CIS-01316; No of Pages 13 Advances in Colloid and Interface Science xxx (2013) xxx–xxx

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Three-phase interactions and interfacial transport phenomena in coacervate/oil/water systems Gregory Dardelle, Philipp Erni Firmenich SA, Corporate Research Division, Materials Science Department, 1217 Meyrin, Geneva, Switzerland

a r t i c l e

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Available online xxxx Keywords: Interfaces Encapsulation Emulsions Wetting Rheology

a b s t r a c t Complex coacervation is an associative liquid/liquid phase separation resulting in the formation of two liquid phases: a polymer-rich coacervate phase and a dilute continuous solvent phase. In the presence of a third liquid phase in the form of disperse oil droplets, the coacervate phase tends to wet the oil/water interface. This affinity has long been known and used for the formation of core/shell capsules. However, while encapsulation by simple or complex coacervation has been used empirically for decades, there is a lack of a thorough understanding of the three-phase wetting phenomena that control the formation of encapsulated, compound droplets and the role of the viscoelasticity of the biopolymers involved. In this contribution, we review and discuss the interplay of wetting phenomena and fluid viscoelasticity in coacervate/oil/water systems from the perspective of colloid chemistry and fluid dynamics, focusing on aspects of rheology, interfacial tension measurements at the coacervate/solvent interface, and on the formation and fragmentation of three-phase compound drops. © 2013 Elsevier B.V. All rights reserved.

Contents 1.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1. Top-down vs bottom-up formation of compound drops and core/shell capsules 1.2. Complex coacervates in nature and in encapsulation technology . . . . . . . 2. Rheology of complex coacervates . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Measurements of the rheological material functions of coacervates . . . . . . 2.2. Interplay of rheology and structure of coacervate droplets . . . . . . . . . . 3. Hydrodynamics and interfacial tensions of coacervate droplets . . . . . . . . . . . 4. From wetting to core/shell formation . . . . . . . . . . . . . . . . . . . . . . 4.1. Apparent contact angles in coacervate/oil/water systems . . . . . . . . . . 4.2. Interfacial energy balance in three-phase compound drops . . . . . . . . . 5. Fluid dynamics of compound drops . . . . . . . . . . . . . . . . . . . . . . . 5.1. Partial coverage — drops with ‘caps’ . . . . . . . . . . . . . . . . . . . . 5.2. Fluid dynamics of fully encapsulated compound drops . . . . . . . . . . . 5.3. Deformation and fragmentation of compound drops . . . . . . . . . . . . 6. Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction 1.1. Top-down vs bottom-up formation of compound drops and core/shell capsules Core/shell capsules are important for controlled delivery and release of active ingredients in applications such as the flavoring of beverages or

E-mail address: philipp.erni@firmenich.com (P. Erni).

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food products, perfume formulation, pharmaceuticals, or inks [1–5]. To obtain particles with a core/shell structure, wherein a barrier layer surrounds the liquid or solid core, two essential fluid processing operations are necessary: (i) the core material must be dispersed in the continuous phase in the form of an emulsion (for liquid active ingredients) or suspension (for solids); and (ii) the shell material must be formed, brought to the active/solvent interface, and stabilized by physical and/or covalent cross-linking. A vast amount of recent literature describes simultaneous, ‘top-down’ dispensing and shell formation via coaxial flow channels or flow focusing

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Please cite this article as: Dardelle G, Erni P, Three-phase interactions and interfacial transport phenomena in coacervate/oil/water systems, Adv Colloid Interface Sci (2013), http://dx.doi.org/10.1016/j.cis.2013.10.001

