pubs.acs.org/Langmuir © 2009 American Chemical Society
Strain-Induced Droplet Retraction Memory in a Pickering Emulsion Lydiane Becu† and Lazhar Benyahia*,‡ ‡
Polym eres, Colloıdes, Interfaces, UMR 6120 Universit e du Maine-CNRS, Avenue Olivier Messiaen, 72085 Le ¨ Mans, France. † Present address: Laboratoire de Physique des Milieux Denses, Groupe des Fluides Complexes, 1 Bd Arago 57078 Metz, France. Received January 14, 2009. Revised Manuscript Received April 9, 2009 We studied the deformation and relaxation of a water droplet covered with polystyrene latex particles (diameter ca. 200 nm) and embedded in an immiscible fluid after a large strain jump. We show that the presence of the solid particles at the droplet interface slows down the retraction kinetics in comparison with a pure water droplet and induces flow singularity not observed with pure water droplets. The terminal relaxation time of the retraction process, defined as the characteristic time required for the droplet to relax to its spherical equilibrium shape, increases linearly with the applied strain. This result implies a memory effect induced by the presence of solid particles at the droplet interface in a solidstabilized or Pickering emulsion.
1. Introduction 1,2
Since the pioneering works of Pickering and Ramsden, it is well-known that solid particles are very efficient to stabilize emulsions,3 blends,4 and even foams.5 Besides the possibility of replacing organic emulsifiers by less hazardous products, interest in the so-called Pickering emulsions lies in their very high stabilization power. Indeed, centimeter size drops can be stabilized by an adsorbed particle layer.6 Such a feature shows that the underlying stabilizing mechanism differs between systems stabilized with relatively large solid particles and surfactant-stabilized systems.7,8 The strong stabilizing ability of solid particles at the interfaces is often thought to arise from the mechanical protection they offer against droplet coalescence.9,10 Several studies have focused on the mechanical properties of flat interfaces under shear11-13 or of particle-laden droplets subjected to compressions/expansions.14-16 However, very few studies have focused on the dynamics of deformation and/or rupture of particle-coated droplets in a shear flow.17
*Corresponding author.
[email protected]. (1) Sherman, P. Emulsion science; Academic Press: London and New York, 1968. (2) Walstra, P. Fundamentals of interface and colloid science; Elsevier: Amsterdam, 2005; Vol. 5. (3) Pickering, S. J. Chem. Soc., Trans. 1907, 91, 2001–2021. (4) Vermant, J.; Cioccolo, G.; Golapan Nair, K.; Moldenaers, P. Rheol. Acta 2004, 43, 529–538. (5) Horozov, T. S. Curr. Opin. Colloid Interface Sci. 2008, 13, 134–140. (6) Arditty, S.; Schmitt, V.; Giermanska-Kahn, J.; Leal-Calderon, F. J. Colloid Interface Sci. 2004, 275, 659–664. (7) Aveyard, R.; Binks, B. P.; Clint, J. H. J. Colloid Interface Sci. 2003, 100, 503–546. (8) Pieranski, P. Phys. Rev. Lett. 1980, 45, 569–572. (9) Ashby, N. P.; Binks, B. P.; Paunov, V. N. Chem. Commun. 2004, 436–437. (10) Stancik, E. J.; Fuller, G. G. Langmuir 2004, 20, 4805–4808. (11) Cicuta, P.; Stancik, E. J.; Fuller, G. G. Phys. Rev. Lett. 2003, 90, 236101. (12) Reynaert, S.; Moldenaers, P.; Vermant, J. Phys. Chem. Chem. Phys. 2007, 9, 6463–6475. (13) Vella, D.; Aussillous, P.; Mahadevan, L. Europhys. Lett. 2004, 68, 212–218. (14) Asekomhe, S. O.; Chiang, R.; Masliyah, J. H.; Elliott, J. A. W. Ind. Eng. Chem. Res. 2005, 44, 1241–1249. (15) Monteux, C.; Kirkwood, J.; Xu, H.; Jung, E.; Fuller, G. G. Phys. Chem. Chem. Phys. 2007, 9, 6344–6350. (16) Xu, H.; Melle, S.; Golemanov, K.; Fuller, G. Langmuir 2005, 21, 10016–10020. (17) Furbank, R. J.; Morris, J. F. Int. J. Multiph. Flow 2007, 33, 448–468.
