pubs.acs.org/Langmuir © 2010 American Chemical Society
Shear-Induced Instabilities in Oil-in-Water Emulsions Comprising Partially Crystallized Droplets Florence Thivilliers-Arvis,† Eric Laurichesse,† Veronique Schmitt,*,† and Fernando Leal-Calderon*,‡ †
Centre de Recherche Paul Pascal, Universit e Bordeaux 1, CNRS, Avenue du Dr Albert Schweitzer, 33600 PESSAC, France, and ‡Laboratoire TREFLE, Universit e Bordeaux 1, CNRS, Avenue des Facult es, 33405 TALENCE, France Received July 7, 2010. Revised Manuscript Received September 8, 2010
We produced triglyceride-in-water emulsions comprising semicrystallized droplets, stabilized by a mixture of protein and low molecular weight surfactant. In these systems, partial (unrelaxed) coalescence could be produced by a thermal treatment referred to as tempering or by the application of a shear. Both primary emulsions and thermally induced gels were submitted to shear strains of variable amplitude, and the resulting transitions were identified. Partial or total destruction of the materials took place and was revealed by the formation of macroscopic clumps. We examined the impact of the initial average droplet size and of the interface composition (controlled by the bulk surfactant-to-protein molar ratio) on the sensitivity to partial coalescence. The evolution under shear occurred via two limiting mechanisms, depending on the susceptibility to partial coalescence. Materials that exhibited fast partial coalescence underwent gelling followed by phase inversion and partial expulsion of the aqueous phase. Alternatively, when the rate of partial coalescence was quite low, large clumps were randomly distributed over the volume and coexisted with a fluid emulsion. The same phenomenology was observed under both oscillatory and steady shear conditions. Interestingly, in oscillatory conditions, clumping was observed above a very well-defined and reproducible value of the strain amplitude independent of the initial state of the system (emulsion or gel).
Introduction Emulsions are metastable materials made of two immiscible fluids with a widespread range of applications, with the most important ones including cosmetics, foods, detergency, adhesives, coatings, paints, surface treatment, road surfacing, and pharmaceutics. Once fabricated, emulsions irremediably evolve toward the total phase separation of the two fluids under the effect of coalescence and Ostwald ripening.1 It is well-known that the presence of crystals in the dispersed phase of oil-in-water (O/W) emulsions can cause specific instabilities.2,3 In the melted state, the spherical and smooth shape of the dispersed droplets is controlled by surface tension. Upon cooling, the surface becomes rough due to the formation of irregularly shaped/oriented crystals which can protrude into the continuous phase. When two semicrystalline droplets approach, the protruding crystals can pierce the thin film *To whom correspondence should be addressed. E-mail:
[email protected] (F.L.-C.);
[email protected] (V.S.). (1) Bibette, J.; Leal-Calderon, F.; Schmitt, V.; Poulin, P. Springer Tracts in modern Physics: Emulsion Science. Basic principles. An overview; Springer-Verlag: Berlin, 2002. (2) Fredrick, E.; Walstra, P.; Dewettinck, K. Adv. Colloid Interface Sci. 2010, 153, 30. (3) Leal-Calderon, F.; Thivilliers, F.; Schmitt, V. Curr. Opin. Colloid Interface Sci. 2007, 12, 206. (4) Boode, K.; Walstra, P. Colloids Surf., A 1993, 81, 121. (5) Boode, K.; Walstra, P.; Degrootmostert, A. E. A. Colloids Surf., A 1993, 81, 139. (6) van Boekel, M.; Walstra, P. Colloids Surf. 1981, 3, 109. (7) van Aken, G. A. Colloids Surf., A 2001, 190, 333. (8) Giermanska-Kahn, J.; Laine, V.; Arditty, S.; Schmitt, V.; Leal-Calderon, F. Langmuir 2005, 21, 4316. (9) Vanapalli, S. A.; Coupland, J. N. Food Hydrocolloids 2001, 15, 507. (10) Thivilliers, F.; Drelon, N.; Schmitt, V.; Leal-Calderon, F. Europhys. Lett. 2006, 76, 332. (11) Thivilliers, F.; Laurichesse, E.; Saadaoui, H.; Leal-Calderon, F.; Schmitt, V. Langmuir 2008, 24, 13364. (12) Boode, K.; Bisperink, C.; Walstra, P. Colloids Surf. 1991, 61, 55.
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between the two droplets, causing a coalescence event.2-15 When a crystal penetrates the second droplet, it will preferably be wetted by the oil rather than by the aqueous phase. The oil will start to flow around the crystal, reinforcing the link until an oil neck enclosing the crystal is created between the two droplets. If the crystallized fraction within the droplets is sufficient, the intrinsic rigidity inhibits relaxation to the spherical shape driven by surface tension. As a consequence, the droplets keep remnants of their original shape after each coalescence event. As time passes, large clusters may grow by the accretion of any other primary droplet or cluster until a rigid gel network is formed. Large surface-active species such as proteins are highly efficient in stabilizing partially crystallized droplets against partial coalescence. The emulsions then exhibit long-term kinetic stability in quiescent storage conditions and can even be submitted to high shear forces without being destroyed. Instead, surfactants with short molecular chain length generally do not provide a sufficient degree of stabilization, and the emulsions may become lumpy a few hours after their preparation. Addition of small amounts of surfactant in protein-stabilized emulsions is a common way to monitor the sensitivity toward partial coalescence.2,11 The displacement of proteins from the oil interface by added surfactants contributes to emulsion instability and induces sensitivity to partial coalescence. In recent articles,10,11 we examined the sensitivity to partial coalescence of initially monodisperse emulsions in quiescent conditions. The dispersed phase was composed of crystallizable (13) Golemanov, K.; Tcholakova, S.; Denkov, N. D.; Gurkov, T. Langmuir 2006, 22, 3560. (14) Giermanska, J.; Thivilliers, F; Backov, R.; Schmitt, V.; Drelon, N.; LealCalderon, F. Langmuir 2007, 23, 4792. (15) Vanapalli, S. A.; Palanuwech, J.; Coupland, J. N. Colloids Surf., A 2002, 204, 227.
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oils whose melting domain extended over several tens of degrees. Oil-in-water emulsions stabilized by a mixture of proteins and low molecular weight surfactants were fabricated at high temperature and quenched at 4 °C. After a period of storage sufficient to achieve oil crystallization, the systems were warmed at a temperature Tp (10 °C < Tp < 40 °C) for a short delay to produce partial melting of the oil, and cooled again at 4 °C. Such treatment referred to as temperature cycling or “tempering” provoked partial coalescence, and the initially fluid emulsions turned into strong gels. It was shown that the coalescence rate and thus the gelling kinetics were controlled by the oil solid fraction, the average droplet size, and the surfactant-to-protein molar ratio. Shear forces due to processing may induce dramatic and irreversible changes in the structure of materials comprising semisolid droplets. For instance, the fabrication of viscoelastic aerated foods such as whipped creams or ice creams is based on the application of intense mechanical agitation which promotes the formation of an interconnected droplet network.7,16 Shearinduced partial coalescence is also exploited to separate fat from the aqueous phase of native dairy creams in order to obtain butter. Many cosmetic products are oil-in-water emulsions containing semicrystalline waxes, and the texture of such materials is controlled by the instabilities occurring under shear. In general, the transformations are only empirically controlled and there is still insufficient knowledge about the evolution scenario depending on the formulation, temperature, and shear history of the sample. Understanding the exact nature of the transitions occurring under shear requires at least four types of information: (i) the rheological conditions (shear history), (ii) the sensitivity to partial coalescence, (iii) the scenario, that is, the evolution of the structure over time, and (iv) the final state of the system. Many papers in the literature have reported destabilization of emulsions made of partially and/or fully crystallized droplets submitted to shear. Most of them deal with the behavior under steady flow con_ or a shear stress ditions.1,2,8,17-21 Typically, either a shear rate (γ) (σ) ramp was applied to emulsions containing triglyceride crystals and the kinetic evolution of the viscosity was registered.18-21 Destabilization due to partial coalescence was observed above some critical value of γ_ or of σ and was reflected by a sudden jump of the viscosity. Constant stress18 or constant shear rate21 experiments have also been carried out, and it was observed that, after an induction period, a sharp increase in the viscosity took place. In this latter case, the shear-sensitivity was characterized by the duration of the induction period. The evolution of the emulsion structure was evidenced by different techniques including droplet sizing,2 back light-scattering,20 and direct microscope imaging.21 The diversity in terms of experimental approaches, materials design (from simple to multiple emulsions), and shear history make it to difficult to compare results and to draw general conclusions about the relation between the rate of partial coalescence and the evolution scenario. The main objective of this paper was to provide insight into how semisolid oil-in-water emulsions which have otherwise longterm stability during quiescent storage may become destabilized by partial coalescence when subjected to moderate shear flows. We were able to tune the rate of partial coalescence at rest by (16) Drelon, N.; Gravier, E.; Boisserie, L.; Omari, A.; Leal-Calderon, F. Int. Dairy J. 2006, 16, 1454. (17) Boode, K. Partial coalescence in oil-in-water emulsions. Ph.D. Thesis, Wageningen, University, 1992 (18) Davies, E.; Dickinson, E.; Bee, R. Food Hydrocolloids 2000, 14, 145. (19) Hinrichs, J.; Kessler, H. J. Food Sci. 1997, 62, 992. (20) Xu, W.; Nikolov, A.; Wasan, D. J. Food Eng. 2005, 66, 97. (21) Guery, J.; Bertrand, E.; Rouzeau, C.; Levitz, P.; Weitz, D. A.; Bibette, J. Phys. Rev. Lett. 2006, 96, 198361.
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Article Table 1. Formulation and Process Parameters for the Preparation of Mother Emulsions Stabilized by Sodium Caseinatea average diameter 5 7 10 12 14 15 16 22 28 34 D ((0.5 μm) oil fraction (wt %) 85 85 85 80 70 70 70 70 70 70 shear rate (103 3 s-1) 12 10.5 5.2 5.2 13 12 10 6 4.5 3.5
a The concentration of sodium caseinate in the aqueous phase was equal to 10 wt %, and the shearing time was set at 10 s for all the emulsions.
varying the temperature, the average droplet diameter, and the interfacial composition. We established a mapping of the shear instabilities over a large range of experimental conditions, always using the same emulsion type and following to the same shear protocol. That way, a straightforward comparison of the data was possible. The state of the system was assessed by visual inspection, observations under the microscope, droplet sizing, and rheological measurements. We identified two limiting destruction phenomena, and we discussed how they are related to the sensitivity to partial coalescence.
