Mathew; Marty-Terrade, Stephanie; Ray, Joydeep; Simone, Elena
    Crystal Growth & Design (2021), 21 (3), 1562-1575CODEN: CGDEFU; ISSN:1528-7483. (American Chemical Society)
    Edible air-in-oil systems, also referred to as oleofoams, constitute a novel promising material for healthier, low-calorie fat replacers in confectionary products. Oleofoams can be formed by whipping oleogels, which are dispersions of fat crystals in an oil phase. Understanding how the properties of the fat crystals (i.e., size, shape, and polymorphism) contained in oleogels affect the microstructure and stability of oleofoams is essential for both the efficient design and manuf. of novel food products. In this work, cocoa butter, one of the main fat phases present in confectionary productions, which is responsible for pleasant texture and mouthfeel properties, was mixed with high oleic sunflower oil and crystd. to obtain an oleogel. This was subsequently whipped to yield a stable, highly aerated oleofoam. The effect of the crystn. conditions (oleogel compn. and cooling rate) on the properties of the oleogels and related oleofoams was investigated with a multitechnique characterization approach, featuring polarized light microscopy, cryogenic SEM, X-ray diffraction, differential scanning calorimetry, and oscillatory rheol. Oleogel crystn. was performed in a lab-scale vessel and was monitored using light turbidimetry as an in situ technique. Results showed that the concn. of cocoa butter in sunflower oil was the parameter that affected most strongly the foamability and rheol. of oleofoam samples. The size and shape of cocoa butter crystals within the oleogel was found to have a less significant effect since crystals were broken or partially melted during the aeration process. Oleofoams whipped from oleogels contg. 15 and 22% wt./wt. cocoa butter displayed an overrun of 200%, corresponding to a calorific d. redn. to one-third, and an increase in both the elastic and viscous moduli compared to their oleogel precursor. This was explained by a structuring effect caused by the aeration process, where cocoa butter β(V) crystal nanoplatelets (CNPs) in the oleogel rearranged to stabilize the air bubbles via a Pickering mechanism. Oleofoams prepd. from 30% wt./wt. cocoa butter oleogels, on the other hand, incorporated less air (overrun between 150 and 180%) and displayed a similar viscoelastic profile to their unwhipped precursors potentially due to air incorporation being limited by the relatively high elastic modulus of the parent oleogels. Nevertheless, the calorific d. of these samples was reduced by a factor of 1.6-2.5 compared to their full-fat analogs.
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    Fameau, A.-L.; Saint-Jalmes, A. Recent advances in understanding and use of oleofoams. Frontiers Sust. Food Systems 2020, 4, 110,  DOI: 10.3389/fsufs.2020.00110
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    Bala Subramaniam, A.; Abkarian, M.; Mahadevan, L.; Stone, H. A. Non-spherical bubbles. Nature 2005, 438, 930,  DOI: 10.1038/438930a
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    16
    Colloid science: Non-spherical bubbles
    Bala Subramaniam, Anand; Abkarian, Manouk; Mahadevan, L.; Stone, Howard A.
    Nature (London, United Kingdom) (2005), 438 (7070), 930CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    Here we show that gas bubbles and liq. drops can exist in stable, non-spherical shapes if the surface is covered with a close-packed monolayer particles.
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  17. 17
    Beltramo, P. J.; Gupta, M.; Alicke, A.; Liascukiene, I.; Gunes, D. Z.; Baroud, C. N.; Vermant, J. Arresting dissolution by interfacial rheology design. Proc. Nat. Acad. Sci. 2017, 114, 10373,  DOI: 10.1073/pnas.1705181114
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    17
    Arresting dissolution by interfacial rheology design
    Beltramo, Peter J.; Gupta, Manish; Alicke, Alexandra; Liascukiene, Irma; Gunes, Deniz Z.; Baroud, Charles N.; Vermant, Jan
    Proceedings of the National Academy of Sciences of the United States of America (2017), 114 (39), 10373-10378CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    A strategy to halt dissoln. of particle-coated air bubbles in water based on interfacial rheol. design is presented. Previously a dense monolayer was believed to be required for such an armored bubble to resist dissoln.; in fact, engineering a two-dimensional yield stress interface achieves such performance at sub-monolayer particle coverage. A suite of interfacial rheol. techniques characterized spherical and ellipsoidal particles at an air/water interface as a function of surface coverage. Bubbles with varying particle coverage were made and their resistance to dissoln. evaluated using a microfluidic technique. A bare bubble only had a single pressure at which a given radius was stable; the authors detd. a range of pressures over which armored bubble dissoln. was arrested. The link between interfacial rheol. and macroscopic dissoln. of ∼100 μm bubbles coated with ∼1 um particles is presented and discussed. Generic design rationale was confirmed using non-spherical particles, which develop significant yield stress at even lower surface coverages. Thus, it can be used to successfully inhibit Ostwald ripening in a multitude of foam and emulsion applications.
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  18. 18
    Kam, S. I.; Rossen, W. R. Anomalous capillary pressure, stress, and stability of solids-coated bubbles. J. Colloid Interface Sci. 1999, 213, 329– 339,  DOI: 10.1006/jcis.1999.6107
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    18
    Anomalous Capillary Pressure, Stress, and Stability of Solids-Coated Bubbles
    Kam, Seung I.; Rossen, William R.
    Journal of Colloid and Interface Science (1999), 213 (2), 329-339CODEN: JCISA5; ISSN:0021-9797. (Academic Press)
    A two-dimensional theor. model for solids-coated, or "armored," bubbles shows how the armor can support a liq.-vapor interface of reduced or reversed curvature between the particles, giving the bubble zero or even neg. capillary pressure. The inward capillary force pulling the particles into the center of the bubble are balanced by large contact forces between the particles in the armor. Thus the bubble is stabilized against dissoln. of gas into surrounding liq., which otherwise would rapidly collapse the bubble. The stresses between particles in such cases are large and could drive sintering of the particles into a rigid framework. Earlier work on solids-coated bubbles assumed that solids can freely enter or leave the bubble surface as the bubble shrinks or expands. In such a case, armored bubbles would not be stable to gas dissoln. into surrounding liq. A new free-energy anal., however, suggests that a shrunken bubble would not spontaneously expel a solid particle from its armor to relieve stress and allow the bubble to shrink further. Implications and limitations of the theory are discussed. (c) 1999 Academic Press.
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    Abkarian, M.; Subramaniam, A. B.; Kim, S.-H.; Larsen, R. J.; Yang, S.-M.; Stone, H. A. Dissolution arrest and stability of particle-covered bubbles. Phys. Rev. Lett. 2007, 99, 188301,  DOI: 10.1103/PhysRevLett.99.188301
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    Dissolution Arrest and Stability of Particle-Covered Bubbles
    Abkarian, Manouk; Subramaniam, Anand Bala; Kim, Shin-Hyun; Larsen, Ryan J.; Yang, Seung-Man; Stone, Howard A.
    Physical Review Letters (2007), 99 (18), 188301/1-188301/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    Expts. show that bubbles covered with monodisperse polystyrene particles, with particle to bubble radius ratios of about 0.1, evolve to form faceted polyhedral shapes that are stable to dissoln. in air-satd. water. We perform Surface Evolver simulations and find that the faceted particle-covered bubble represents a local min. of energy. At the faceted state, the Laplace overpressure vanishes, which together with the pos. slope of the bubble pressure-vol. curve, ensures phase stability. The repulsive interactions between the particles cause a redn. of the curvature of the gas-liq. interface, which is the mechanism that arrests dissoln. and stabilizes the bubbles.
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    Pitois, O.; Buisson, M.; Chateau, X. On the collapse pressure of armored bubbles and drops. Eur. Phys. J. E 2015, 38, 48,  DOI: 10.1140/epje/i2015-15048-9
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    Taccoen, N.; Lequeux, F.; Gunes, D. Z.; Baroud, C. N. Probing the mechanical strength of an armored bubble and its implication to particle-stabilized foams. Phys. Rev. X 2016, 6, 011010,  DOI: 10.1103/PhysRevX.6.011010
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    21
    Probing the mechanical strength of an armored bubble and its implication to particle-stabilized foams
    Taccoen, Nicolas; Lequeux, Francois; Gunes, Deniz Z.; Baroud, Charles N.
    Physical Review X (2016), 6 (1), 011010/1-011010/11CODEN: PRXHAE; ISSN:2160-3308. (American Physical Society)
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  1. Saikat Saha, Paul F. Luckham, Valeria Garbin. Non-linear response of colloid monolayers at high-frequency probed by ultrasound-driven microbubble dynamics. Journal of Colloid and Interface Science 2023, 630 , 984-993. https://doi.org/10.1016/j.jcis.2022.10.093
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  • Abstract

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    Figure 1

    Figure 1. (a) Schematic of an air bubble stabilized by wax crystals in oil. (b) Wax-coated bubble viewed using bright-field microscopy. (c) The same bubble viewed under crossed polarizers. (d) Schematic of the experimental setup.

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    Figure 2

    Figure 2. Dissolution of wax crystal-coated bubble in oil (SI Video 1). Schematic of bubble dissolution, occurring over t ∼ 103–104 s, with points qualitatively marking the dissolution steps observed in the following panels (i–vii). The image sequence is taken using crossed polarizers.

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    Figure 3

    Figure 3. Crystal expulsion after ultrasound-driven bubble oscillations. The schematic illustrates the protocol of oscillating a crystal-coated bubble with ultrasound pulses of different amplitudes. Images at selected times, labeled (i)–(viii), are shown below. (a) High-speed image sequence of a wax-coated bubble with initial radius R0 = 101 μm undergoing oscillations of moderate amplitude, with x0 = ΔR/R0 ≈ 3%, N = 50 cycles (SI Video 2). The frames show (i) the initial non-spherical shape of the bubble before oscillations, the (ii) maximum and (iii) minimum during oscillations at f = 25 kHz, and (iv) the final, almost spherical shape after a total of N = 260 cycles of oscillations of small amplitude. (b) High-speed image sequence with crossed polarizers, for large-amplitude oscillations, with x0 ≈ 20% (SI Video 3): (v) initial state before oscillations, (vi) maximum and (vii) minimum during oscillations at f = 25 kHz, and (viii) final state after N = 100 cycles of oscillations, showing expulsion of crystals around the bubble.

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    Figure 4

    Figure 4. Bubble dissolution after ultrasound-driven bubble oscillations. The schematic shows the protocol including large-amplitude radial oscillations, followed by unperturbed dissolution. The labels correspond to the image sequence shown in panels i–vii. The initial radius of the bubble before oscillations (i) is R0 = 102 μm. Oscillations are driven by ultrasound at f = 25 kHz for N = 480 cycles, with amplitude of oscillations x0 > 20% (shown in SI Video 4). (ii) t = 0 s represents the start time of recording as the bubble dissolves (ii–vii).

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  • References

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      4
      Formation and Microstructures of Whipped Oils Composed of Vegetable Oils and High-Melting Fat Crystals
      Mishima, Shoko; Suzuki, Atsushi; Sato, Kiyotaka; Ueno, Satoru
      Journal of the American Oil Chemists' Society (2016), 93 (11), 1453-1466CODEN: JAOCA7; ISSN:0003-021X. (Springer)
      This paper reports the exptl. results of processes used for the formation of whipped oils composed of vegetable oils (salad oil) and high-melting fat crystals [fully hydrogenated rapeseed oil rich in behenic acid (FHR-B)]. No emulsifier was added to form this whipped oil. Microprobe FT-IR spectroscopy, synchrotron radiation microbeam X-ray diffraction (SR-μ-XRD), polarized optical microscopy, and differential scanning calorimetry (DSC) were employed to observe fine fat crystal particles of the most stable polymorph of β (β-fat crystal), FHR-B, and their adsorption at the air-oil surface before, during, and after the formation of the whipped oil. The results obtained revealed the following: (1) The prepn. of an organogel composed of salad oil and small fibrous β-fat crystals using a special tempering procedure was a prerequisite for forming whipped oil. (2) The β-fat crystals were adsorbed at the air-oil surface to encapsulate the air bubbles during the formation process of whipped oil. (3) The values of overrun of the whipped oil reached >200 % after an aeration time of 30 min at 20 °C. (4) The SR-μ-XRD expts. demonstrated that the lamellar planes of the β-fat crystals near the air-oil surface were arranged almost parallel to the air-oil surface plane. The present study provides the first evidence that tiny fat crystal particles may cause aeration in liq. oils without the addn. of other whip-assisting substances such as emulsifier crystals.
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    5. 5
      Gunes, D. Z.; Murith, M.; Godefroid, J.; Pelloux, C.; Deyber, H.; Schafer, O.; Breton, O. Oleofoams: Properties of crystal-coated bubbles from whipped oleogels–Evidence for Pickering stabilization. Langmuir 2017, 33, 1563– 1575,  DOI: 10.1021/acs.langmuir.6b04141
      5
      Oleofoams: Properties of Crystal-Coated Bubbles from Whipped Oleogels-Evidence for Pickering Stabilization
      Gunes, D. Z.; Murith, M.; Godefroid, J.; Pelloux, C.; Deyber, H.; Schafer, O.; Breton, O.
      Langmuir (2017), 33 (6), 1563-1575CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      The authors report evidence that lipidic crystals made of a high fraction of fully sol. monoglyceride (MG) in oil do not only adsorb at the oil-air interface but also can easily form a jammed, closely packed layer of crystals around the bubbles of a foam produced by whipping (Pickering effect). Very fine bubbles, soft textures, or firmer ones such as for shaving foams could be obtained, with a high air fraction (up to 75%), which is unprecedented. A thin, jammed layer of crystals on bubbles can cause bubbles to retain nonspherical shapes in the absence of bulk effects for times much longer than the characteristic capillary relaxation time for bare bubbles, which is actual evidence for Pickering-type interfacial stabilization. By comparing to foams obtained by depressurization, we show that whipping is necessary for bubble wrapping with a layer of crystals. The origin of high stability against Ostwald ripening at long times is also discussed. These Pickering whipped foams have rheol. properties dominated by interfacial or film contributions, which is of high interest for food and cosmetics applications because of their high moduli. This system can be considered to be a model of the crystn. behavior of MG in oil, which is similar to that in many fats.
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    6. 6
      Heymans, R.; Tavernier, I.; Danthine, S.; Rimaux, T.; Van der Meeren, P.; Dewettinck, K. Food-grade monoglyceride oil foams: the effect of tempering on foamability, foam stability and rheological properties. Food Funct. 2018, 9, 3143– 3154,  DOI: 10.1039/C8FO00536B
      6
      Food-grade monoglyceride oil foams: the effect of tempering on foamability, foam stability and rheological properties
      Heymans, Robbe; Tavernier, Iris; Danthine, Sabine; Rimaux, Tom; Van der Meeren, Paul; Dewettinck, Koen
      Food & Function (2018), 9 (6), 3143-3154CODEN: FFOUAI; ISSN:2042-6496. (Royal Society of Chemistry)
      Foams with a continuous oil phase may be stabilized using cryst. particles. Those systems are compelling because of their potential in edible oil structuring, modifying sensorial properties and creating healthier food products. This study aimed to relate oleogel (unwhipped state) properties to oil foam (whipped state) properties using a monoglyceride-sunflower oil model system. The properties of crystal-oil mixts. were influenced by time and temp. during prepn. and storage. Therefore, oleogels were prepd. using different tempering protocols and their resulting microstructure was investigated with rheol., differential scanning calorimetry and X-ray diffraction. The corresponding oil foams were characterized in terms of foamability and foam stability. The properties of both systems were studied immediately after prepn. as well as after 4 wk of storage. We demonstrated that there is a large influence of the time-temp. history on the foam properties. Partially crystd. mixts. were shown to form weaker structures which capture more air because of their lower viscosity and as crystn. would preferentially take place at the interface. They were characterized by larger bubbles and were less stable and firm. It is proposed that their rheol. properties are mainly dominated by interfacial contributions. Fully crystd. and stored monoglyceride-oil mixts. were seen to form stronger gel networks which included less air, contained smaller air bubbles and were stable during storage. It is hypothesized that these samples also included an important bulk gelation contribution.
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    7. 7
      Liu, Y.; Binks, B. P. A novel strategy to fabricate stable oil foams with sucrose ester surfactant. J. Colloid Interface Sci. 2021, 594, 204– 216,  DOI: 10.1016/j.jcis.2021.03.021
      7
      A novel strategy to fabricate stable oil foams with sucrose ester surfactant
      Liu, Yu; Binks, Bernard P.
      Journal of Colloid and Interface Science (2021), 594 (), 204-216CODEN: JCISA5; ISSN:0021-9797. (Elsevier B.V.)
      Can a mixt. of sucrose ester surfactant in vegetable oil be aerated to yield stable oleofoams. Is foaming achievable from one-phase mol. solns. and/or two-phase crystal dispersions. Does cooling a foam after formation induce surfactant crystn. and enhance foam stability. Concg. on extra virgin olive oil, we first study the effect of aeration temp. and surfactant concn. on foamability and foam stability of mixts. cooled from a one-phase oil soln. Based on this, we introduce a strategy to increase foam stability by rapidly cooling foam prepd. at high temp. which induces surfactant crystn. in situ. Differential scanning calorimetry, X-ray diffraction, infra-red spectroscopy, surface tension and rheol. are used to elucidate the mechanisms. Unlike previous reports, both foamability and foam stability decrease upon decreasing the aeration temp. into the two-phase region contg. surfactant crystals. At high temp. in the one-phase region, substantial foaming is achieved (over-run 170%) within minutes of whipping but foams ultimately collapse within a week. We show that surfactant mols. are surface-active at high temp. and that hydrogen bonds form between surfactant and oil mols. The generic nature of our findings is demonstrated for a range of vegetable oil foams with a max. over-run of 330% and the absence of drainage, coalescence and disproportionation being achievable.
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    8. 8
      Binks, B. P.; Vishal, B. Particle-stabilized oil foams. Adv. Colloid Interface Sci. 2021, 291, 102404,  DOI: 10.1016/j.cis.2021.102404
      8
      Particle-stabilized oil foams
      Binks, Bernard P.; Vishal, Badri
      Advances in Colloid and Interface Science (2021), 291 (), 102404CODEN: ACISB9; ISSN:0001-8686. (Elsevier B.V.)
      A review. The area of oil foams although important industrially has received little academic attention until the last decade. The early work using mol. surfactants for stabilization was limited and as such it is difficult to obtain general rules of thumb. Recently however, interest has grown in the area partly fuelled by the understanding gained in the general area of colloidal particles at fluid interfaces. We review the use of solid particles as foaming agents for oil foams in cases where particles (inorg. or polymer) are prepd. ex situ and in cases where crystals of surfactant or fat are prepd. in situ. There is considerable activity in the latter area which is particularly relevant to the food industry. Discussion of crude oil/lubricating oil foams is excluded from this review.
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    9. 9
      Goibier, L.; Pillement, C.; Monteil, J.; Faure, C.; Leal-Calderon, F. Emulsification of non-aqueous foams stabilized by fat crystals: Towards novel air-in-oil-in-water food colloids. Food Chem. 2019, 293, 49– 56,  DOI: 10.1016/j.foodchem.2019.04.080
      9
      Emulsification of non-aqueous foams stabilized by fat crystals: Towards novel air-in-oil-in-water food colloids
      Goibier, Lucie; Pillement, Christophe; Monteil, Julien; Faure, Chrystel; Leal-Calderon, Fernando
      Food Chemistry (2019), 293 (), 49-56CODEN: FOCHDJ; ISSN:0308-8146. (Elsevier Ltd.)
      We designed Air-in-Oil-in-Water (A/O/W) emulsions. First, Air-in-Oil foams were fabricated by whipping anhyd. milk fat. The max. overrun was obtained at 20 °C. The foams contained 30-35 vol% air and were stabilized solely by fat crystals. To refine the bubble size, foams were further sheared in a Couette's cell. The av. bubble size reached a value as small as 6.5 μm at a shear rate of 5250 s-1. The nonaq. foams were then dispersed in a viscous aq. phase contg. sodium caseinate to obtain A/O/W emulsions. The shear rate was varied from 1000 to 7500 s-1, allowing to obtain Air-in-Oil globules whose av. diam. ranged from 15 to 60 μm. To avoid globule creaming, the aq. phase was gelled by incorporating hydroxyethyl cellulose. Homogeneous emulsions were obtained with fat globules contg. around 22 vol% of residual air. The systems were kinetically stable for at least 3 wk at 4 °C.
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    10. 10
      Saha, S. Micromechanics of Particle-Coated Bubbles: Deformation from Quasistatic to Millisecond Timescales. Ph.D. Thesis, Imperial College London, 2020.
      There is no corresponding record for this reference.
    11. 11
      Saha, S.; Saint-Michel, B.; Leynes, V.; Binks, B. P.; Garbin, V. Stability of bubbles in wax-based oleofoams: decoupling the effects of bulk Oleogel rheology and interfacial rheology. Rheol. Acta 2020, 59, 255– 266,  DOI: 10.1007/s00397-020-01192-x
      11
      Stability of bubbles in wax-based oleofoams: decoupling the effects of bulk oleogel rheology and interfacial rheology
      Saha, S.; Saint-Michel, B.; Leynes, V.; Binks, B. P.; Garbin, V.
      Rheologica Acta (2020), 59 (4), 255-266CODEN: RHEAAK; ISSN:0035-4511. (Springer)
      Oleofoams are dispersions of gas bubbles in a continuous oil phase and can be stabilized by crystals of fatty acids or waxes adsorbing at the oil-air interface. Because excess crystals in the continuous phase form an oleogel, an effect of the bulk rheol. of the continuous phase is also expected. Here, we evaluate the contributions of bulk and interfacial rheol. below and above the m.p. of a wax forming an oleogel in sunflower oil. We study the dissoln. behavior of single bubbles using microscopy on a temp.-controlled stage. We compare the behavior of a bubble embedded in an oleofoam, which owes its stability to both bulk and interfacial rheol., to that of a bubble extd. from the oleofoam and resuspended in oil, for which the interfacial dilatational rheol. alone provides stability. We find that below the m.p. of the wax, bubbles in the oleofoam are stable whereas bubbles that are only coated with wax crystals dissolve. Both systems dissolve when heated above the m.p. of the wax. These findings are rationalized through independent bulk rheol. measurements of the oleogel at different temps., as well as measurements of the dilatational rheol. properties of a wax-coated oil-air interface.
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    12. 12
      Liascukiene, I.; Amselem, G.; Landoulsi, J.; Gunes, D. Z.; Baroud, C. N. Intermittent dynamics of bubble dissolution due to interfacial growth of fat crystals. Soft Matter 2021, 17, 10042– 10052,  DOI: 10.1039/D1SM00902H
      12
      Intermittent dynamics of bubble dissolution due to interfacial growth of fat crystals
      Liascukiene, Irma; Amselem, Gabriel; Landoulsi, Jessem; Gunes, Deniz Z.; Baroud, Charles N.
      Soft Matter (2021), 17 (44), 10042-10052CODEN: SMOABF; ISSN:1744-6848. (Royal Society of Chemistry)
      Foams are inherently unstable objects, that age and disappear over time. The main cause of foam aging is Ostwald ripening: smaller air bubbles within the foam empty their gas content into larger ones. One strategy to counter Ostwald ripening consists in creating armored bubbles, where solid particles adsorbed at the air/liq. interface prevent bubbles from shrinking below a given size. Here, we study the efficiency of coating air bubbles with fat crystals to prevent bubble dissoln. A monoglyceride, monostearin, is directly crystd. at the air/oil interface. Expts. on single bubbles in a microfluidic device show that the presence of monostearin fat crystals slows down dissoln., with an efficiency that depends on the crystal size. Bubble ripening in the presence of crystals exhibits intermittent dissoln. dynamics, with phases of arrest, when crystals jam at the interface, followed by phases of dissoln., when monostearin crystals are ejected from the interface. In the end, crystals do not confer enough mech. strength to the bubbles to prevent them from fully dissolving.
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    13. 13
      Tavernier, I.; Moens, K.; Heyman, B.; Danthine, S.; Dewettinck, K. Relating crystallization behavior of monoacylglycerols-diacylglycerol mixtures to the strength of their crystalline network in oil. Food Res. Int. 2019, 120, 504– 513,  DOI: 10.1016/j.foodres.2018.10.092
      13
      Relating crystallization behavior of monoacylglycerols-diacylglycerol mixtures to the strength of their crystalline network in oil
      Tavernier, Iris; Moens, Kim; Heyman, Bart; Danthine, Sabine; Dewettinck, Koen
      Food Research International (2019), 120 (), 504-513CODEN: FORIEU; ISSN:0963-9969. (Elsevier B.V.)
      Diacylglycerols (DAGs) are interesting oil structuring mols. as they are structurally similar to triacylglycerols (TAGs), but are metabolized differently which results in wt. loss and improved blood cholesterol levels upon dietary replacement of TAGs with DAGs. Many com. products consist of a mixt. of monoacylglycerols (MAGs) and DAGs, yet the effect of MAGs on the crystn. behavior of DAGs is still to be unraveled. Two types of com. MAGs, one originating from hydrogenated palm stearin and one of hydrogenated rapeseed oil, were added in concns. 1, 2 and 4% to 20% DAGs derived from hydrogenated soybean oil. Using differential scanning calorimetry, it was shown that the presence of MAGs delayed the onset of DAG crystn. Rheol. anal. revealed that MAGs also hindered crystal network development. Synchrotron X-ray diffraction anal. demonstrated that the addn. of MAGs suppressed the formation of the β form and stimulated the development of the β' form. Likely, MAGs mainly hindered the crystn. of 1,3-DAGs, which are responsible for the development of the β form, and stimulated the crystn. of the 1,2-DAGs, which can crystallize in the α and β' forms. The presence of two polymorphic forms resulted in a decrease of the crystal network strength, as was derived from oscillatory rheol. measurements. This research implies a different effect of monoacylglycerols on both the nucleation and crystal growth of 1,2- and 1,3-DAG isomers. This insight is not only relevant for oleogelation research, but also for emulsifying agents which often contain blends of MAGs, 1,2-DAGs and 1,3-DAGs.
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    14. 14
      Metilli, L.; Lazidis, A.; Francis, M.; Marty-Terrade, S.; Ray, J.; Simone, E. The effect of crystallization conditions on the structural properties of oleofoams made of cocoa butter crystals and high oleic sunflower oil. Cryst. Growth Des. 2021, 21, 1562– 1575,  DOI: 10.1021/acs.cgd.0c01361
      14
      The effect of crystallization conditions on the structural properties of oleofoams made of cocoa butter crystals and high oleic sunflower oil
      Metilli, Lorenzo; Lazidis, Aris; Francis, Mathew; Marty-Terrade, Stephanie; Ray, Joydeep; Simone, Elena
      Crystal Growth & Design (2021), 21 (3), 1562-1575CODEN: CGDEFU; ISSN:1528-7483. (American Chemical Society)
      Edible air-in-oil systems, also referred to as oleofoams, constitute a novel promising material for healthier, low-calorie fat replacers in confectionary products. Oleofoams can be formed by whipping oleogels, which are dispersions of fat crystals in an oil phase. Understanding how the properties of the fat crystals (i.e., size, shape, and polymorphism) contained in oleogels affect the microstructure and stability of oleofoams is essential for both the efficient design and manuf. of novel food products. In this work, cocoa butter, one of the main fat phases present in confectionary productions, which is responsible for pleasant texture and mouthfeel properties, was mixed with high oleic sunflower oil and crystd. to obtain an oleogel. This was subsequently whipped to yield a stable, highly aerated oleofoam. The effect of the crystn. conditions (oleogel compn. and cooling rate) on the properties of the oleogels and related oleofoams was investigated with a multitechnique characterization approach, featuring polarized light microscopy, cryogenic SEM, X-ray diffraction, differential scanning calorimetry, and oscillatory rheol. Oleogel crystn. was performed in a lab-scale vessel and was monitored using light turbidimetry as an in situ technique. Results showed that the concn. of cocoa butter in sunflower oil was the parameter that affected most strongly the foamability and rheol. of oleofoam samples. The size and shape of cocoa butter crystals within the oleogel was found to have a less significant effect since crystals were broken or partially melted during the aeration process. Oleofoams whipped from oleogels contg. 15 and 22% wt./wt. cocoa butter displayed an overrun of 200%, corresponding to a calorific d. redn. to one-third, and an increase in both the elastic and viscous moduli compared to their oleogel precursor. This was explained by a structuring effect caused by the aeration process, where cocoa butter β(V) crystal nanoplatelets (CNPs) in the oleogel rearranged to stabilize the air bubbles via a Pickering mechanism. Oleofoams prepd. from 30% wt./wt. cocoa butter oleogels, on the other hand, incorporated less air (overrun between 150 and 180%) and displayed a similar viscoelastic profile to their unwhipped precursors potentially due to air incorporation being limited by the relatively high elastic modulus of the parent oleogels. Nevertheless, the calorific d. of these samples was reduced by a factor of 1.6-2.5 compared to their full-fat analogs.
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    15. 15
      Fameau, A.-L.; Saint-Jalmes, A. Recent advances in understanding and use of oleofoams. Frontiers Sust. Food Systems 2020, 4, 110,  DOI: 10.3389/fsufs.2020.00110
      There is no corresponding record for this reference.
    16. 16
      Bala Subramaniam, A.; Abkarian, M.; Mahadevan, L.; Stone, H. A. Non-spherical bubbles. Nature 2005, 438, 930,  DOI: 10.1038/438930a
      16
      Colloid science: Non-spherical bubbles
      Bala Subramaniam, Anand; Abkarian, Manouk; Mahadevan, L.; Stone, Howard A.
      Nature (London, United Kingdom) (2005), 438 (7070), 930CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      Here we show that gas bubbles and liq. drops can exist in stable, non-spherical shapes if the surface is covered with a close-packed monolayer particles.
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    17. 17
      Beltramo, P. J.; Gupta, M.; Alicke, A.; Liascukiene, I.; Gunes, D. Z.; Baroud, C. N.; Vermant, J. Arresting dissolution by interfacial rheology design. Proc. Nat. Acad. Sci. 2017, 114, 10373,  DOI: 10.1073/pnas.1705181114
      17
      Arresting dissolution by interfacial rheology design
      Beltramo, Peter J.; Gupta, Manish; Alicke, Alexandra; Liascukiene, Irma; Gunes, Deniz Z.; Baroud, Charles N.; Vermant, Jan
      Proceedings of the National Academy of Sciences of the United States of America (2017), 114 (39), 10373-10378CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      A strategy to halt dissoln. of particle-coated air bubbles in water based on interfacial rheol. design is presented. Previously a dense monolayer was believed to be required for such an armored bubble to resist dissoln.; in fact, engineering a two-dimensional yield stress interface achieves such performance at sub-monolayer particle coverage. A suite of interfacial rheol. techniques characterized spherical and ellipsoidal particles at an air/water interface as a function of surface coverage. Bubbles with varying particle coverage were made and their resistance to dissoln. evaluated using a microfluidic technique. A bare bubble only had a single pressure at which a given radius was stable; the authors detd. a range of pressures over which armored bubble dissoln. was arrested. The link between interfacial rheol. and macroscopic dissoln. of ∼100 μm bubbles coated with ∼1 um particles is presented and discussed. Generic design rationale was confirmed using non-spherical particles, which develop significant yield stress at even lower surface coverages. Thus, it can be used to successfully inhibit Ostwald ripening in a multitude of foam and emulsion applications.
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    18. 18
      Kam, S. I.; Rossen, W. R. Anomalous capillary pressure, stress, and stability of solids-coated bubbles. J. Colloid Interface Sci. 1999, 213, 329– 339,  DOI: 10.1006/jcis.1999.6107
      18
      Anomalous Capillary Pressure, Stress, and Stability of Solids-Coated Bubbles
      Kam, Seung I.; Rossen, William R.
      Journal of Colloid and Interface Science (1999), 213 (2), 329-339CODEN: JCISA5; ISSN:0021-9797. (Academic Press)
      A two-dimensional theor. model for solids-coated, or "armored," bubbles shows how the armor can support a liq.-vapor interface of reduced or reversed curvature between the particles, giving the bubble zero or even neg. capillary pressure. The inward capillary force pulling the particles into the center of the bubble are balanced by large contact forces between the particles in the armor. Thus the bubble is stabilized against dissoln. of gas into surrounding liq., which otherwise would rapidly collapse the bubble. The stresses between particles in such cases are large and could drive sintering of the particles into a rigid framework. Earlier work on solids-coated bubbles assumed that solids can freely enter or leave the bubble surface as the bubble shrinks or expands. In such a case, armored bubbles would not be stable to gas dissoln. into surrounding liq. A new free-energy anal., however, suggests that a shrunken bubble would not spontaneously expel a solid particle from its armor to relieve stress and allow the bubble to shrink further. Implications and limitations of the theory are discussed. (c) 1999 Academic Press.
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    19. 19
      Abkarian, M.; Subramaniam, A. B.; Kim, S.-H.; Larsen, R. J.; Yang, S.-M.; Stone, H. A. Dissolution arrest and stability of particle-covered bubbles. Phys. Rev. Lett. 2007, 99, 188301,  DOI: 10.1103/PhysRevLett.99.188301
      19
      Dissolution Arrest and Stability of Particle-Covered Bubbles
      Abkarian, Manouk; Subramaniam, Anand Bala; Kim, Shin-Hyun; Larsen, Ryan J.; Yang, Seung-Man; Stone, Howard A.
      Physical Review Letters (2007), 99 (18), 188301/1-188301/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      Expts. show that bubbles covered with monodisperse polystyrene particles, with particle to bubble radius ratios of about 0.1, evolve to form faceted polyhedral shapes that are stable to dissoln. in air-satd. water. We perform Surface Evolver simulations and find that the faceted particle-covered bubble represents a local min. of energy. At the faceted state, the Laplace overpressure vanishes, which together with the pos. slope of the bubble pressure-vol. curve, ensures phase stability. The repulsive interactions between the particles cause a redn. of the curvature of the gas-liq. interface, which is the mechanism that arrests dissoln. and stabilizes the bubbles.
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    20. 20
      Pitois, O.; Buisson, M.; Chateau, X. On the collapse pressure of armored bubbles and drops. Eur. Phys. J. E 2015, 38, 48,  DOI: 10.1140/epje/i2015-15048-9
      There is no corresponding record for this reference.
    21. 21
      Taccoen, N.; Lequeux, F.; Gunes, D. Z.; Baroud, C. N. Probing the mechanical strength of an armored bubble and its implication to particle-stabilized foams. Phys. Rev. X 2016, 6, 011010,  DOI: 10.1103/PhysRevX.6.011010
      21
      Probing the mechanical strength of an armored bubble and its implication to particle-stabilized foams
      Taccoen, Nicolas; Lequeux, Francois; Gunes, Deniz Z.; Baroud, Charles N.
      Physical Review X (2016), 6 (1), 011010/1-011010/11CODEN: PRXHAE; ISSN:2160-3308. (American Physical Society)
      Bubbles are dynamic objects that grow and rise or shrink and disappear, often on the scale of seconds. This conflicts with their uses in foams where they serve to modify the properties of the material in which they are embedded. Coating the bubble surface with solid particles has been demonstrated to strongly enhance the foam stability, although the mechanisms for such stabilization remain mysterious. In this paper, we reduce the problem of foam stability to the study of the behavior of a single spherical bubble coated with a monolayer of solid particles. The behavior of this armored bubble is monitored while the ambient pressure around it is varied, in order to simulate the dissoln. stress resulting from the surrounding foam. We find that above a crit. stress, localized dislocations appear on the armor and lead to a global loss of the mech. stability. Once these dislocations appear, the armor is unable to prevent the dissoln. of the gas into the surrounding liq., which translates into a continued redn. of the bubble vol., even for a fixed overpressure. The obsd. route to the armor failure therefore begins from localized dislocations that lead to large-scale deformations of the shell until the bubble completely dissolves. The crit. value of the ambient pressure that leads to the failure depends on the bubble radius, with a scaling of ΔPcollapse ∞ R-1, but does not depend on the particle diam. These results disagree with the generally used elastic models to describe particle-covered interfaces. Instead, the exptl. measurements are accounted for by an original theor. description that equilibrates the energy gained from the gas dissoln. with the capillary energy cost of displacing the individual particles. The model recovers the short-wavelength instability, the scaling of the collapse pressure with bubble radius, and the insensitivity to particle diam. Finally, we use this new microscopic understanding to predict the aging of particle-stabilized foams, by applying classical Ostwald ripening models. We find that the smallest armored bubbles should fail, as the dissoln. stress on these bubbles increases more rapidly than the armor strength. Both the exptl. and theor. results can readily be generalized to more complex particle interactions and shell structures.
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    22. 22
      Poulichet, V.; Garbin, V. Cooling particle-coated bubbles: Destabilization beyond dissolution arrest. Langmuir 2015, 31, 12035– 12042,  DOI: 10.1021/acs.langmuir.5b03480
      22
      Cooling Particle-Coated Bubbles: Destabilization beyond Dissolution Arrest
      Poulichet, Vincent; Garbin, Valeria
      Langmuir (2015), 31 (44), 12035-12042CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      The effect of changes in temp. on the lifetime of particle-coated air microbubbles in water has been studied. A decrease in temp. destabilized particle-coated microbubbles beyond dissoln. arrest. A quasi-steady model describing the effect of the change in temp. on mass transfer suggested that the dominant mechanism of destabilization was the increased soly. of the gas in the liq., leading to a condition of undersatn. Expts. at const. temp. confirmed that undersatn. alone could drive destabilization of particle-coated bubbles, even for vanishing Laplace pressure. Dissoln. of a particle-coated bubble could lead either to buckling of the coating or to gradual expulsion of particles, depending on the particle-to-bubble size ratio, with potential implications for controlled release.
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    23. 23
      Achakulwisut, K.; Tam, C.; Huerre, A.; Sammouti, R.; Binks, B. P.; Garbin, V. Stability of clay particle-coated microbubbles in alkanes against dissolution induced by heating. Langmuir 2017, 33, 3809– 3817,  DOI: 10.1021/acs.langmuir.7b00429
      23
      Stability of Clay Particle-Coated Microbubbles in Alkanes against Dissolution Induced by Heating
      Achakulwisut, Kanvara; Tam, Chak; Huerre, Axel; Sammouti, Rafaella; Binks, Bernard P.; Garbin, Valeria
      Langmuir (2017), 33 (15), 3809-3817CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      The authors investigated the dissoln. and morphol. dynamics of air bubbles in alkanes stabilized by fluorinated colloidal clay particles when subjected to temp. changes. A model for bubble dissoln. with time-dependent temp. reveals that increasing the temp. enhances the bubble dissoln. rate in alkanes, opposite to the behavior in water, because of the differing trends in gas soly. Exptl. results for uncoated air bubbles in decane and hexadecane confirm this prediction. Clay-coated bubbles in decane and hexadecane are shown to be stable in air-satd. oil at const. temp., where dissoln. is driven mainly by the Laplace pressure. When the temp. increases from ambient, the particle-coated bubbles are prone to dissoln. as the oil phase becomes undersatd. The interfacial layer of particles is obsd. to undergo buckling and crumpling, without shedding of clay particles. Increasing the concn. of particles is shown to enhance the bubble stability by providing a higher resistance to dissoln. When subjected to complex temp. cycles, for which the effect of time-dependent temp. is dominant, the clay-coated bubbles can resist long-term dissoln. in conditions under which uncoated bubbles dissolve completely. These results underpin the design of ultrastable oil foams stabilized by solid particles with improved shelf life under changing environmental conditions.
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    24. 24
      Poulichet, V.; Garbin, V. Ultrafast desorption of colloidal particles from fluid interfaces. Proc. Nat. Acad. Sci. 2015, 112, 5932– 5937,  DOI: 10.1073/pnas.1504776112
      24
      Ultrafast desorption of colloidal particles from fluid interfaces
      Poulichet, Vincent; Garbin, Valeria
      Proceedings of the National Academy of Sciences of the United States of America (2015), 112 (19), 5932-5937CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      Ultrafast desorption of colloid monolayers from the interface of particle-stabilized bubbles is described. The bubbles were driven into periodic compression-expansion using ultrasound waves, causing significant deformation and microstructural changes in the particle monolayer. Using high-speed microscopy the authors uncover different particle expulsion scenarios depending on the mode of bubble deformation, including highly directional patterns of particle release during shape oscillations. Complete removal of colloid monolayers from bubbles is achieved in under a millisecond. This method should find a broad range of applications, from nanoparticle recycling in sustainable processes to programmable particle delivery in lab-on-a-chip applications.
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    25. 25
      Kloek, W.; Van Vliet, T.; Meinders, M. Effect of bulk and interfacial rheological properties on bubble dissolution. J. Colloid Interface Sci. 2001, 237, 158– 166,  DOI: 10.1006/jcis.2001.7454
      25
      Effect of Bulk and Interfacial Rheological Properties on Bubble Dissolution
      Kloek, William; van Vliet, Ton; Meinders, Marcel
      Journal of Colloid and Interface Science (2001), 237 (2), 158-166CODEN: JCISA5; ISSN:0021-9797. (Academic Press)
      This paper describes theor. calcns. of the combined effect of bulk and interfacial rheol. properties on dissoln. behavior of a bubble in an infinite medium at satd. conditions. Either bulk or interfacial elasticity can stop the bubble dissoln. process, and stability criteria are defined for the elastic cases. In the case of an elastic interface with dilation modulus Ed and a bubble with an initial radius R0 and initial interfacial tension σ0, the bubble is stabilized as it has shrunk to a relative radius of ε=R/R0 = exp(-σ0/2Ed). In case of an elastic bulk with modulus G a bubble will shrink until GR0 = 4σ0ε3/(1-5ε4+4ε3) is fulfilled. Bulk and interfacial viscosity can retard the dissoln. process if their magnitude exceeds a certain crit. value but will never completely stop bubble dissoln. (c) 2001 Academic Press.
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    26. 26
      Mishra, K.; Grob, L.; Kohler, L.; Zimmermann, S.; Gstöhl, S.; Fischer, P.; Windhab, E. J. Entrance flow of unfoamed and foamed Herschel–Bulkley fluids. J. Rheol. 2021, 65, 1155– 1168,  DOI: 10.1122/8.0000286
      26
      Entrance flow of unfoamed and foamed Herschel-Bulkley fluids
      Mishra, Kim; Grob, Lucas; Kohler, Lucas; Zimmermann, Simon; Gstohl, Stefan; Fischer, Peter; Windhab, Erich J.
      Journal of Rheology (Melville, NY, United States) (2021), 65 (6), 1155-1168CODEN: JORHD2; ISSN:0148-6055. (American Institute of Physics)
      The present study investigates extrusion processing of unfoamed and foamed cocoa butter (CB) crystal-melt suspensions (CMSs) with varying crystal vol. fraction ΦSFC. Capillary rheometry derived flow curves were fitted with the Herschel-Bulkley-Papanastasiou (HBP) model, and the derived yield stress to wall shear stress ratio τ0/τw of CB CMSs was compared for the various ΦSFC. Foamed CB CMSs behaved fluidlike for ΦSFC≤11.9% and according to a brittle porous solid for ΦSFC > 11.9%. The dimensionless entrance pressure loss nen/α as a function of dimensionless shear stress τ* was higher for foamed compared to unfoamed CB CMSs at ΦSFC≤11.9% and lower for foamed compared to unfoamed CB CMSs at ΦSFC > 11.9%. The ΦSFC dependent difference in nen/α was attributed to the crystal confinement in the die entrance flow, which is increased in the case of elastic gas bubble deformation and decreased in the case of plastic gas pore collapse. The computational fluid dynamics simulated flow of unfoamed and foamed CB CMSs through an abrupt circular 20:1 contraction with the HBP model was compared with the exptl. results from quant. entrance flow visualization (QEFV). Furthermore, the QEFV derived half center incidence angle θ as well as the entrance flow shear and elongational rates γef and εef were derived and used to establish a model predicting the Bagley entrance pressure lossδPBag and calc. an entrance flow characteristic shear and elongational viscosities ηandηeef. (c) 2021 American Institute of Physics.
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    27. 27
      Fameau, A.-L.; Lam, S.; Arnould, A.; Gaillard, C.; Velev, O. D.; Saint-Jalmes, A. Smart nonaqueous foams from lipid-based oleogel. Langmuir 2015, 31, 13501– 13510,  DOI: 10.1021/acs.langmuir.5b03660
      27
      Smart Nonaqueous Foams from Lipid-Based Oleogel
      Fameau, Anne-Laure; Lam, Stephanie; Arnould, Audrey; Gaillard, Cedric; Velev, Orlin D.; Saint-Jalmes, Arnaud
      Langmuir (2015), 31 (50), 13501-13510CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      Oil foams are composed of gas bubbles dispersed in an oil phase. These systems are scarcely studied despite their great potential in diverse fields such as the food and cosmetic industries. Contrary to aq. foams, the prodn. of oil foams is difficult to achieve due to the inefficiency of surfactant adsorption at oil-air interfaces. Herein, we report a simple way to produce oil foams from oleogels, whose liq. phase is a mixt. of sunflower oil and fatty alcs. The temp. at which the oleogel formed was found to depend on both fatty alc. chain length and concn. The air bubbles in the oleogel foam were stabilized by fatty alc. crystals. Below the melting temp. of the crystals, oleogel foams were stable for months. Upon heating, these ultrastable foams collapsed within a few minutes due to the melting of the crystal particles. The transition between crystal formation and melting was reversible, leading to thermoresponsive nonaq. foams. The reversible switching between ultrastable and unstable foam depended solely on the temp. of the system. We demonstrate that these oleogel foams can be made to be photoresponsive by using internal heat sources such as carbon black particles, which can absorb UV light and dissipate the absorbed energy as heat. This simple approach for the formulation of responsive oil foams could be easily extended to other oleogel systems and could find a broad range of applications due to the availability of the components in large quantities and at low cost.
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    28. 28
      Duncan, P. B.; Needham, D. Test of the Epstein-Plesset model for gas microparticle dissolution in aqueous media: Effect of surface tension and gas undersaturation in solution. Langmuir 2004, 20, 2567– 2578,  DOI: 10.1021/la034930i
      28
      Test of the Epstein-Plesset Model for Gas Microparticle Dissolution in Aqueous Media: Effect of Surface Tension and Gas Undersaturation in Solution
      Duncan, P. Brent; Needham, David
      Langmuir (2004), 20 (7), 2567-2578CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      The gas from a free air bubble will readily dissolve in water, driven by two main factors: the concn. (undersatn.) of dissolved gas in the aq. soln. and the surface tension of the gas bubble-water interface via a Laplace overpressure in the bubble that this creates. This paper exptl. and theor. investigates each of these effects individually. To study the effects of surface tension, single- and double-chain surfactants were utilized to control and define interfacial conditions of the microbubble in satd. soln. To study the effect of undersatn., solid distearoylphosphocholine lipid was utilized to coat the gas microparticle with, essentially, a wax monolayer and to achieve zero tension in the surface. The exptl. work was performed using a micromanipulation technique that allows one to create and micromanipulate single air microparticles (5-50 μm radius range) in infinite diln. and to accurately record the size of the particle as it loses vol. due to the dissoln. process. The micropipet technique has shown to be an improvement over other previous attempts to measure dissoln. time with a 3.2% av. exptl. error in gas microparticle dissoln. time. An ability to study a gas microparticle in infinite diln. in an isotropic diffusion field is in line with the theor. assumptions and conditions of the Epstein-Plesset model. The Epstein-Plesset model on av. underpredicted the exptl. detd. dissoln. time by 8.6%, where the effect of surface tension was considered with a range of surface tensions from 72 down to 25 mN/m. The Epstein-Plesset model on av. overpredicted the dissoln. time by 8.2%, where the effect of undersatn. was considered for a microparticle with zero tension in the surface (zero Laplace pressure) and a range of gas saturations from 70% to 100%. Compared to previous attempts in the literature, this paper more appropriately and accurately tests the Epstein-Plesset model for the dissoln. of a single microbubble and an air-filled microparticle in aq. soln.
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    29. 29
      Epstein, P. S.; Plesset, M. S. On the stability of gas bubbles in liquid-gas solutions. J. Chem. Phys. 1950, 18, 1505– 1509,  DOI: 10.1063/1.1747520
      29
      The stability of gas bubbles in liquid-gas solutions
      Epstein, P. S.; Plesset, M. S.
      Journal of Chemical Physics (1950), 18 (), 1505-9CODEN: JCPSA6; ISSN:0021-9606.
      cf. J. Applied Mechanics 16, 277-82(1949). With the neglect of translational motion of the bubble, approx. solutions can be found for the rate of soln. by diffusion of a gas bubble in an undersatd. liquid-gas soln.; approx. solutions are also presented for the rate of growth of a bubble in an oversatd. liquid-gas soln. The effect of surface tension on the diffusion process is also considered.
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    30. 30
      Plesset, M. S.; Prosperetti, A. Bubble dynamics and cavitation. Annu. Rev. Fluid Mech. 1977, 9, 145– 185,  DOI: 10.1146/annurev.fl.09.010177.001045
      30
      Bubble dynamics and cavitation
      Plesset, Milton S.; Prosperetti, Andrea
      Annual Review of Fluid Mechanics (1977), 9 (), 145-85CODEN: ARVFA3; ISSN:0066-4189.
      A review is given on the bubble dynamics in connection with boiling heat transfer. The discussion includes the processes and results for acoustic cavitatiton and flow cavitation. 166 Refs.
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    31. 31
      Bolaños-Jiménez, R.; Rossi, M.; Fernandez Rivas, D.; Kähler, C. J.; Marin, A. Streaming flow by oscillating bubbles: quantitative diagnostics via particle tracking velocimetry. J. Fluid Mech. 2017, 820, 529– 548,  DOI: 10.1017/jfm.2017.229
      31
      Streaming flow by oscillating bubbles: quantitative diagnostics via particle tracking velocimetry
      Bolanos-Jimenez, Rocio; Rossi, Massimiliano; Rivas, David Fernandez; Kaehler, Christian J.; Marin, Alvaro
      Journal of Fluid Mechanics (2017), 820 (), 529-548CODEN: JFLSA7; ISSN:0022-1120. (Cambridge University Press)
      Oscillating microbubbles can be used as microscopic agents. Using external acoustic fields they are able to set the surrounding fluid into motion, erode surfaces and even to carry particles attached to their interfaces. Although the acoustic streaming flow that the bubble generates in its vicinity has been often obsd., it has never been measured and quant. compared with the available theor. models. The scarcity of quant. data is partially due to the strong three-dimensional character of bubble-induced streaming flows, which demands advanced velocimetry techniques. In this work, we present quant. measurements of the flow generated by single and pairs of acoustically excited sessile microbubbles using a three-dimensional particle tracking technique. Using this novel exptl. approach we are able to obtain the bubble's resonant oscillating frequency, study the boundaries of the linear oscillation regime, give predictions on the flow strength and the shear in the surrounding surface and study the flow and the stability of a two-bubble system. Our results show that velocimetry techniques are a suitable tool to make diagnostics on the dynamics of acoustically excited microbubbles.
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  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.langmuir.1c03171.

    • High-magnification optical micrographs and characterization of crystal shape and size in bulk oleogel network and at oil/air interface (PDF)

    • Dissolution of a wax-coated bubble in oil, imaged with crossed polarizers (MP4)

    • Small-amplitude oscillations (driven by ultrasound at 25 kHz) of a wax-coated bubble in oil (MP4)

    • Large-amplitude oscillations (driven by ultrasound at 25 kHz) of a wax-coated bubble in oil, imaged with crossed polarizers (AVI)

    • Dissolution of a wax-coated bubble after pre-treatment with large-amplitude ultrasound-driven oscillations, imaged with crossed polarizers (MP4)

    • Large-amplitude oscillations (driven by ultrasound at 25 kHz) of a wax-coated bubble showing buckling (AVI)


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