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A simple way to study this phenomenon is to consider an isolated droplet embedded in a matrix of another liquid subjected to a flow. In this case, the droplet will increase its length L and decrease its width B in the gradient/velocity plane. Taylor18,19 first related the anisotropy of the deformed droplet, represented by the parameter D = (L - B)/(L + B), to the ratio of viscous to interfacial forces, represented by the capillary number Ca = : γηmR0/Γ, and to the viscosity ratio K = ηd/ηm. L and B (m) are, respectively, the major and the minor axes of the deformed droplet in the velocity/shear gradient plane. ηd and ηm (Pa.s) are the Newtonian viscosity of the droplet and the matrix, respectively, R0 (m) is the initial radius of the droplet, Γ (N/m) : is the interfacial tension, and γ (s-1) the shear rate. The main results on the deformation and breakup of Newtonian droplets imbedded in a Newtonian matrix are summarized in reviews by, e.g., Ralisson, Grace, and Stone.20-22 However, very few results were reported on non-Newtonian systems or droplets with a modified interface. Several studies have focused on the retraction of highly deformed droplets after large strain jumps.23-27 Recently, it has been shown that the relaxation of a deformed droplet submitted to a step strain γ0 is a universal process driven by the droplet geometry.27 The kinetics of the relaxation is characterized by two relaxation regimes, and the transition between them occurs when 1n(L/B) ≈ 0.5 independently of K and γ0. During the first step of the deformation, the droplet is highly deformed (1n(L/B) > 0.5) and looks like a cylinder with spherical ends. The apparent Henky strain of the elongated droplet, εL = 1n(λL), where λL = L/2R0 is the stretching ratio of the droplet, decreases linearly with a characteristic time τ1. When the deformation of the droplet becomes small (1n(L/B) < 0.5), i.e., when it is slightly deformed (18) Taylor, G. I. Proc. R. Soc. London, Ser. A 1932, 138, 41–48. (19) Taylor, G. I. Proc. R. Soc. London, Ser. A 1934, 146, 201–523. (20) Grace, H. P. Chem. Eng. Commun. 1982, 14, 225–227. (21) Rallison, J. M. Ann. Rev. Fluid Mech. 1984, 16, 45–66. (22) Stone, H. A. Ann. Rev. Fluid Mech. 1994, 26, 95–102. (23) Bentley, B. J.; Leal, L. G. J. Fluid Mech. 1986, 167, 241–283. (24) Acrivos, A.; Lo, T. S. J. Fluid Mech. 1978, 86, 641–672. (25) Tjahjadi, M.; Ottino, J. M.; Stone, H. A. AlChE J. 1994, 40, 385–394. (26) Guido, S.; Villone, M. J. Colloid Interface Sci. 1999, 209, 247–250. (27) Assighaou, S.; Benyahia, L. Phys. Rev. E: Stat. Phys., Plasmas, Fluids 2008, 77, 036305.
Published on Web 04/30/2009
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from a spherical shape to an ellipsoidal shape, the Henky strain tends toward zero exponentially with a characteristic decay time τ2.28,29 Moreover, both relaxation times are proportional with a single proportionality constant independent of K and γ0: τ1 = 4.4τ2. τ2 is given by the equation30 τ2 ¼
ηm R0 ð19K þ 16Þð2K þ 3Þ 40ðK þ 1Þ Γ
ð1Þ
Assighaou et al.27 related the universal droplet retraction process to the curvature gradient at the interface, i.e., the gradient of the Laplace’s pressure. Recently, the simulation results of Renardy et al.31 showed good agreement with the previous experimental findings. In this work, we performed the same experiments with the difference that the droplets are now water droplets covered with solid particles and immersed in a liquid PDMS matrix. We recorded the relaxation of the droplets for different applied strains and compared it with the particle-free case. The presence of solid particles at the droplet interface induced important modifications on both morphology and droplet retraction kinetics. In particular, we found a strain-induced memory effect of the droplet retraction process after a large strain jump.
2. Experimental Section The droplet was subjected to strain steps in a homemade counter-rotating shearing device.32 The imposed strain is achieved in less than 0.1 s, which is very small in comparison to the droplet retraction or the particle diffusion at the water/PDMS interface (see below). We thus consider that the droplet undergoes affine deformation when the strain step is applied. The apparatus consists of two glass plates directly attached to two motors. The displacement of the two motors in opposite directions allows the deformed droplet to stay in the visualization frame. A CCD camera (Sony XT-ST50CE) with a zoom (Nachet M002020) was used to image the droplet during its deformation at a 17 frames/s acquisition rate, in combination with a cold light source (Leica 5 CLS150 E). The device allowed observations in both the gradient/velocity and vorticity/velocity planes. A complete description of the device can be found in ref 32. The continuous phase was a silicon oil of viscosity ηm = 100 Pa. s (PDMS 47 V100000, Rhodia), deposited between the glass plates. For all the experiments, the gap between the plates was adjusted to between 4.5 and 5 mm. For particle-free droplets, the dispersed phase was pure water (Millipore) with viscosity ηd = 10-3 Pa.s. For particle-coated droplets, the dispersed phase with viscosity ηd = 10-3 Pa.s was an aqueous suspension containing Cp = 2.5 wt % fluorescent latex microspheres purchased from Polysciences Inc. The particles are polystyrene latex with a radius r = 100 nm (Fluoresbrite Multifluorescent microspheres, ref 24050-5). The polystyrene latex spheres are internally dyed using solvent swelling/dye entrapment. The three highly hydrophobic dyes used remain trapped in the beads in aqueous environments. The contact angle of polystyrene latex spheres, defined in the water phase, is about 175.7 If we consider the density of polystyrene equal to 1.05 (as given by the manufacturer), the number of particles per mL is around 5.7 1012. Thus, a droplet of Ø = 600 μm will count approximately 20 times the amount of particles needed for a close-packed single adsorbed layer. From the (28) Guido, S.; Greco, F.; Villone, M. J. Colloid Interface Sci. 1999, 219, 298–309. (29) Sigillo, I.; Santo, L. d.; Guido, S.; Grizzuti, N. Polym. Eng. Sci. 1997, 37, 1540–1549. (30) Oldroyd, J. G. Proc. R. Soc. London, Ser. A 1953, 218, 122–132. (31) Renardy, Y.; Renardy, M.; Assighaou, S.; Benyahia, L. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, 2009; Accepted. (32) Assighaou, S.; Benyahia, L.; Du, G. P.-L. Rheologie 2007, 11, 45–55.
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Figure 1. Localization of solid particles in a water droplet embedded in a PDMS matrix. The solid particles stack preferentially at the droplet interface. ambient diffusion constant D = kT/6ηπr given by the StokesEinstein relationship, where k is the Boltzmann constant and η is the viscosity of the medium (droplet or matrix), the time td required for a particle to diffuse over its own radius is 6r2/D in water (3D) and 4r2/D at the PDMS/water interface (2D).33 Thus, td ≈ 28 10-3 s in water and td ≈ 1864 s at the PDMS/ water interface. To estimate the time required to achieve a complete coverage of the droplet surface by diffusion, we need to estimate the characteristic distance from the interface. This corresponds approximately to the width lc of a shell close to the interface that contains the number of particles Nc sufficient to completely cover the interface Nc ≈ Cp (R3 - (R - lc)3)/r3.34 For a droplet of radius R = 300 μm, we find lc ≈ 4.7 μm, and the time necessary for complete coverage of the droplet is estimated as 6lc2/D ≈ 61 s. A droplet was formed by injection of the dispersed phase in the PDMS matrix with a microsyringe. The radius of the droplet R0 did not exceed 300 μm so that wall effects on the droplet during the flow could be neglected.35 Due to the very high viscosity of the PDMS phase, buoyancy effects were negligible over the time of the experiments. All experiments were carried out in a room thermostatted at 20 C. Observations of a particle-coated droplet formed in a PDMS matrix were carried out on a confocal microscope (Leica TCSSP2) in the fluorescence mode (Figure 1). The incident light was emitted by a laser beam at 543 nm, which excited the dyes at the surface of particles. The fluorescence light was recorded between 550 and 690 nm. The picture shows that fluorescent particles (in white) are preferentially localized at the interface between the droplet and the matrix, similarly to droplets that constitute Pickering emulsions.
3. Results 3.1. Relaxation of a Pure Water Droplet. We first studied the relaxation of a pure water droplet embedded in a PDMS matrix. This experiment is similar to that carried out in ref 27 for a much lower viscosity ratio K = 10-5 and a larger interfacial tension Γ (given below). After applying a step strain γ0 to the continuous phase, the droplet tilted along the flow direction with an angle θ that remained constant during the whole relaxation process as reported earlier27,36 (Figure 2). For large strain jumps, the highly deformed droplet resembled a flat ellipsoid (Figure 2a). At the first stage of its relaxation, the droplet adopted an (33) Tarimala, S.; Ranabothu, S. R.; Vernetti, J. P.; Dai, L. L. Langmuir 2004, 20, 5171–5173. (34) Borrell, M.; Leal, L. G. Langmuir 2007, 23, 12497–12502. (35) Chan, P. C. H.; Leal, L. G. J. Fluid Mech. 1979, 92, 131–170. (36) Yamane, H.; Takahashi, M.; Hayashi, R.; Okamoto, k. J. Rheol. 1998, 42, 567–580.
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Figure 2. Snapshots of droplets embedded in a PDMS matrix during the retraction after a step strain γ0 = 4.3. Up: pure water droplet at t = 0.5 (a), 1 (b), 2.2 (c), 3.5 (d), and 6 s (e). Down: particle-coated droplet at t = 1.5 (f), 12 (g), 25 (h), 57 (i), and 123 s (j). The contrast is much higher in the latter case, since the particle-coated droplets are opaque.
“eye-like” shape with thin tips (Figure 2b,c) as reported previously (see, for example, ref 22). Quickly, the tips lost their acuteness and the droplet took an ellipsoidal shape (Figure 2d) at the end of the relaxation process. The main axis of the ellipsoid continued to decrease until the droplet returned back to its spherical equilibrium shape (Figure 2e). This type of morphology, often referred as to “slender drop”, has already been reported in experiments and simulations for Ca f ¥ and K f 0, which correspond to the present experimental conditions (see ref 37 and references therein). For a pure water droplet, the total duration of the relaxation lasted more than 6 s in the explored strain range. Here, we did not observe the sphero-cylinder shape reported in ref 27 probably because of the very low viscosity ratio K. However, the evolution of the Henky strain εL with time during droplet retraction, plotted for different applied strains γ0 in Figure 3, allowed for the identification of two successive regimes. The time in Figure 3 is shifted by an arbitrary value d that depends on γ0 to stress the universal character of the clean-interface droplet relaxation. The regimes are indicated in Figure 3: a linear decay of characteristic time τ1 = 0.73 s (left solid line) and an exponential decay of characteristic time τ2 = 0.76 s (right solid line). The transition between these regimes occurred for εL ≈ 0.2 instead of εL ≈ 0.34 reported in the previous study.27 The interfacial tension Γ = 40 mN/m of a clean water-PDMS interface was deduced from the terminal relaxation time τ2 of the pure water droplet through eq 1. This value is within 5% of the one determined by a pendant drop method. 3.2. Relaxation of a Particle-Coated Droplet. Images taken during the relaxation process of a particle-coated droplet subjected to a step strain γ0 = 4.3 are shown in Figure 2. As for the pure water case, initial deformation of the droplet was characterized by a slender shape (Figure 2f). A strong localization of the swelling in the center of the droplet was observed, which led to the formation of much more acute tips of the droplet (Figure 2g,h) showing thus a flow singularity. The droplet took a diamond-like shape that differred strongly from the “eye-like” shape adopted by the pure water droplet. At the end of the relaxation, the droplet became ellipsoidal until it reached the original spherical shape (Figure 2i,j). The total time necessary for the particle-coated droplet to relax back to its equilibrium shape was 126 s, i.e., 20 times longer than for pure water droplets. The evolution of the Henky strain εL during the relaxation process is shown in Figure 4. The inset in Figure 4a compares the evolution of the Henky strain for pure water and particle-coated droplets after a step strain γ0 = 4.3 and shows the strong slowdown induced by the solid particles. An initial linear regime, (37) Favelukis, M.; Lavrenteva, O. M.; Nir, A. J. Non-Newton. Fluid Mech. 2005, 125, 49–59.
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Figure 3. Relaxation of a pure water droplet embedded in a PDMS matrix for several step strains γ0. The solid lines represent, respectively, the initial linear (left) and terminal exponential (right) relaxation processes. The horizontal lines indicate the transition range between the two regimes.
identical for all applied strains γ0, was identified (solid line in Figure 4a). As a consequence, the curves could be superimposed in that regime by shifting the time scale with an arbitrary value d. The characteristic time τ1 was 22 s, which means that the relaxation process in this regime was 30 times slower in the presence of particles. The exponential decay observed toward the end of the relaxation process had a characteristic time τ2 that depended on the applied step strain γ0 (Figure 4b). In this regime, the Henky strain curves could not be superimposed. Figure 4b points out that τ2 increased with γ0: the higher the initial deformation of the droplet, the slower the terminal relaxation process. The terminal relaxation times τ2 are reported in Figure 5 as a function of γ0 for particle-coated (closed circles) and pure water (open circles) droplets. As mentioned previously, τ2 = 0.76 s was independent of γ0 in the case of pure water, whereas τ2 increased linearly from τ2 = 3 to τ2 = 18 s for the particle-coated droplet (Figure 5). For all applied step strains γ0, the terminal exponential relaxation was always slower in the presence of particles.
4. Discussion In this work, we have shown that the relaxation process of a water droplet immersed in a PDMS matrix is significantly affected by the presence of solid particles at the water/PDMS interface. The presence of particles induces (1) a change in the droplet morphology which adopts a diamond shape at intermediate relaxation times and exhibits a flow singularity; (2) a slowdown of the relaxation kinetics (in particular, an increase of the initial relaxation time τ1 from 0.73 to 22s); and (3) a linear increase of the terminal relaxation time τ2 with the applied strain γ0. In the Langmuir 2009, 25(12), 6678–6682
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Figure 4. Relaxation of a particle-coated droplet embedded in a PDMS matrix for several step strains γ0. (a) Evolution of the apparent Henky strain on a linear scale. The solid line represents the linear relaxation of characteristic time τ1. The inset shows a comparison of a pure water droplet (squares) and a particle-coated droplet (circles) for a strain γ0 = 4.9. (b) Evolution of the apparent Henky strain on a semilogarithmic scale. The solid line represents one of the exponential relaxations of characteristic times τ2.
Figure 5. Exponential relaxation time τ2 as a function of the
applied strain γ0 for a pure water (open circles) and a particlecoated droplet (close circles). Evolution of the interfacial area (square) normalized by the initial droplet area of a particle-coated droplet versus the applied strain. The straight lines are guides to the eye.
following, we will discuss the possible origins of the kinetics effects. Figure 1 shows that solid particles are preferentially localized at the interface, which can reduce the gradient of pressure across the interface15 driving the shape relaxation.27 Moreover, adsorbed particles can induce a transition from a fluid interface only characterized by an interfacial tension toward a solid-like particle film that possesses an elastic modulus.6,11-13,38 Elasticity of the solid-like film can counteract the relaxation of the droplet driven by interfacial forces and thus delay the kinetics of the retraction process. A surprising feature is that the droplet appears to retain some memory of its initial deformation, since the kinetics of the terminal relaxation depended on the applied strain γ0. A similar observation has been also reported by Macaubas et al.39 in the case of a liquid crystalline polymer droplet embedded in a PDMS matrix. They explained this phenomenon by a gradual change of the rheological properties of the liquid droplet induced by its deformation under the external applied strain. In our case, this dependency suggests that some properties of the particle-coated interface involved in the relaxation mechanism are modified by γ0. The deformation of a droplet induces an (38) Arditty, S.; Schmitt, V.; Lequeux, F.; Leal-Calderon, F. Eur. Phys. J. B 2005, 44, 381–393. (39) Macaubas, P. H. P.; Kawamoto, H.; Takahashi, M.; Okamoto, K.; Takigawa, T. Rheol. Acta 2007, 46, 921–932.
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increase of the water-PDMS interface: for strains γ0 between 0.24 and 4.3, the projected area of the deformed droplet, deduced from observations in the velocity/vorticity plane, increases linearly by a factor of 20 (Figure 5), while in the meantime, the total relaxation time increases from 65 to 126 s. This increase of the interfacial area is likely to lead to modifications of the particle layer: in particular, particles suspended in the volume of the water droplet could adsorb at the newly created water-PDMS interface. The adsorption of particles on the newly created interface is limited by the migration of particles to the interface. The characteristic time for particles to self-diffuse to the interface and completely cover the equilibrium droplet surface has been estimated as 61 s. However, the flow initiated by the droplet deformation and the strong elongation of the droplets that will confine the particles closer to the interface might accelerate the migration process and thus the adsorption of particles at the interface. Adsorption cannot be considered an instantaneous process (especially on an extended surface obtained at higher strains), and we cannot rule out that a complete coverage of the newly created interface with particles could be kinetically limited. The time available for the particles to adsorb at the excess interface increases with γ0, which favors an increase of the particle coverage. As the droplet relaxes back to its spherical shape, the water/PDMS interface, and thus the area available per particle, decreases again. In the case where particles stay strongly attached to the interface, compression of the particle layer occurs and that induces an increase of the particle surface coverage. This densification of the particle layer will become more effective as the particle coverage at the start of the relaxation process is higher, i.e., as γ0 is larger. For Henky strains sufficiently small, the particle surface coverage will notably increase with γ0. The subsequent decrease of the interfacial tension Γ15 with γ0 will lead to an increase of both τ1 and τ2 via eq 1. Moreover, since the elastic modulus increases with the surface coverage,6,11-13,38 the retraction of the droplet will be delayed more effectively as γ0 increases. The increase of the layer density may be possible because partial crystallization of an additional number of particles confined in the vicinity of the interface occurs.38 Recently, several studies have described particle-laden droplets that never relax to a spherical shape after a deformation because the compression of the particles creates a rigid layer40,41 able to maintain a (40) Bon, S. A. F.; Mookhoek, S. D.; Colver, P. J.; Fischer, H. R.; van der Zwaag, S. Eur. Polym. J. 2007, 43, 4839–4842. (41) Studart, A. R.; Shum, H. C.; Weitz, D. A. J. Phys. Chem. B 2009, 113, 3914–3919.
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nonspherical shape. However, in our experiments the droplet always retracts back to its spherical equilibrium shape and no buckling transition of the interface was observed, which shows that the particle layer does not reach a close-packing configuration.13-16 This suggests that, in our experiments, the adsorption of particles on the newly created interface at the first stage of the deformation has been limited. Another possibility is that all particles may not stay adsorbed at the interface. The energy required to remove a particle from the interface is given by E = rπ2Γ(1 ( cos θ)2, where the sign in the brackets is negative (respectively positive) for desorption in PDMS (respectively water). However, due to the very high viscosity of the PDMS phase and to the limited existence time of the interfacial area created during the deformation of the droplet, the contact angle θ of the particles adsorbed on the newly created interface is likely to be very small despite the hydrophobic character of the particles. The energy of detachment may be similarly too low to prevent their desorption into the water phase during the relaxation process.8 Moreover, the ability of a low dielectric constant medium-water interface to bind nonwetting particles through image charge interactions has been recently demonstrated.42 In that case, the particles sit at the interface and are not attached to it. Such a phenomenon could occur in our system due to the large dielectric constant difference between PDMS and water. The presence of desorbing particles may still induce a slowdown of the relaxation process. As γ0 increases, more particles are adsorbed and the time available for their adsorption to take place increases. As a consequence, their contact angles, and thus energy of attachment, should (42) Leunissen, M. E.; van Blaaderen, A.; D. Hollingsworth, A. D.; Sullivan, M.; Paul M. Chaikin, P. M. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 2585–2590.
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increase, so that the relaxation of the droplet slows down with increasing γ0. In all cases, interfacial modifications induced by the application of an increasing strain led to a slowdown of the relaxation process. The linear increase of the interfacial area with γ0 is in agreement with the linear increase of τ2 with γ0. This suggests that the interfacial changes described are proportional to the waterPDMS interface area at the beginning of the retraction process and thus, as a first approximation, to the initial number of solid particles present at the water/oil interface. The presented results reveal an interesting case where the adsorption/desorption of the particles is counterbalanced by the droplet retraction. The adsorption of the solid particles should increase the relaxation time of the droplet retraction, and this may increase with the applied strain. The retraction of the droplet favors the particle desorption and prevents the layer from buckling and/or retraction arrest.
5. Conclusions The relaxation of particle-coated droplets was studied after different shear strain jumps. The presence of solid particles at the interface induces a strong modification of the droplet morphology as well as a slowing down of the relaxation kinetics. The results suggest a probable modification of the particle layer during the initial deformation of the droplet, which creates a dependency of the terminal relaxation time τ2 on γ0. A change of interfacial rheological properties and/or Laplace’s pressure inside the droplet could account for the slowdown of the kinetics (up to a factor of 30). The observed linear increase of the terminal relaxation time τ2 with γ0 implies a strain-induced memory effect during the droplet retraction process.
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