Materials and Methods Emulsion Preparation. The emulsions were of the oil-inwater type. The dispersed phase was partially crystallized at room temperature. We used natural anhydrous milk fat provided by Flechard (France) (density ≈ 0.9 g.cm-3) which is composed of a mixture of triglycerides with a wide range of melting temperatures from -40 °C to þ40 °C.11,22-24 We adopted that oil because the crystallized fraction could be tuned easily and continuously with temperature, with the matrix always remaining homogeneous.22-24 The emulsions were stabilized by sodium caseinate (Mw ≈ 23 300 g 3 mol-1), purchased from Aldrich, and by the nonionic surfactant Tween 20, from Fluka (Mw = 1228 g 3 mol-1, critical micellar concentration ≈ 8.10-5 mol 3 L-1 ≈ 10-2 wt % at T = 25 °C). We first prepared crude polydisperse emulsions stabilized by sodium caseinate alone, by dropwise incorporation of the liquefied oil into the aqueous phase containing 10 wt % sodium caseinate at 45 °C under manual shaking. Quasi-monodisperse emulsions were obtained by shearing the polydisperse ones within a narrow gap (100 μm) in a Couette cell (concentric cylinders’ geometry, Ademtech, France) at 45 °C for 10 s. The final droplet size was determined by the shear rate, the interfacial tension, and the viscosity ratio between the dispersed and continuous phases.25 Thus, the shear rates and initial formulations were adapted to obtain average droplet sizes ranging from 2 to 34 μm. All details concerning the composition and average diameter of the mother emulsions are provided in Table 1. Daughter emulsions were obtained by diluting the mother emulsions with an aqueous solution containing Tween 20 and/or sodium caseinate at T = 45 °C. That way, we could control the overall concentrations of protein and surfactant, the mass fraction of dispersed oil, φ, and the average drop size, D. In all experiments, φ was equal to 45 wt %. At that mass fraction, the shear instabilities occurred within relatively short periods of time ranging from minutes to hours. The mixture of surface-active species was characterized in terms of the bulk surfactant-to-protein molar ratio, Rm, which sets the interfacial composition. All the prepared emulsions contained 0.3 wt % sodium azide (NaN3 from Aldrich) in the aqueous phase as a bactericide agent. Crystallization. The same crystallization conditions as in ref 11 were adopted. The emulsions were always cooled down to 4 °C in a thermostatically controlled chamber in order to induce partial crystallization of the oil droplets. Crystallization of milk (22) Lavigne, F.; Ollivon, M. OCL 1997, 4, 212. (23) Lopez, C.; Bourgaux, C.; Lavigne, F.; Lesieur, P.; Ollivon, M. J. Dairy Sci. 2001, 84, 756. (24) Lopez, C.; Lavigne, F.; Lesieur, P.; Bourgaux, C.; Ollivon, M. J. Dairy Sci. 2001, 84, 2402. (25) Mabille, C.; Leal-Calderon, F.; Bibette, J.; Schmitt, V. Europhys. Lett. 2003, 61, 708.
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Figure 1. Proportion of solid oil as a function of temperature. fat is a slow process involving variable polymorphic forms. Metastable soft crystals first appear at short times. Then, they progressively evolve toward stable and rigid crystalline forms over a characteristic time scale of 10 h.23,24 This is why our emulsions were always stored at low temperature for at least 15 h. The proportion of solid oil in the end of the crystallization process was of the order of (57 ( 3)% as deduced from NMR measurements (Figure 1).10,16 The samples could be stored at 4 °C for several weeks without undergoing any type of instability. Optical Microscopy. The emulsions were directly observed with an inverted optical microscope (Zeiss Axiovert X100, resolution of 200 nm) equipped with a digital camera (SONY) and a homemade Peltier module for observations at 4 °C. Emulsion Characterization. The size distributions of the emulsions were deduced from static light-scattering measurements performed at 45 °C with a Mastersizer 2000 Hydro SM device (Malvern) using Mie theory. The aqueous phase used for dilution contained sodium dodecyl sulfate from Sigma-Aldrich at the critical micellar concentration (8 10-3 mol 3 L-1). This surfactant was used because of its ability to dissociate protein aggregates that might reversibly bridge emulsion drops.16 The emulsions were characterized by their volume-averaged diameter defined as: D = (ΣiNiDi4)/(ΣiNiDi3), where Ni is the total number of droplets with diameter Di. Tempering Protocol. Tempering was applied to some emulsions immediately after loading them in a rheometer’s cell (see below). Starting from 4 °C, the samples were warmed up to 25 °C at þ5 °C 3 min-1, held at 25 °C during a variable period of time tp (from 1 to 35 min), and finally cooled down to 4 °C at -5 °C 3 min-1. Such treatment resulted in the formation of gels, and by varying the holding time tp we could trigger the final gel elasticity, G0 gel. The gels were kinds of percolated materials resulting from the spontaneous formation of irreversible interdroplet bonds due to partial coalescence, and the final elasticity was influenced by the bulk concentration of such irreversible bonds.10,11 Shear Protocol. The samples (either primary emulsions or thermally induced gels) were sheared by means of a strain-controlled rheometer (ARES-LS TA Instruments). The experiments were carried out using two distinct geometries: either parallel-plate (gap = 1 mm, diameter of the rotating plate = 50 mm) or coneplate (gap = 50 μm, cone angle = 0.04 rad, cone diameter = 50 mm). The surfaces were made rough in order to avoid wall slipping, and the cells were equipped with an anti-evaporating device. Both steady flow and oscillatory experiments were conducted. Experiments in steady flow conditions were carried at constant _ In a typical oscillatory experiment, a shear strain shear rate γ. characterized by its amplitude γ0 and its pulsation ω was applied to the sample and the resulting shear stress σ was measured. As long as the strain amplitude belongs to the linear regime, the measured 16784 DOI: 10.1021/la1027288
Thivilliers-Arvis et al. stress is sinusoidal and characterized by its amplitude σ0 and by the same pulsation ω with a phase shift δ relative to the strain. The elastic G0 and loss G00 moduli defined as G0 = σ0/γ0 cos(δ) and G00 = σ0/γ0 sin(δ) are representative of the stored and dissipated parts of the energy, respectively. In the linear regime, these two parameters fully characterize the sample viscoelasticity. Above a threshold value of the applied strain, the deformation may perturb or modify the structure, the material begins to flow, and G0 and G00 become dependent on the applied strain. The materials were submitted to shear at constant temperature, Td. We sometimes observed partial expulsion of solid oil from the rheometer’s gap under continuous flow especially when the shear rate γ_ was larger than ∼1 s-1. This is why most of the experiments were conducted in oscillatory conditions. The parallel-plate geometry was adopted because of the large gap (1 mm) in order to avoid problems related to confinement. The temperature was raised from 4 °C to Td (4 °C < Td < 40 °C) at þ20 °C 3 min-1. After 60 s at Td, a sinusoidal strain, γ(t) = γ0,p cos(ωt), out of the linear regime (γ0,p > 0.1%) was applied during a well-defined period of time, τ. Due to the parallel-plate geometry, the local strain imposed by the rheometer was proportional to the radial position r and γ0,p was actually the highest strain amplitude achieved at the periphery of the rotating plate. The applied strain amplitude γ0(r) at a distance r from the center was given by γ0(r) = γ0,p(r/rp), where rp is the radius of the rotating plate (=25 mm). After the deformation step, the sample was cooled down to 4 °C at -20 °C 3 min-1 and finally the rotating plate was slowly removed in order to visually assess the final state of the material.
Results and Discussion We propose to describe the impact of several parameters (average droplet size, temperature, Rm) under flow conditions, based on the knowledge gained under quiescent conditions.10,11 We first examine the behavior under shear of a primary emulsion and of thermally induced gels deriving from it, at Td =25 °C and at constant initial diameter D=(15 ( 0.5) μm. Figure 2 shows a microscope image and the droplet size distribution of the primary emulsion. In ref 11, it was demonstrated that the emulsions at rest became unstable at 25 °C and that the kinetics of partial coalescence could be triggered by the surfactant-to-protein bulk ratio, Rm, at constant initial droplet diameter. For D = 15 μm and Td = 25 °C, the rate of partial coalescence in quiescent conditions increased dramatically over a narrow composition interval, 0.2 < Rm < 1. The experiments described in sections 1 and 2 aim at determining the shear-induced evolution scenario under low (Rm=0) and fast (Rm=2.7) coalescence rates. 1. Fast Partial Coalescence (Rm = 2.7; Td = 25 °C). The overall contents of the surface-active species relative to the aqueous phase were 0.5 wt % for Tween 20 and 3.5 wt % for sodium caseinate, corresponding to Rm = 2.7. The systems were submitted to oscillatory strains of variable amplitude from 0.1 to 1100% (maximum attainable value), for a period of time, τ, ranging from 2 to 300 min. The materials did not exhibit any visible sign of destabilization when the strain amplitude, γ0,p, was lower than ∼300%. However, above that characteristic value of γ0,p and after an induction period, we observed a spectacular evolution of the texture. Large macroscopic clumps reminiscent of the semisolid oil phase used to fabricate the emulsions were present at the periphery of the rheometer’s plate. Figure 3 is a snapshot showing the characteristic aspect of the materials. The central part was milky, while the periphery comprised macroscopic yellowish clumps surrounded by a clear aqueous phase (not visible in the image). Some clumps remained stuck on the bottom plane, which explains the presence of large voids. In order to probe the structure of the different zones, an aqueous drop containing a hydrophilic bluish dye (methyl blue from Sigma-Aldrich at 0.5 wt %) was gently deposited at two different locations of the sample surface: on the Langmuir 2010, 26(22), 16782–16790
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Figure 2. (a) Microscope image of the primary emulsion diluted to facilitate the observation (scale bar = 50 μm); (b) size distribution of the same emulsion.
Figure 3. Final aspect of a gel submitted to a sinusoidal strain. Initial elastic modulus: G0 gel = (25 ( 2) 103 Pa (D = 15 μm; tp= 15 min); φ = 45 wt %; ω = 1 rad 3 s-1; γ0,p = 400%; Td = 25 °C; τ = 20 min. Aqueous phase composition: 0.5 wt % Tween 20; 3.5 wt % sodium caseinate (Rm = 2.7). The dotted circle represents the boundary between the gelled and clumped zones. In this example, rp = 25 mm and r* = (17 ( 1.5) mm. Threshold strain: γ* 0 = (270 ( 20)%.
central white part and on a macroscopic yellowish clump (Figure 4a). The dye was used as a diffusive tracer to probe the continuity of the aqueous path. The drop deposited in the central part spread instantaneously, and the diffusive expansion of the bluish spot was already significant after 10 min (Figure 4b). Moreover, diffusion also progressed in the vertical direction, through the entire thickness, since the spot was visible at the opposite side when the sample was turned upside-down. It can therefore be deduced that the central part of the sample is a water-continuous gel. This was confirmed by observations under the microscope (Figure 5) which revealed the presence of large clusters in which the initial droplets remained recognizable. In contrast, the stained drop deposited on the macroscopic yellowish clump neither spread laterally nor diffused vertically over time (Figure 4c and d), proving that this part of the sample was no longer water-continuous. The clumps were warmed at 45 °C and observed under the microscope, revealing a significant amount of water droplets dispersed within the oil continuous phase. As stated above, the clumps were surrounded by a small amount of a transparent aqueous solution, proving that both phase inversion and phase separation occurred at the sample periphery. The boundary between the gelled and the clumped zones was observed at a relatively well-defined radial position r* Langmuir 2010, 26(22), 16782–16790
(Figure 3), suggesting that the clumped state was attained above some threshold value, γ0*, of the strain amplitude given by γ0* = γ0,p(r*/rp). From the position of the boundary in Figure 3, we deduce γ0* = (270 ( 20)%. We examined the kinetics of the clumping process and especially the influence of the initial droplet connectivity, reflected in the initial material’s elasticity. At constant temperature and droplet fraction, the elasticity is mainly reflecting the concentration of irreversible bonds due to partial coalescence within the material. The latter could be tuned by varying the duration of the tempering plateau (tp). A set of experiments was carried out with the primary emulsion as the starting material. When submitted to γ0,p = 400% for 20 min at 25 °C, the initially fluid emulsion turned into a gel whose elastic modulus at 25 °C was about 5 103 Pa (value measured in the linear regime), with no apparent sign of clumping. It is worth noting that, in the absence of shear, gelling occurred spontaneously, with the gel elastic modulus after the same period of time (20 min) being of the order of 200 Pa.11 Thus, the applied strain clearly accelerated the spontaneous gelling process. In a different experiment, an emulsion was submitted to a sinusoidal strain of amplitude γ0,p = 1100% for 20 min. The coexistence of the gelled and clumped states was observed, and from the radial position, r*, of the boundary, we deduced γ*0 = (680 ( 50)%. The boundary further progressed toward the center of the plate as the strain was applied for longer time until a stationary value corresponding to γ0* = (250 ( 50)% was reached for τ > 50 min. When the starting material was a strong gel (G0 gel > 25 103 Pa), the same stationary value of γ0* was attained in less than 2 min. It can therefore be concluded that the initial droplet connectivity had no impact on the stationary value of γ*. 0 However, it determined the kinetics of the clumping process: the characteristic delay to attain the stationary state was much longer for emulsions than for strong gels. Such difference may be explained by the fact that clumping required the formation of interdroplet irreversible bonds which were already present in the strong gel and not in the primary emulsion. This hypothesis was confirmed by performing an experiment with a weak gel characterized by G0 gel = (5 ( 1) 103 Pa. In terms of initial droplet connectivity, this system corresponded to an intermediate situation between the primary emulsion (almost zero droplet connectivity) and strong gels (high droplet connectivity). As expected, the same order of magnitude for γ0* was found after an intermediate period of time of about 10 min. Figure 6 represents the kinetic evolution of γ*0 for the previously mentioned systems. The induction period to reach the stationary state became shorter as the initial droplet connectivity increased. Within experimental uncertainty, the value of γ*0 in the stationary regime was found to be independent of the strain DOI: 10.1021/la1027288
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Figure 4. (a) Aqueous drop containing a hydrophilic bluish dye deposited at two different locations of the sample surface: on the central white part and on a macroscopic yellowish clumps. (b) Diffusive expansion of the bluish spot after 10 min. (c) Diffusion of spot across the sample. (d) The drop deposited on the macroscopic clump neither spread nor diffused over time (the sample was turned upside-down).
Figure 5. Microscope image of the gel located in the central part of Figure 4 diluted to facilitate the observation (scale bar = 100 μm). The observation was carried out at 4 °C.
amplitude, γ0,p, imposed at the sample periphery. This was confirmed in Figure 7 where we reported some characteristic images taken in the stationary state after applying strains of variable amplitude to a strong gel. Clumping was not observed for γ0,p < 200% (Figure 7a and b). For γ0,p = 500%, clumping occurred above a well-defined radial position r* (Figure 7c) corresponding ( 20)%. Finally, for γ0,p =1000% (Figure 7d), the to γ*=(230 0 boundary between the gelled and clumped states was displaced toward a smaller r* value corresponding to roughly the same threshold strain amplitude, γ*0 = (270 ( 20)%. In addition, we observed that the threshold strain amplitude for clumping was only weakly varying with the pulsation ω, as revealed by Figure 8. The threshold strain amplitude followed an effective power-law variation with a rather low exponent: γ*0 µ ω-0.2. 2. Slow Partial Coalescence (Rm = 0; Td = 25 °C). We explored the behavior of an emulsion with the same average diameter as in section 1 (D=15 μm), but stabilized solely by sodium caseinate (Rm = 0). Under quiescent conditions, partial coalescence did not take place over a characteristic time period of 16786 DOI: 10.1021/la1027288
Figure 6. Evolution of the threshold strain for clumping as a function of the duration τ for various initial configurations: strong gel (2; G0 gel = (30 ( 5) 103 Pa; D = 15 μm; tp= 27 min), weak gel (3; G0 gel = (5 ( 1) Pa; D = 15 μm; tp= 10 min); emulsion (O; D = 15 μm). φ = 45 wt %; ω = 1 rad 3 s-1; γ0,p = 1100%. Aqueous phase composition: 0.5 wt % Tween 20; 3.5 wt % sodium caseinate (Rm = 2.7). Lines are only guides to the eyes.
several weeks. Figure 9a shows that the emulsion remained fluid and homogeneous when submitted to an oscillatory strain of amplitude γ0,p = 400% at Td = 25 °C for 20 min. A second trial was carried out by applying a larger strain amplitude γ0,p = 1100%. In this case, the treatment resulted in the formation of large yellowish clumps, randomly distributed over the periphery of the plate and surrounded by a white fluid emulsion (Figure 9b). The macroscopic clumps coexisted with a fluid emulsion whose size distribution was nearly identical to the initial one, as revealed by droplet sizing measurements. Compared to the previous case (Rm = 2.7), the process of destruction was only partial. It occurred at discrete clumping sites randomly distributed over the Langmuir 2010, 26(22), 16782–16790
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Figure 7. Final state of a gel submitted to a sinusoidal strain (ω= 1 rad 3 s-1) of variable amplitude for τ=2 min at Td=25 °C. Initial gel elastic modulus: G0 gel=(35 ( 5) 103 Pa (D=15 μm; tp=30 min). φ = 45 wt %. Aqueous phase composition: 0.5 wt % Tween 20; 3.5 wt % sodium caseinate (Rm=2.7). (a) γ0,p = 50%, no clumping; (b) γ0,p= 200%, no clumping; (c) γ0,p = 500%, γ0* = (230 ( 20)%; (d) γ0,p= 1000%, γ*=(270 ( 20)%. The dotted circles represent the boundary 0 between the gelled and clumped regions. Plate radius: rp = 25 mm.