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G. Dardelle, P. Erni / Advances in Colloid and Interface Science xxx (2013) xxx–xxx

geometries in microfabricated channels [3,6–8]. Similar flow geometries have been described [9] and commercialized previously with larger capillary or channel dimensions; in the area of encapsulation technology, the methods are often called ‘prilling’ (i.e. the continuous ejection of gel-forming droplets via dripping from a capillary into continuous fluid for rapid gelation [10]) or ‘laminar jet breakup’. Typically, the capillary is vibrated at a controlled frequency via accoustic actuation. Interestingly, the attention that the microfluidic approach to capsule formation has received in academics since the first articles on droplet formation using microporous membranes are not mirrored in actual usage of the technology in industrial applications [11,12]. This lack of industrial implementation is somewhat surprising, since the average article on microfluidic or membrane-based droplet formation typically starts or ends with claims to industrial relevance. Here, our objective is to review a bottom-up route for the formation of core/shell capsules that does not rely on microfluidics and discuss the formation of core/shell capsules via complex coacervation, i.e. the bottom-up formation of capsules by coating of an ensemble of oil droplets or solid particles with a polymeric layer material in a large-volume, three phase flow, from the perspective of fluid dynamics and interfacial transport phenomena (Fig. 1). 1.2. Complex coacervates in nature and in encapsulation technology For applications in foods, consumer products, or for pharmaceutics the choice of capsule wall materials is naturally restricted to ingredients in line with legislation constraints [5,13–15]; self-assembled, biopolymerbased capsules are among the most important delivery systems in this field [1,2,16,17].

More recently, coacervates have also been used as scaffolds to form interpenetrating silica/biopolymer materials. Classical precipitated silica has been used as a carrier for such molecules for decades [18], but the protection of the entrapped volatiles is typically poor. In contrast, coacervatetemplated silica/biopolymer hybrid capsules have recently been shown to dramatically improve retention of volatile encapulated oils [2]. Complex coacervation is a liquid/liquid phase separation occurring in colloidal systems, resulting in the formation of two liquid phases [19–21]: a hydrocolloid-rich phase (coacervate phase) and a dilute continuous phase. The coacervate appears as amorphous liquid droplets exhibiting affinity for oil/water or solid/liquid interfaces. Formation of microcapsules by complex coacervation typically involves the steps of (i) phase separation and formation of coacervate nodules; (ii) emulsification/ dispersion of the active ingredient; (iii) deposition and coalescence of coacervate nodules onto the active/continuous phase interface, (iv) physical gelation and, optionally, (v) covalent crosslinking of the wall. Whereas step (i) has been covered in much detail in the literature over the last decades [22–26], there is far less quantitative knowledge available regarding the engineering/processing of the (micro)capsule genesis described in the steps (ii)–(v). Lemetter et al. [16] studied microcapsule formation by complex coacervation from a process engineering perspective and specifically focused on the role of different processing parameters, such as the Reynolds number in the reactor, cooling rate profiles, and the type of emulsification. These authors emphasized the different microcapsule morphologies that arise in coacervate capsule formation, including (a) mononucleated capsules (one core, surrounded by a shell), (b) polynucleated capsules (several cores contained within one shell, and (c) ‘grape’ type, aggregated capsules.

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Fig. 1. Overview of the core/shell capsule formation process by complex coacervation. (a) Schematic of a core/shell encapsulation process by complex coacervation, starting with oil-in-water emulsification, coacervate formation by protein/polyanion phase separation, coacervate deposition at the substrate/water interface, and subsequent physical gelation of the protein followed by covalent cross-linking. (b) Typical coacervate capsules obtained with different processing conditions and compositions (left); gelatin/Acacia gum coacervate phase immediately after phase-separation (middle left) and after gelation (middle right); typical ternary phase diagram [2,5,19] for coacervates formed by a protein (such as gelatin) and a weakly anionic polysaccharide (such as Acacia gum) (right).

Please cite this article as: Dardelle G, Erni P, Three-phase interactions and interfacial transport phenomena in coacervate/oil/water systems, Adv Colloid Interface Sci (2013), http://dx.doi.org/10.1016/j.cis.2013.10.001

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Formation of capsule walls from phase-separated, polymer-rich droplets dispersed in their own solvent relies on the interfacial energy balance between the three phases (oil/solvent/polymer-rich phase). The relation between the interfacial energies must be such that the polymer-rich phase wets the oil/solution interface at least partially. This coating process results in a compound suspension of oil or solid cores contained within larger drops of phase-separated polymer. Recently, coacervation has also received attention for its role in biological adhesion [27], studied for example for the case of coacervates formed by hyaluronic acid and recombinant mussel adhesive proteins [28,29]. On the other hand, coacervates also form between synthetic polyelectrolytes such as poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) [30]. Despite its relevance for the formation of core/shell capsules and for biological adhesion, the interplay between surface thermodynamics, rheology of the coacervate phase, and multiphase flow has not been studied in much detail. Moreover, while the mechanisms underlying the formation of core/shell capsules with shells formed by phaseseparated biopolymer gels have been known for decades, there appears to remain some confusion about the sequences of physical processes leading to capsule formation. Even for many scientists familiar with polyelectrolyte complex formation, the particular mechanisms of capsule formation remain unclear, and the interested reader trying to get an overview of the subject is treated to descriptions as varied ‘complex formation at the interface’, ‘precipitation of a coacervate at the interface’, ‘coacervation at the oil/water interface’, ‘layer formation by coacervation’ or ‘deposition of polyelectrolyte complexes at the oil/water interface’ for one and the same process. Why is there such confusion on a topic that, in principle, has been known in the scientific and patent literature for over 60 years? Part of it might be due to the interdisciplinary nature of this topic: coacervation is largely discussed in the colloid science community. Conversely, the physical principles for the formation of composite droplets, double emulsions and core/shell capsules have traditionally been discussed in the context of chemical engineering or multiphase fluid dynamics. Therefore, in this article we start from the view that coacervate core/ shell capsule formation is a three-phase, free-interface flow problem, where one of the three phases has been formed by associative phase separation, and we investigate the interplay between the flow, threephase-wetting, and the rheology of coacervate phases. Since the physicochemical aspects of complex coacervate formation have been summarized in several excellent overview articles [22–24,31–35], we will discuss only the key results and insights relevant for the three-phase wetting process in core/shell capsule formation with coacervates. Coacervates can be formed between a variety of biopolymers. Typically, the polyanion is a weakly anionic polyampholyte; strongly anionic polysaccharides, such as the carrageenans, cause precipitation rather than coacervation. An overview of the most important polymer pairs for complex coacervation is provided in an overview article by de Kruif et al. [22]. Some prominent examples are β-lactoglobulin/Acacia gum [36–38], β-lactoglobulin/pectin [39], gelatin/Acacia gum, or chitosan/Acacia gum [40,41]. For reference, we also point out the case of simple coacervation, wherein a single polymer phase-separates by a change in solvent conditions, i.e. gelatin in water/ethanol mixtures [42]; their structure, rheology and physicochemical properties appear to be more closely related to precipitates formed by strongly charged polyampholytes rather than being an actual phase-separated liquid phase. 2. Rheology of complex coacervates 2.1. Measurements of the rheological material functions of coacervates Given their relevance for biological or bioinspired adhesion and for industrial encapsulation processes, the rheology of complex coacervates

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has surprisingly received little attention. Those studies that have addressed their rheology tend to focus on the viscosity as a function of parameters such as the ionic strength, pH, polymer concentration or the mass or molar ratios of the constituent polymers. Spruijt et al. [43] described the role of the electrolyte concentration for the rheology of electrostatic complexes using two equally long and oppositely charged polyelectrolytes and found that the small amplitude oscillatory shear rheology can be condensed shifted onto a master-curve by a time/salt superposition (similar to time/temperature or time/ concentration superposition, and not accounting for swelling). The same authors also discussed interfacial tension measurements of coacervate phases against their polymer-depleted solvent using an atomic force microscopy technique. The rheology of coacervates formed by whey proteins and Acacia gum was investigated by Weinbreck et al. [44]; they measured the steady shear viscosities and the dynamic moduli at varying shear rates and frequencies. Varying the polymer concentration, pH value and the protein-to-polysaccharide ratios, they found that the averaged viscosity values of the coacervate phase as a function of pH follow a trend parallel to the polymer content in the coacervate phase, with a pronounced maximum around pH 4.0. For the coacervates with the highest viscosity, coinciding with what the authors described as the point of the strongest electrostatic interactions, they found that the flow curves showed a hysteresis effect between the up- and down curves. While we prefer not to use hysteresis curves for the data shown later in this work (mostly because they blur the line between the separate effects of shear history, shear thinning and time-dependence), they do indeed suggest that significant structure formation takes place in the coacervate, as is typically the case for weakly aggregated suspensions or gels. However, Weinbreck et al. also found clear evidence that in their whey protein/ Acacia gum coacervates at room temperature the elastic modulus G′ was only very weak and the viscous modulus G″ was dominant. Their data for G″ at 25°C and at different pH values suggest a predominantly viscous scaling law of the shear modulus with frequency. From a different perspective, Thomasin et al. [45] discussed the role of viscosity for coacervate-based encapsulation but did not mention the effects of fluid elasticity. As we will show below, elastic stresses need to be taken into account especially for coacervates wherein one of the polymers is a gel-forming protein. Gelatin/Acacia gum complex coacervates for rheometry and for all experiments described in the following were formed as described in more detail elsewhere [2] with a 1:1 mass ratio of the polymers at pH 4.5 and ionic strength 5 mM. Phase separation was induced by dilution with water to yield final polymer concentrations of 1.7% w/w each. With the data shown in Figs. 2 and 3 the rheological response of complex coacervates formed from gelatin and a weak polyanion can be decomposed into a contribution stemming mostly from the protein (percolation related to the concentration of collagen triple helices) [46] and reflected in the temperature profile of the loss tangent G″/G′(T), and into a contribution of the polyanion, reflected merely in the increase in the overall level of the modulus. To a minor extent, the presence of the polyanion slightly softens the percolation threshold of the gelatin, indicated by a less abrupt change in the slope of dG′/dT. 2.2. Interplay of rheology and structure of coacervate droplets In a few studies the role of coacervate rheology for the encapsulation process is mentioned [47–49], as previously summarized by Weinbreck et al. [17,44]. In particular, high coacervate viscosity was mentioned as a reason for improved coalescence stability of the capsules by Burgess [48]. Later, Liberatore et al. [50] studied the role of shear flow for the formation of polyelectrolyte/mixed micelle coacervates. Hwang et al. [29] investigated the viscosity and friction properties of mussel-inspired coacervates formed between recombinant proteins and hyaluronic acid using the surface force apparatus (SFA).

Please cite this article as: Dardelle G, Erni P, Three-phase interactions and interfacial transport phenomena in coacervate/oil/water systems, Adv Colloid Interface Sci (2013), http://dx.doi.org/10.1016/j.cis.2013.10.001

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Fig. 2. Rheology of the coacervate phase forming the shell of the core/shell structure. (a) Temperature sweep experiment for complex coacervates formed from gelatin and Acacia gum, showing the transition from the liquid coacervate to physical gel. Inset: frequency sweep experiment performed after gelation. (b) Steady shear viscosity, plotted as a function of the shear stress at various temperatures. The coacervate phase transitions from a weakly shear-thinning viscous fluid behavior at temperatures above 37 °C to strong shear thinning in an intermediate regime, followed by the onset of an apparent yield stress for temperatures below 30 °C.

Priftis et al. [51] also studied the rheological properties of complex coacervates formed by (ethylene-imine)/poly(D,L-glutamic acid) and observed the typical reduction in the viscosity with salt concentration [22] also found for biopolymers. Depending on the acid:base ratio, these authors also found weakly shear-thinning behavior in steady shear flow, and demonstrated that for the dynamic oscillatory rheology the frequency sweep data form a master-curve via a superposition of frequency and salt concentration. Coacervates formed by bovine serum albumin (BSA) and poly (diallyldimethylammonium chloride) were investigated by Bohidar et al. [25] in dynamic oscillatory shear flow and observed mostly liquid-like rheological responses with dominant viscous moduli G″ N G′ and typical Newtonian scaling G″∝ω and G′∝ω2 at high oscillation frequencies ω. At the lower end of the frequency range, a slow relaxation shoulder with increased G′(ω) occurred. This onset of weak elasticity at low frequencies suggests that despite the predominantly viscous response a very weak network is present, a view supported by the reduction in these elastic effects with increasing strain amplitude. Similar frequency sweep data were also presented for coacervates formed from Acacia gum and chitosan by Espinosa-Andrews et al. [41]. Kizilay et al. [34] pointed out the effect of shear rate on the size of coacervate droplets; the effect of different levels of shear rate on the

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breakup or coalescence of coacervate droplets has been studied by Sanchez and coworkers [52]. Interestingly, the connection of the vast literature on the thoroughly studied field of morphology development in immiscible polymer blends [53] seems to be rarely made in studies of coacervates [54], even though the underlying competition between interfacial forces, shear forces, droplet breakup and (re-)coalescence is identical. In contrast, these similarities are discussed more explicitly in the literature on water-in-water biopolymer blends formed by segregative phase separation [55–61]. Recent measurements by Priftis et al. [62] on coacervates formed between the synthetic polypeptides poly(L-lysine hydrochloride) and poly(L-glutamic acid sodium salt) with a capillary adhesion technique using the surface force apparatus (SFA) reveal that the interfacial tensions are consistently below 1 mN/m. These authors discussed the role of operational parameters (such as compression force and separation speed), solution conditions and total polymer chain length for the interfacial tension. As in the work of Spruijt et al. [63], increasing salt concentrations decrease the interfacial tension. The possible role of non-Newtonian fluid rheology for the force balance used in the interfacial tension measurements remains an interesting topic for further studies. We expect that these effects are also relevant for coacervates formed with globular proteins [17,31,33] although in that case gel

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Fig. 3. Comparison of the rheology of a coacervate phase (protein: gelatin, polyanion: Acacia gum) and with the corresponding protein-only system at comparable protein concentration, ionic strength and pH value. Both the temperature dependence and the dynamic behavior of the coacervate phase directly reflect that of the protein component, but in the coacervate the gel transition is softened and the overall level of the shear stress is increased. In contrast, the polyanion merely increases the level of the viscosity above the gel temperature, but does not alter the elasticity due to gelation of the protein component.

Please cite this article as: Dardelle G, Erni P, Three-phase interactions and interfacial transport phenomena in coacervate/oil/water systems, Adv Colloid Interface Sci (2013), http://dx.doi.org/10.1016/j.cis.2013.10.001

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formation will proceed by different mechanisms [64] as compared to gelatin. The latter form thermoreversible gels upon cooling, essentially determined by the recombination of disordered gelatin chains to reform portions of the original collagen triple helix [65]. Coacervates formed with globular protein will therefore not melt upon heating, and the relatively simple rheological behavior found for gelatin-based coacervates above the gel point does not exist in those cases. 3. Hydrodynamics and interfacial tensions of coacervate droplets The interfacial tension between complex coacervates and their coexisting aqueous solvent σcoa/w is important for the hydrodynamics and formation of core/shell droplets and capsule. However, there is surprisingly little experimental or theoretical work available on actual values, experimental measurements, or theoretical predictions for interfacial tensions in coacervate/solvent systems. The main experimental challenge is the low level of σcoa/w. Since common methods, such as pendant drop or drop volume tensiometry, rely on the presence of a well-defined droplet and on a balance between interfacial tension and gravity, they are difficult to apply, and it is challenging to even form droplets that can be analyzed quantitatively by these (quasi-)static methods. In an early study, De Ruiter and Bungenberg de Jong [66] describe tensiometry measurements using a capillary method and found very small interfacial tensions of the order of ≈1 μN/m. More recently Spruijt et al. [63] used atomic force microscopy (AFM) to record force–distance curves for liquid bridges formed by coacervates adhering to solid surfaces. They obtained values for the coacervate/solvent interfacial tension of the order of tens to hundreds of μN/m. Similar values were found using the surface force apparatus [62]. Like coacervates, phase-separated polymer mixtures formed by segregative phase-separation have similarly low interfacial tensions [54,60,61,67–69]; interfacial tensions in those systems have been studied in more detail using rheo-optics, microscopic droplet deformation or retraction methods, or spinning drop tensiometry. Antonov and coworkers [54] investigated associative phase separation and flow-induced structures in gelatin/Acacia gum systems using rheo-SALS and used droplet deformation and breakup models to estimate the interfacial tension; they obtained values of the order of tens of μN/m. Hydrodynamics has also been shown to influence the formation of coacervates: Liberatore et al. [50] studied shear-induced phase-separation for polyelectrolyte/mixed micelle coacervates using rheo-small angle light scattering (SALS) experiments. Those coacervate systems possess a critical temperature or shear rate and tend to undergo a transition from Newtonian to shear thinning as the temperature increases, and the corresponding SALS patterns [70] reveal flow-induced deformation of the coacervate droplets. Although the authors did not make this comparison, those phenomena appear to be rather similar to the flow-induced morphologies of uncompatibilized, immiscible twophase polymer blends. Seen from this perspective and applied to the coacervate systems of interest here, 'shear-induced phase separation' is likely an initial phase separation (quiescent or under shear) followed by shear-induced coalescence of non-compatibilized coacervate droplets. The shape of liquid filaments immersed in a second fluid evolves under the action of the capillary pressure, which acts as a driving force in the capillary thinning dynamics of the filament [9,71–75]. This flow occurs spontaneously if the configuration of a liquid bridge is brought beyond its static Rayleigh–Plateau stability limit [76] and is driven by the surface tension. It is slowed down by a combination of viscous and elastic stresses in the fluid [77–80]. Therefore, the material properties of the filament can be extracted from its thinning dynamics. They are typically measured as the change in the radius at the neck of the liquid bridge over time R(t). If boundary conditions are properly chosen and adapted to the material, capillary thinning can be used as a rheometric flow [76–78,81–83]. A necessary condition for microfilament rheometry is that the viscocapillary time scale tvc = η0R0/σ be sufficiently long that a

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well-defined capillary-driven flow can be established and fluid inertia should be negligible [76,83] (σ is the interfacial tension, R0 is the radius of the filament, and η0 is the zero shear viscosity). From the definition of tvc it is clear that this can be achieved either via high viscosity levels or low interfacial tensions. This corresponds pffiffiffiffiffiffiffiffiffiffiffiffi to high values of the Ohnesorge number Oh ¼ t vc =t r ¼ η0 = ρR0 σ N1 , a dimensionless group comparing tvc with the Rayleigh time scale tr = (R30ρ/σ)1/2 [84–87]. Interestingly, although the zero shear viscosity and the interfacial tension both appear in tvc, it seems that the capillary thinning technique is mostly used to extract apparent viscosities of complex fluids whereas the interfacial (or surface) tension is measured (or assumed) independently and used as a constant to calculate the rheological properties from appropriate fluid models. Conversely, capillary thinning dynamics in principle also allows to proceed the other way around and estimate the interfacial tension if the rheological properties are known [60,61,73,88]. This approach is particularly interesting if a sample is Newtonian, its viscosity is known, and the capillary thinning dynamics occur on a time scale that is experimentally accessible and within the boundary conditions needed for the model calculations to be applicable — all of which happens to be the case for an aqueous coacervate thread immersed in its coexisting, polymerdepleted solvent. We therefore suggest that the dynamics of microfilaments are very useful to estimate the interfacial properties of phaseseparated polymer solutions and other low-interfacial-tension systems. It is crucial to ensure that viscoplastic behavior is absent — if the fluid thread possesses even a weak yield stress (i.e. due to gel formation of the protein or peptide component), or if it tends to form a gel under the conditions of the measurement, standard viscous or viscoelastic fluid models will underpredict the interfacial tension. If those properties cannot be neglected, quasi-static experiments wherein the liquid thread slowly expands step by step, are preferable: postulating a stress balance between the yield stress (conserves the filament) and the capillary pressure (tends to break up the filament) at the precise radius at which the filament breaks provides at least an estimate of the apparent interfacial tension. In Fig. 5, a capillary thinning experiment performed with a complex coacervate filament floating in its coexisting aqueous solvent is shown. The polymer-rich phase was formed by coacervation with gelatin and Acacia gum, separated by sedimentation and reinjected into the polymer-depleted water phase. The temperature is T = 60 °C, which is above the gelation temperature of the protein. In first approximation, the rate of thinning of the coacervate filament is controlled by the radius of curvature, the coacervate/water interfacial tension, and the viscosities. The floating coacervate thread immersed in the coexisting solvent thins down and breaks with the radius decreasing linearly with time, as predicted for Newtonian fluids [89]. Due to the low interfacial tension, the critical time scale for filament thinning and breakup is of the order of seconds, tc ≈ 2.2 s; the numerical front factor f follows from Papageorgiou's similarity solution for capillary thinning of Newtonian fluid filaments [76,89] (a typical approximation is f = 0.0709): R σ ¼f ðt −t Þ: R0 η0 c

ð1Þ

The linear decrease of the radius of the coacervate filament with time suggests that a Newtonian fluid model can be used to describe the behavior [89]; with the zero shear viscosity η0 known from independent rheology measurements, the data shown here can be used to extract an estimate for the coacervate/solvent interfacial tension σ ≈ 87 ± 5 μN/m. These experiments were performed far above the gelling temperature of the protein component in the coacervate. Using the zero shear viscosity to approximate the fluid rheology to first order is a simplification; for systematic studies on compositional effects on the interfacial tension, more detailed fluid models (e.g. weakly shear thinning) are needed. On the other hand, the advantage of this liquid filament method

Please cite this article as: Dardelle G, Erni P, Three-phase interactions and interfacial transport phenomena in coacervate/oil/water systems, Adv Colloid Interface Sci (2013), http://dx.doi.org/10.1016/j.cis.2013.10.001

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to estimate interfacial tension is that the role of fluid rheology is more directly visible in the experiments — indeed, non-Newtonian rheology or even fluid elasticity or a yield stress remains more obscure and difficult to differentiate in AFM or SFA-based methods [63]. If these fluid rheology effects play a role in coacervates, they most likely lead to an over-prediction of the interfacial tension. For simply shear thinning, the effect might be weak or negligible; in contrast, if even a very weak yield stress is present, the effect on the measured apparent interfacial tension might be quite dramatic. Our view is that interfacial tension measurements on coacervates always must be accompanied by rheology measurements. Additionally, for measuring techniques between solid surfaces it is vital to account for the effect of confinement on fluid rheology. Confining a complex fluid to the length scale of its structural elements can lead to drastic changes in the rheological material functions [90,91]. Depending on the polymer chemistries used, these effects are likely to occur at several length scales in coacervates. 4. From wetting to core/shell formation 4.1. Apparent contact angles in coacervate/oil/water systems Complex coacervates are traditionally used to encapsulate hydrophobic core materials. This process is indeed a combination of a ‘topdown’ emulsification or dispersing process for the core material (here: oil droplets) with a ‘bottom-up’ coacervate formation process. The result is a three-phase emulsion consisting of (i) oil droplets, (ii) aqueous coacervate droplets rich in polymer, and (iii) a continuous, polymerdepleted continuous aqueous phase. To form core/shell drops, the polymer-rich phase needs to coat the core oil droplets or solid particles, resulting in a composite emulsion of oil droplets contained within larger coacervate drops. