Dynamics of Liquid Oil that Flows Inside Aqueous ... - ACS Publications

Article Cite This: J. Phys. Chem. B XXXX, XXX, XXX−XXX

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Dynamics of Liquid Oil that Flows Inside Aqueous Wet Foam Azusa Kusaka,† Junko Sonoda,‡ Hitoshi Tajima,‡ and Takaya Sakai*,† †

Material Science Research Laboratories, Kao Corporation, 1334, Minato, Wakayama-shi, Wakayama 640-8580, Japan Skin Care Research Laboratories, Kao Corporation, 2-1-3, Bunka, Sumida-ku, Tokyo 131-8501, Japan

‡

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S Supporting Information *

ABSTRACT: Wet soap foam spontaneously imbibes liquid oil without defoaming when it is brought into contact. The kinetics behind this recently observed phenomenon was studied experimentally, with focus on the origin of the suction force and on the oil front dynamics. Using an aqueous foam with an air volume fraction slightly greater than the critical value ϕC, we show that the pumping pressure of oil and/or miscible liquid into the wet foam is attributed to the interfacial distortion of the bubble surfaces. Two distinct regimes along time t were observed in the oil imbibition dynamics. The proceeding oil front evolves with t1/2 dependency in the early imbibition time in accordance with the classical theory of penetration of a porous medium, whereas it departs into t1/3 at late imbibition time. The latter process is attributed to the elongation of an oil branch trapped inside the foam when pumping of the exterior oil has ceased.

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INTRODUCTION Foam is commonly found in various industrial fields, from daily commercial products, such as food, beverages, detergents, and cosmetics, to environmental applications, such as enhanced oil recovery (EOR).1,2 Aqueous foam is a complex colloidal system composed of two phases: a densely packed dispersion of gas bubbles and a surfactant solution surrounding them. Meanwhile, an immiscible third phase, such as oil, may also interact with the aqueous foam. Typical examples are cosmetic cleansers that include soap designed to remove sebum on the skin surface and EOR, where foam is applied to extract oils from the underground pores. Generally, oil is well known for its antifoaming ability. In contrast, an oil drop can be stably trapped in an aqueous foam film withour defoaming when the oil neither spontaneously penetrates nor spreads at the air/ water interface. The penetration and spreading of an oil drop at the air/water interface is characterized by two coefficients defined by the balance of the interfacial tensions at air/water, oil/water, and air/oil interfaces, denoted by γaw, γow, and γao, respectively.3 The entering coefficient, E = γaw + γow − γao, defines the position of the oil drop (i.e., whether it remains inside the aqueous phase or penetrates the air/water interface). The other coefficient is the spreading coefficient, S = γaw − γow − γao, which indicates whether an emerged oil drop spontaneously spreads to cover the air/water interface. In certain circumstances where both E and S are negative, the oil drops dispersed in a surfactant solution are trapped inside the aqueous phase. When the oil-laden solution is foamed, these trapped oil drops can enhance the foam stability.4,5 Meanwhile, studies on the dynamic interactions that occur when aqueous foam contacts oil have been limited to simple © XXXX American Chemical Society

model systems; early examples were the elongation of an oil drop in a single Plateau border microchannel or in a foam film architecture6,7 until the pioneering work of Sonoda et al.8 Through experimental observations of wet soap foam brought into direct contact with various oils, they found that almost all types of oil could spontaneously proceed into the aqueous network of the Plateau borders without causing the foam to rupture. They also pointed out that the rate of the oil imbibition into the foam was directly proportional to the decrease of the dynamic oil/water interfacial tension. Although their study indicates that oil imbibition into aqueous foam is dominated by the adsorption rate of the surfactant molecules at the oil/water interface, the driving force of the oil imbibition remains to be elucidated. The dynamics of oil imbibition into aqueous foam was reported a few years after the publication of the report of Sonoda et al.; however, these studies were performed on very dry foams with liquid fractions close to zero.9,10 Such dry foams consist of a network of well-developed slender Plateau borders and polygonal bubble cells, where the liquid volume between the foam film is negligible. However, foams in commercial products, such as detergents and shampoos, are rather wet. Wet foam contains of a considerable volume of water in swollen Plateau borders; thus, the bubbles are rounder than those in dry foams. In this study, we experimentally investigate the imbibition of oil into the wet foam of a soap solution, focusing on the origin of the driving force and imbibition dynamics. Oil imbibition is Received: June 18, 2018 Revised: August 4, 2018 Published: August 9, 2018 A

DOI: 10.1021/acs.jpcb.8b05819 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B observed even in very wet foam at an air/liquid fraction slightly greater than the critical value. Our observations show that the driving pressure to pump the oil volume inside wet foam is the osmotic pressure of highly concentrated bubbles. Interestingly, two distinct regimes in time t were observed in oil imbibition dynamics: initially, the imbibition length is proportional to t1/2 and consistent with the classical penetration theory of a porous medium, whereas it dissociates into t1/3 in the late imbibition time.

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EXPERIMENTAL SECTION Materials. A stock surfactant solution was prepared by neutralizing a solution of myristic acid (Kao Corp.) and palmitic acid (Kao Corp.) with potassium hydroxide (Asahi Glass Co., Ltd.) while stirring at 70 °C such that a 1:1 mass ratio of the potassium salt of myristic acid to that of palmitic acid was obtained and both were present at 5 wt %. No additives, such as thickening or stabilizing agents, were employed because the foam was sufficiently stable during the period of the experiment. The surface tension of the solution above the critical micelle concentration was determined to be 21.9 mN/m in our previous study.8 A water-soluble dye, Green No. 1 (Kishi Kasei Co., Ltd.), was used to visualize the imbibing surfactant solution. Isopropyl myristate (2-methylethyl tetradecanoate, Kao Corp.) with a viscosity of 6.6 mPa s was used as a model oil in imbibing experiments. Its low oil/water interfacial tension of 0.8 mN/m results in negative E and S values; hence, the oil is trapped inside the aqueous phase of the foam film after contact with an aqueous foam. That is, the oil does not wet the air/ water interface of the foam film as it proceeds. A small quantity of oil-soluble colorant (Oil Orange SS, Tokyo Chemical Industry Co., Ltd.) was added to the oil for visualization of the oil phase. All materials were used as received without further purification. Experiments were conducted at an air-conditioned room temperature of 25 °C. Preparation of Foam. Foam was prepared by stirring 10 g of the stock surfactant solution in a 500 mL beaker using a mixer (7000 rpm, blade size: 1 cm × 2 cm, Pencil Mixer DX, ASONE Co., Ltd.). Foaming levels were varied by changing the stirring time. Subsequently, a foam drop approximately 8 mm in diameter was placed onto a glass slide (76 mm × 26 mm, thickness 1.0 mm, 2926WSLID-P-N, Asahi Glass Co., Ltd.) and four spacers were positioned around the sample (Maitakku Label, ML-120, 8 mm diameter, 0.11 mm thickness, Nichiban Co., Ltd.), which enabled us to observe the foam in two-dimensional (2D), as shown in Figure 1. The foam structure was then observed in detail through a digital microscope (VHX-1000, Keyence Co., Ltd.) with transmitted light. The radius of 200 bubbles was measured using still images to obtain the average bubble size for each foam sample. The initial air volume fraction, ϕ, of the foam samples was determined by measuring the volume V and the mass of the foam M. The air fraction of the foam is derived as

Figure 1. Schematic of the foam. For oil imbibition experiments, oil is injected from the gap between the glass plates by a PIPETMAN instrument.

microscope with a magnification of 100× at a frame rate of 15 frames per second. Still images were obtained from the resulting videos as a function of time after the moment of initial contact. The front position of the imbibing oil phase relative to the foam surface was measured using image analysis (WinROOF v7.0). Imbibition of a miscible surfactant solution into wet foam was also conducted and analyzed using the same procedure.

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RESULTS Foam Structure. Figure 2 shows the microscopy images of the bubbles in the foam of various liquid fractions. Low air content resulted in a bubbly liquid, where large spherical bubbles were dispersed in the bulk liquid phase. As the liquid decreased and the air fraction ϕ increased, the bubbles became smaller and more concentrated. At ϕ > 85%, the air/water interfaces of the bubbles gradually distorted because of densely packed neighboring bubbles. The distortion of the interfaces caused the bubbles to assume polygonal shapes, where each bubble was surrounded by a flat foam film between adjacent bubbles. When the air fraction was further increased to ϕ =91%, the liquid volume in the foam film substantially decreased, the foam consisted of a network of slender Plateau borders, and all bubble cells were polygonal. Oil Imbibition by Wet Foam. Figure 3 shows the imbibition of the oil into the aqueous foam of various air fractions. Refer to the Supporting Information S1 and S2 for videos of the experimental observations. For the aqueous foam with an air fraction less than 80%, the approaching oil stopped as it contacted the foam surface and did not proceed into the foam, as shown in Figure 3a,b, and S1. In contrast, Figure 3c,d, and S2 show that the oil was spontaneously imbibed into the aqueous foam without defoaming when the air fraction of the foam was greater than 85%. The rate of oil imbibition was higher in the drier foam: at 5 s, the oil front distance from the foam surface was 0.7 mm for ϕ = 85% and 1.2 mm for ϕ = 91%. We focus on the foam with ϕ > 85% to further investigate the oil imbibition dynamics. In Figure 4, the position of the imbibing oil front relative to the foam surface was plotted as a function of elapsed time t on a log−log scale. The approaching oil contacted the foam and started imbibition at t = 0. The area weighted average radii of the bubbles were 154, 120, 121, and 102 μm for the foam with air fractions of 85, 88, 89, and 91%, respectively. As evident in Figure 4, the imbibition dynamics were faster in the early time and then collapsed in the late time. The plots were well fitted with t1/2 in the former period and

(M / ρ )

ϕ = 1 − ϕ1 = 1 − V l , where ϕl is the liquid fraction and ρl is the density of the foaming liquid. Imbibition Experiments. The foam samples were brought into contact with immiscible oil following the procedure described in the literature.8 A volume of 50 μL of the oil with colorant added for visualization was carefully injected between the slides using a PIPETMAN instrument. The dynamic contact of the foam and the oil was recorded using a digital B

DOI: 10.1021/acs.jpcb.8b05819 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

Figure 2. Transmitted-light microscopy images of the bubble shapes of the foam samples with air fractions of (a) 75%, (b) 80%, (c) 85%, and (d) 91%. The arrows in (c) show where bubbles are deformed by neighboring bubbles.

Figure 3. Oil imbibition at 5 s after the initial contact of the oil and the foam with the air fraction of (a) 75%, (b) 80%, (c) 85%, and (d) 91%. The arrows indicate the position of the oil front. Oil imbibition was observed only in (c) and (d).

In fact, drainage was observed at the boundary of the foam and the solution. For ϕ > 85%, the solution was imbibed into the foam; as expected, thedrier foam resulted in faster imbibition. Light reflection and diffusion of the imbibing front caused a variation in the reading width of the front length in the case of the miscible solution, although we note that the rate of imbibition of the foaming solution was higher compared to that of oil: the imbibition length into the foam with ϕ = 91% was 1.2 mm in 5 s for the oil, whereas it exceeded 2 mm for the solution.

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DISCUSSION Foam Structure and Air Fraction. As shown in Figure 2, spherical bubbles in the aqueous foam gradually deformed at ϕ > 85% and developed polygonal shapes as ϕ was increased further. For this 2D disordered foam, the shape of the bubble cells is determined by the air-to-liquid ratio, and the critical air volume fraction ϕC is 84%; at ϕ > 84%, the bubbles are jammed and the air/water interfaces are distorted by contact among neighboring bubbles.11 The measured air fraction of bubble distortion was ϕ = 85%, in good agreement with the theoretical value. Driving Force of Oil Imbibition. As shown in Figure 3, the oil was imbibed into the aqueous foam without defoaming when ϕ > ϕC. What is the origin of the driving force responsible for the spontaneous pumping of immiscible oil into the aqueous wet foam? Sonoda et al.8 pointed out that the oil imbibition is not attributable to simple capillary action because the oil does not wet the air/water interface of the Plateau borders as it proceeds. We emphasize that the oil is not solubilized in the water phase during the imbibition because the oil/water interface was distinct and the two phases were obviously separated throughout the experiment.

Figure 4. Oil front position as a function of elapsed time after contact. The front position of the imbibing oil is represented relative to the surface of the foam of each air fraction. The error bars indicate the interval of 95% confidence. The solid and dashed lines correspond to slopes of 1/2 and 1/3, respectively.

with t1/3 in the latter period. Greater air fractions tended to result in faster imbibition; however, imbibition was slow for the driest foam. Miscible Liquid Imbibition by Wet Foam. Figure 5 shows typical images of the foam samples with various air fractions brought into contact with the foaming surfactant solution of the same concentration and with water-soluble dye added for visualization. The imbibition behavior also depended significantly on ϕ, as shown previously for immiscible oil. The approaching solution did not enter the wet foam with ϕ < 80%. C

DOI: 10.1021/acs.jpcb.8b05819 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

Figure 5. Imbibition of surfactant solution 5 s after initial contact with foam with an air fraction of (a) 75%, (b) 80%, (c) 85%, and (d) 91%. The arrows indicate the position of the imbibing front.

Figure 6. Change in bubble shapes of the foam with an initial air fraction of 91% (a) before and (b) after imbibition of the oil phase under transmitted light. After imbibition, the oil (orange in color) fills in the spaces of spherical bubbles that were initially polygonal.

For ϕ < ϕC such that all bubbles are spherical, foam does not imbibe the liquid as in Figures 3 and 5a,b because the osmotic pressure of the foam is relieved. When the foam solution is mixed for a longer period, new air/water interfaces are created by excess shear and the air fraction of the foam increases. The bubbles are no longer spherical at ϕ > ϕC; they are distorted into polygonal shapes because of the dense packing of neighboring bubbles, thereby generating the osmotic pressure. When the oil or miscible liquid contacts the foam in this state, the bubbles reduce their interfacial energy by turning rounder to pump a liquid volume inside under the osmotic pressure, as shown in Figures 3 and 5c,d. The creation of a new interface at the invading oil front would not require substantial work compared to the reduction of the bubbles’ surface area because the oil/water interfacial tension was as low as 0.8 mN/m, whereas the air/water interfacial tension was 22 mN/m. The driving osmotic pressure is magnified for drier foam; therefore, the faster imbibition for greater ϕ in Figure 4 is reasonable. Meanwhile, the slow imbibition for the driest foam of ϕ = 91% is explained by low permeability, owning to its particularly small bubble size (Figure 3). Imbibition Dynamics. As evident in Figure 4, two distinct time regimes were observed during oil imbibition into wet soap foam. For a quantitative argument, we consider a flow in a porous medium. Because the oil/water interfacial tension is extremely low, we assume that the decrement of the oil/water interfacial tension is instantaneous and that the ratedetermining effect is negligible. Another assumption is that the air/water interface of the Plateau border microchannel is immobile because of long-chain fatty acids of myristic and palmitic acid salts;13 hence, the flow is assumed to be zero at the surface of the wall. We note that the oil flow in a Plateau

As previously mentioned and shown in Figure 2, the air/ water interfaces of the bubbles become distorted into polygonal shapes as the air fraction of the foam increases. The interfacial area of these distorted bubbles is extended compared to that of the spherical bubbles of the same volume. According to Princen’s concept of the osmotic pressure of highly concentrated bubbles or emulsions, such foam suctions the liquid into its Plateau borders to minimize the surface area by turning the bubbles rounder.12 Thus, a volume of liquid, either miscible solution or immiscible oil, can be pumped into the foam interior only when the bubbles are polygonal, as observed in Figures 3 and 5. The osmotic pressure decreases as the foam becomes wetter and is relieved at ϕC, where all bubbles are spherical. In our observations, indeed, polygonal bubbles became rounder and the foam film thickened with imbibed liquid after the immersion, as shown in Figure 6. Overall Mechanism of Oil Imbibition by Wet Foam. As Sonoda et al.8 indicated, the oil/water interfacial tension is rapidly reduced when the approaching oil contacts the outermost layer of the liquid film on the foam surface because of excess surfactant molecules. The decrement of the oil/water interfacial tension allows the oil phase to extend into the aqueous foam film and subsequent network of the Plateau borders. Here, the oil is surrounded by a thin layer of water phase, the so-called pseudoemulsion film, which prevents direct contact between the oil and the air/water interface.3 The rate of imbibition should be faster when the dynamic oil/water interface is lower. For miscible liquids imbibing the aqueous foam, no effective interfacial tension exists at the invading front; thus, the imbibition rate is enhanced compared to that of the oil phase.10 D

DOI: 10.1021/acs.jpcb.8b05819 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

Figure 7. Predicted evolution of the oil front on the basis of the classical model of penetration into a porous medium.

Figure 8. Configuration of the oil front at early (t = 60 s) and late (t = 120 and 300 s) times in the foam of the initial volume fraction of 91%. The oil (yellow in color) proceeds from the right to left. At late time, sharp oil branches are developed at the leading front, as indicated by the arrows.

border is slow: typically 300 μm/s at maximum. Given that the size of a Plateau border is approximately 30 μm, the Reynolds number Re is very small (Re ≪ 1), meaning that the flow is governed by the balance of the driving pressure and the viscous dissipation. Darcy’s law relates the mean velocity in the Plateau border microchannels ⟨uPB⟩ to the average velocity of the fluid in the foam um as u ⟨uPB⟩ = m ϕl (1)

Π = Patm

(3)

where R is the area weighted average bubble radius and ϕCl=1ϕC is the critical liquid fraction of the foam (0.16 for a 2D disordered foam).14 Also, the permeability of the foam is estimated using the cross section s of the Plateau border and a geometric constant KC as11 k foam =

and k ΔP um = − foam η L

ÑÉÑ ÅÄÅ 1/2 ÑÑ γaw (1 − ϕl)1/2 ÅÅÅijj ϕCl yzz ÅÅjj zz − 1ÑÑÑ −p= ÑÑ 1/2 Å R (1 − ϕCl) ÅÅÅjk ϕl z{ ÑÑ ÑÖ ÅÇ

sK Cϕl 3

(4)

The cross section of the Plateau border s in 2D foam is related to measurable values of the area of the bubble A and the liquid fraction of the foam as s ∼ Aϕl/2. The value of KC = 0.02 for a Plateau border with an immobile surface is duplicated because 2D foam confined between the plates has two Plateau border cross sections on its unit face as a flow channel. The position of the oil front derived using the aforementioned relations is plotted in Figure 7 for various liquid fractions of foam. As shown in the figure, the classical model of penetration in a porous medium with t1/2 dependency is in good agreement for the early time dynamics of oil imbibition into wet soap foam. As previously noted, in the early time, the foam structure was quite stable: breakage and coalescence of bubbles were rarely observed. A slight discrepancy in the predicted values may arise from the fact that the cross section

(2)

where kfoam is the permeability of the foam, η is the viscosity, ΔP = Patm − p is the driving pressure difference, where Patm and p are the pressure of the atmosphere and the liquid, respectively, and L is the length to which the oil penetrated the foam. The position of the invading oil front at time t is dL obtained by modifying eqs 1 with 2 as ⟨uPB⟩ = dt , yielding the imbibition dynamics, L ∼ t1/2. Using the simple aforementioned relations, we can predict the imbibition length at time t if we know the magnitude of the pressure difference and the foam permeability. For the pressure difference ΔP, we employed the osmotic pressure Π of the foam, which is written as E

DOI: 10.1021/acs.jpcb.8b05819 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B of the Plateau border can be convex or flat rather than concave when the oil undergoes imbibition,15 which could result in different permeability. The observed t1/2 dynamics is consistent with the penetration of a soap solution into the foam under microgravity.16 Similar t1/2 dynamics has been reported for the early times of oil imbibition into three-dimensional dry foam when gravitational effects are negligible; the dynamics eventually fell into t1/4 at late times in their experiment.9 The early time dynamics of t1/2 is followed by t1/3 in the late time regime. To explain this observation, we compare the oil front configurations at early and late times. As shown in Figure 8, the invading oil front is distributed uniformly relative to the foam surface in the t1/2 regime, whereas sharp oil tentacles are branched from the leading frontier at late time. This behavior is reminiscent of the elongation of an oil slug in a single Plateau border observed by Piroird and Lorenceau7 driven by the balance of the viscous dissipation and local Laplace pressure; the rate of elongation of an oil drop along the Plateau border follows a t1/3 law when the volume of the oil is unchanged. Here, the late time imbibition that we observed is reasonably considered a process of elongation of the trapped oil in the foam instead of the suction of the exterior oil. That is, at late time, the pumping osmotic pressure is dismissed in the vicinity of the foam surface, as indicated by the bubbles becoming spherical (Figure 6) because of the imbibed oil phase, whereas interior polygonal bubbles still generate the suction pressure. Under such conditions, the oil trapped in the foam would be extended inward along a Plateau border as though a branch following the t1/3 dependency, whereas oil is no longer supplied into the foam. Although more precise modeling of imbibition dynamics in wet foam is beyond the scope of the present study, we note that numerous factors can be considered for accurate analysis; examples include drainage, swelling of the Plateau border microchannel that may alter the curvature and width of the flow path backward from the imbibition front, and the oil/ water interfacial tension limiting the imbibition rate when it is not negligible. A comprehensive analysis of wet foam imbibition dynamics is expected in future studies.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +81-73-426-5065. Fax: +81-73-426-5067. ORCID

Azusa Kusaka: 0000-0002-9239-6238 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors did not receive specific grant from any funding agency in the public, commercial, or not-for-profit sectors. They thank Dr. Kaoru Tsujii for fruitful discussions.

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REFERENCES

(1) Yamada, H.; Komatsu, H.; Tanaka, M. Influence of Bubble Size on Rheological Properties of Soap Foam. J. Soc. Cosmet. Chem. 1981, 33, 131−140. (2) Farajzadeh, R.; Andrianov, A.; Krastev, R.; Hirasaki, G. J.; Rossen, W. R. Foam−Oil Interaction in Porous Media: Implications for Foam Assisted Enhanced Oil Recovery. Adv. Colloid Interface Sci. 2012, 183−184, 1−13. (3) Denkov, N. D. Mechanisms of Foam Destruction by Oil-Based Antifoams. Langmuir 2004, 20, 9463−9505. (4) Koczo, K.; Lobo, L.; Wasan, D. Effect of Oil on Foam Stability: Aqueous Foams Stabilized by Emulsions. J. Colloid Interface Sci. 1992, 150, 492−506. (5) Lee, J.; Nikolov, A.; Wasan, D. Stability of Aqueous Foams in the Presence of Oil: On the Importance of Dispersed vs Solubilized Oil. Ind. Eng. Chem. Res. 2013, 52, 66−72. (6) Piroird, K.; Lorenceau, E.; Biance, A. -L Oil Repartition in a Foam Film Architecture. Soft Matter 2014, 10, 7061−7067. (7) Piroird, K.; Lorenceau, E. Capillary Flow of Oil in a Single Foam Microchannel. Phys. Rev. Lett. 2013, 111, No. 234503. (8) Sonoda, J.; Sakai, T.; Inomata, Y. Liquid Oil that Flows in Spaces of Aqueous Foam without Defoaming. J. Phys. Chem. B 2014, 118, 9438−9444. (9) Mensire, R.; Piroird, K.; Lorenceau, E. Capillary Imbibition of Aqueous Foams by Miscible and Nonmiscible Liquids. Phys. Rev. E 2015, 92, No. 053014. (10) Mensire, R.; Ault, J. T.; Lorenceau, E.; Stone, H. A. PointSource Imbibition into Dry Aqueous Foams. Europhys. Lett. 2016, 113, No. 44002. (11) Cantat, I.; Cohen-Addad, S.; Elias, F.; Graner, F.; Höhler, R.; Pitois, O.; Rouyer, F.; Saint-Jalmes, A. In Foams: Structure and Dynamics; Cox, S. J., Ed., OUP: Oxford, 2013. (12) Princen, H. M.; Kiss, A. D. Osmotic Pressure of Foams and Highly concentrated Emulsions. 2. Determination from the Variation in Volume Fraction with Height in an Equilibrated Column. Langmuir 1987, 3, 36−41. (13) Golemanov, K.; Denkov, N. D.; Tcholakova, S.; Vethamuthu, M.; Lips, A. Surfactant Mixtures for Control of Bubble Surface Mobility in Foam Studies. Langmuir 2008, 24, 9956−9961. (14) Princen, H. M. Osmotic Pressure of Foams and Highly Concentrated Emulsions. 1. Theoretical Considerations. Langmuir 1986, 2, 519−524. (15) Neethling, S. J.; Morris, G.; Garrett, P. R. Modeling Oil Droplets in Plateau Borders. Langmuir 2011, 27, 9738−9747. (16) Caps, H.; Cox, S. J.; Decauwer, H.; Weaire, D.; Vandewalle, N. Capillary Rise in Foams under Microgravity. Colloids Surf., A 2005, 261, 131−141.

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CONCLUSIONS Wet foam imbibes liquid oil under osmotic pressure due to surface distortion of the bubbles, even at an air fraction slightly greater than the critical value. The imbibition dynamics is well described with the classical penetration theory of a porous medium in the early imbibition time, with a dependency of t1/2, followed by the evolution into t1/3 in the late imbibition time. Our study indicates that oils trapped in a confined space where mechanical force is hardly achieved, such as fatty dirt on fabric and crude oil in underground pores, can be removed or collected effectively using foam with a controlled air volume fraction. This fact might be of great interest in many industrial operations such as cleansing processes and in environmental applications such as EOR and soil remediation.

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for visualization was used as the model oil in this trial (AVI) (AVI)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.8b05819. Oil imbibition into the aqueous foam of fatty acid potassium salt solution with air fractions of (S1) 80% and (S2) 91%; isopropyl myristate with colorant added F

DOI: 10.1021/acs.jpcb.8b05819 J. Phys. Chem. B XXXX, XXX, XXX−XXX