Figure 8. Evolution of the threshold strain for clumping as a function of the pulsation ω. Initial gel elastic modulus: G0 gel = (40 ( 5) 103 Pa (D = 15 μm; tp = 35 min); φ = 45 wt %; γ0,p = 1000%; Td = 25 °C. Aqueous phase composition: 0.5 wt % Tween 20; 3.5 wt % sodium caseinate (Rm = 2.7). All the gels underwent N = ωτ/2π = 191 cycles. The dotted line is a ω-0.2 power-law fit.
volume. The images of Figure 9a and b suggest that the clumps are formed above some critical value of the strain amplitude. However, because of the random nature of the process, it was difficult to measure the threshold strain amplitude with sufficient accuracy. For Rm = 0, the application of a tempering cycle did not produce partial coalescence and it was thus not possible to explore the influence of the initial connectivity like in the fast coalescence limit (section 1). The surfactant-to-protein ratio was varied between 0 and 2.7, and the shear duration τ was maintained constant and equal Langmuir 2010, 26(22), 16782–16790
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to 20 min. The images of Figure 9b and c are reflecting two limiting situations in terms of destabilization scenario. The proportion of macroscopic clumps is rather low at Rm = 0 (Figure 9b) and increases with Rm until the same stationary regime as the one described in section 1 is recovered (Figure 9c). It is clear from Figure 9b and c that the rate of destabilization (clump formation) increases with Rm. Besides the kinetics, the scenario is also evolving, and from the whole set of observations we deduce that the shearinduced destabilization of the emulsions may follow two limiting pathways. In the first one, referred to as mechanism 1, gelling first occurs and is followed by massive clumping (Rm = 2.7, fast partial coalescence). In the second one, referred to as mechanism 2, randomly distributed clumps made of partially coalesced droplets appear and coexist with an emulsion whose size distribution is identical to the initial one (Rm = 0, slow partial coalescence). A schematic representation of both mechanisms is proposed in Figure 10. 3. Influence of Temperature. The solid fraction within the oil phase was determined by the temperature, according the curve reported in Figure 1. Partial coalescence did not occur at rest over a period of at least 1 month at temperatures exceeding the upper limit of the oil melting domain (T > 35 °C). It also did not occur at sufficiently low temperature, allowing us to store the emulsions at 4 °C for several weeks without any destabilization. In ref 10, we examined the destabilization of emulsions at rest submitted to a tempering cycle. The kinetics of partial coalescence was determined by measuring the evolution of the bulk elastic modulus in the linear regime. The experimental data were interpreted within the frame of percolation theory, using the characteristic rate of partial coalescence, k, as the unique fitting parameter. We deduced the evolution of the coalescence rate at rest as a function of the solid fraction, S, for emulsions with D = 15 μm and Rm = 2.7. The experimental curve k = f(S) was nearly parabolic, reflecting the fact that partial coalescence requires the simultaneous presence of solid and liquid oil within the droplets. The rate of partial coalescence was very low for S < 5% or S > 25%, and it reached a maximum value for S ≈ 10-15% (Figure 11). We observed the shear-induced evolution of emulsions and strong gels (D = 15 μm and Rm = 2.7), using the temperature to trigger the rate of partial coalescence. Destabilization occurred following mechanism 1 for 15 °C < Td < 35 °C. In Figure 12, we report the evolution of γ0* in the stationary regime as a function of the temperature (lower horizontal axis) and of the bulk solid fraction (upper horizontal axis) for both emulsions (full circles) and strong gels (open triangles). For Td < 15 °C, γ0* exceeded the maximum achievable strain amplitude (1100%) and we could not observe clumping even after 5 h. In section 2, it was demonstrated that the scenario of destabilization at constant temperature shifted from mechanism 1 to mechanism 2 as the rate of partial coalescence, controlled by means of Rm, decreased. The same transition is likely to occur at constant Rm as the temperature is lowered. However, it was not observable in the oscillatory regime because of the intrinsic limitation of the strain amplitude ( 35 °C since the solid fraction within the oil phase was so low ( 35 °C was very small and we did not observe any type of shear instability. The experimental curves in Figure 12 exhibit the same nearly parabolic shape whatever the initial state of the material. The minimum value of γ*0 corresponds to a solid fraction of 10-15%. By comparing the data of Figures 11 and DOI: 10.1021/la1027288
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Figure 9. Final state of emulsions submitted to a sinusoidal strain. D = 15 μm; φ = 45 wt %; Td = 25 °C, ω = 1 rad 3 s-1, τ = 20 min. Aqueous phase composition: 3.5 wt % sodium caseinate; variable amounts of Tween 20 (0% for Rm = 0 and 0.5 wt % for Rm = 2.7). (a) Rm = 0, γ0,p = 400%; (b) Rm = 0, γ0,p = 1100%; (c) Rm = 2.7, γ0,p = 1100%.
Figure 10. Diagram schematizing the kinetic evolution of the emulsions submitted to low shear following the two identified limiting mechanisms.
Figure 11. Evolution of the rate of coalescence at rest (no applied shear) as a function of the solid content in the oil phase. D = 15 μm, Rm = 2.7, φ = 45 wt %. Adapted from ref 10. The solid line is a guide for the eyes. Reproduced with permission from EDP Sciences (Bicontinuous emulsion gels induced by partial coalescence: Kinetics and mechanism, 2006, 76 (2); http://epljournal.edpsciences.org/).
12, it appears that the critical strain amplitude for clumping and the rate of partial coalescence are correlated. The evolution of both parameters with the solid fraction (or equivalently the temperature) is non-monotonous, and they both 16788 DOI: 10.1021/la1027288
Figure 12. Evolution of the threshold strain for clumping as a function of temperature (lower horizontal axis) and of the oil solid fraction (upper horizontal axis) for both emulsions (b; D = 15 μm) and strong gels (4; G0 gel = (40 ( 5) 103 Pa; D = 15 μm; tp = 35 min). φ = 45 wt %; ω = 1 rad 3 s-1; γ0,p = 1100%; τ = 20 min for gels and 5 h for emulsions. Aqueous phase composition: 0.5 wt % Tween 20; 3.5 wt % sodium caseinate (Rm = 2.7).
attain an extremum value for the same solid content, namely, S ≈ 10-15%. We thus conclude that, for clumping to occur, Langmuir 2010, 26(22), 16782–16790
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the amplitude of the sinusoidal strain has to overcome some threshold value which depends on the rate of partial coalescence: the faster the coalescence rate, the smaller γ0*. 4. Influence of Droplet Size and Interfacial Composition. So far, the measurements were carried out with emulsions with the same initial diameter. We then tried to determine the mechanism of destruction as a function of Rm and D, at constant temperature. The deformation protocol was modified in order to apply very large strains and to check the generality of the phenomenology under different flow conditions. We adopted a cone and plate geometry, and the emulsions were sheared in steady flow conditions, at low shear rate in order to avoid the expulsion of the material from the rheometers’ gap. The primary emulsions were loaded at 4 °C, and the temperature was increased up to Td = 25 °C at þ20 °C 3 min-1. After 60 s at Td, the emulsions were submitted to a constant shear rate, γ_ = 0.4 s-1 during 1 h. In such conditions, the strain achieved at the end of the process, γ = 1.44 105 %, was constant over the sheared volume. The short time evolution of the viscosity was also monitored. Although the signal was noisy, it was possible to infer the mechanism from its qualitative evolution. Indeed, for mechanism 1, the viscosity raised a few seconds after the application of the shear (>1 Pa 3 s) as a consequence of the fast gelling process. However, for mechanism 2, the viscosity remained comparatively low (∼0.1 Pa 3 s) and almost constant during at least 5 min before it increased sharply due to the growth of large clumps within the gap. Once the shear was interrupted, the sample was cooled down to 4 °C at -20 °C 3 min-1. The scenario of destruction was concomitantly assessed from both the evolution of the viscosity and from the visual inspection of the material in the final state. We first checked that the adopted protocol did not modify the mechanism of destruction compared to the previous one based on the application of an oscillatory strain. Preliminary trials were conducted with emulsions characterized by D = 15 μm and two different Rm values: 0 and 2.7. For Rm = 2.7, the evolution occurred according mechanism 1. As could be expected, the clumped state was obtained and expanded over the whole volume because the applied strain was spatially homogeneous. For Rm = 0, macroscopic clumps randomly distributed over the volume were observed, resulting from mechanism 2. These observations are fully consistent with those reported under oscillatory conditions. Figure 13 represents the behavior of the emulsions under flow over a wide range of D and Rm values. The diagram comprises two well identified regions, and a transition line has been drawn in between them as a guide for the eyes. Emulsions characterized by large average droplet diameters and/or large surfactant-to-protein molar ratios underwent destruction following mechanism 1. Alternatively, for small average droplet diameters and/or low Rm values, destruction occurred through mechanism 2. In this latter case, the extent of clumping was quite low, but it increased significantly at the approach of the transition line. Figure 13 is actually reflecting differences not only in terms of destruction scenario but also in terms of kinetics. Indeed, all systems evolving trough mechanism 1 were fully destabilized after 1 h. However, the systems evolving through mechanism 2 were only partially clumped. Although we did not follow the evolution for longer periods of time, the clumps are expected to further grow until they totally incorporate the oil phase and they finally coexist with an almost clear aqueous phase. The data of Figure 13 corroborate and generalize the empirical link between the shear-induced destabilization scenario and the susceptibility to partial coalescence at rest: emulsions undergoing fast (respectively slow) partial coalescence evolve following mechanism 1 (respectively 2). As mentioned in section 1, the quiescent Langmuir 2010, 26(22), 16782–16790
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Figure 13. Destruction scenario of emulsions submitted to a constant shear rate, γ_ = 0.4 s-1 for 1 h at Td = 25 °C, as a function of the average drop diameter and the bulk surfactant-to-protein molar ratio, Rm: (]) mechanism 1, gelling followed by massive clumping; (b) mechanism 2, coexistence of a fluid emulsion and solid clumps. Aqueous phase composition: 3.5 wt % sodium caseinate; variable amounts of Tween 20. φ = 45 wt %.
stability of emulsions with D = 15 μm and variable Rm values stored at 25 °C for a given period of time was examined in ref 11. The curve G0 gel = f(Rm) after a tempering cycle exhibited a sharp transition for 0.2 < Rm < 1, revealing a dramatic increase of the coalescence rate. Interestingly, in Figure 13, the transition between the two scenarios occurs within approximately the same Rm range for D = 15 μm. The observed transition is most probably provoked by the competitive adsorption of Tween 20 and casein molecules at the oil-water interface. Casein layers are generally viscoelastic and almost always tangentially immobile.26-28 Such interfacial viscoelasticity combined with the large electro-steric repulsive forces existing between casein layers29 efficiently protects the droplets against partial coalescence. It is well-known that the presence of nonionic surfactant Tween 20 modifies the interfacial properties. At very low concentration, Tween 20 preferentially adsorbs at the oil-water interface and locates in the defects of the protein film.30 As Rm increases, surfactant adsorption induces a progressive displacement of casein31,32 and the interactions between proteins are weakened.33 The interface becomes more fluid, allowing the diffusion of the adsorbed species even if the protein is not totally displaced.34 Moreover, from interferometry,34 it has been shown that air-water-air films stabilized with pure casein are much thicker than those stabilized by casein-surfactant mixtures. Last but not least, the presence of hydrophilic surfactant in water is known to promote the transfer of lipid crystals from the oil to the water phase.35 This phenomenon is applied for industrial lipid purification and is known as the Lanza process.36 Although we did not observe the presence of (26) Bressy, L.; Hebraud, P.; Schmitt, V.; Bibette, J. Langmuir 2003, 19, 598. (27) Dickinson, E.; Radford, S. J.; Golding, M. Food Hydrocolloids 2003, 17, 211. (28) Dickinson, E. Colloids Surf., B 2001, 20, 197. (29) Dimitrova, T.; Leal-Calderon, F. Langmuir 2001, 17, 3235. (30) Mackie, A.; Gunning, A.; Wilde, P.; Morris, V. Langmuir 2000, 16, 2242. (31) Courthaudon, J. L.; Dickinson, E.; Dalgleish, D. J. Colloid Interface Sci. 1991, 145, 390. (32) Girardet, J.; Humbert, G.; Creusot, N.; Chardot, V.; Campagna, S.; Courthaudon, J.; Gaillard, J. J. Colloid Interface Sci. 2001, 243, 515. (33) Chen, J.; Dickinson, E. Food Hydrocolloids 1995, 9, 35. (34) Kr€agel, J.; W€ustneck, R.; Husband, F.; Wilde, P.; Makievski, A.; Grigoriev, D.; Li, J. Colloids Surf., B 1999, 12, 399. (35) Spicer, P. T.; Hartel, R. W. Aust. J. Chem. 2005, 58, 655. (36) Lanza, F. German Patent 191238, 1905
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crystals in the aqueous phase, Tween 20 favors the protrusion of crystals at the interface. This hypothesis was verified by examining the topography of a macroscopic interface between the aqueous and oil phases by atomic force microscopy.11 To summarize, in the presence of surfactant, the interface becomes more fluid, the range of the repulsive interactions between the droplet surfaces is reduced, and the number of protruding crystals and the characteristic protrusion length phase increase. All these factors concomitantly promote partial coalescence as observed experimentally. In the same vein, the relative stability of emulsions with variable average diameters D and constant surfactant-to-protein molar ratio Rm = 2.7 was examined in ref 11. The final elasticity, G0 gel, raised over almost 4 decades between 9 and 15 μm again revealing the existence of a very sharp transition between a regime where gelling by partial coalescence is intrinsically slow (D < 9 μm) and a regime where partial coalescence is fast and leads to a rigid gel over a very short period of time (D g 12 μm). In Figure 13, for Rm = 2.7, the transition between the two scenarios occurs within exactly the same diameter range, that is, D ≈ 10 μm. Different explanations can be proposed to interpret the influence of the average droplet diameter. On the one hand, because of the Laplace pressure, two large globules will deform more strongly than smaller ones, resulting in a larger area of the thin film formed between the droplets.1 The probability that a protruding crystal (large enough to reach the second surface) is located in that film is therefore higher. On the other hand, the characteristic crystal size is an increasing function of the drop diameter.11,37 Since partial coalescence requires protrusion of crystals over distances larger than the film thickness, gelling should occur above a critical drop size. In ref 11, we observed two crystallized emulsions with average diameters of 5 and 15 μm by optical microscopy under crossed analyzers after a storage period of 15 h at T = 4 °C. Drops with D = 5 μm were poorly birefringent compared to D = 15 μm, most probably reflecting the much smaller crystal size. By combining optical microscope observation between crossed analyzers and differential scanning calorimetry measurements coupled to X-ray diffraction (XRD), Lopez et al.37 demonstrated that the emulsion drop diameter affects the crystal size and/ or the structure of the crystal network in milk fat-in-water emulsions. The formation of crystalline structures was revealed by the presence of diffractions peaks. The decrease in the average fat drop size induced a decrease in small-angle XRD peak maximum intensity correlated with an increase in peak width. This was interpreted as resulting from defaults in the organization of triglyceride molecules in the crystals, either directly due to the curvature of the oil-water interface from which crystals are supposed to grow, or indirectly due to the faster relaxation that can induce the formation of crystals of smaller size. Indeed, the decrease of droplet size induces a faster supercooling relaxation and, consequently, a higher disorder and/or a smaller size of triglyceride crystals within the emulsion droplets.37,38
The two shear-induced mechanisms (Figure 10) are actually reflecting the impact of the energy barrier for coalescence. Mechanism 1 is somehow reminiscent of a spinodal decomposition for which there is a very low energy barrier. This is reflected by the fact that partial coalescence occurs even in the absence of shear. Large length-scale structures are formed as the process is initiated, and then coarsen as the transition proceeds until the separation of the crystallized oil (with water trapped in it) and of the aqueous phase. Conversely, the rate of partial coalescence in mechanism 2 is low because a significant energy barrier has to be overcome. Destruction is initiated by a few nuclei resulting from shear-induced partial coalescence events. We believe the process initially involves a reduced number of droplets, most probably the largest ones in the size distribution which are also the most susceptible to coalescence.11 The isolated nuclei further grow by the accretion of any other primary droplet or cluster. The process favors the diverging growth of already formed clusters because of their larger surface area and consequently larger “capture” capacity. Within this scheme, it is likely that the largest clumps rapidly incorporate smaller ones, which explains that only a few large clumps are actually observed.
(37) Lopez, C.; Bourgaux, C.; Lesieur, P.; Bernadou, S.; Keller, G.; Ollivon, M. J. Colloid Interface Sci. 2002, 254, 64. (38) Coupland, J. N. Curr. Opin. Colloid Interface Sci. 2002, 7, 445.
Acknowledgment. The authors are very grateful to Le Conseil Regional d’Aquitaine for financial support.
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Conclusion In this paper, we examined partial coalescence phenomena in which crystals form oil continuous bridges between oil-in-water droplets. For that purpose, we used monodisperse emulsions in which we tuned the average droplet size, the solid fraction, and the interfacial composition. We designed a set of rheological experiments that allowed us to trigger coalescence phenomena by the application of shear. We also designed observation methods to visualize oil-continuous regions that result from coalescence of droplets. Quantitative relations between the composition of the droplets (in terms of solid fraction) and the parameters that characterize the instability onset (magnitude, duration or frequency of the applied strain) were obtained. With these tools, we identified two mechanisms for the propagation of coalescence through the network of partially coalesced droplets. The evolution of the primary emulsions according one of them is mainly determined by the rate of partial coalescence. Emulsions prone to fast coalescence undergo gelling before massive macroscopic clumping occurs. Instead, in emulsions exhibiting low coalescence rates, randomly distributed clumps are formed and grow over time. The coalescence rate and thus the destabilization pathway depend on the crystal size and/or protrusion at the oil-water interface, with both properties being controlled by the average droplet size and by the protein-to-surfactant molar ratio which sets the interfacial composition. The phenomena that were described are generic and occur in a wide range of oil-in-water emulsions based on partially crystallized oils (food, cosmetic products). This paper may provide some guidance for the formulation of such materials and the control of the shear instabilities.
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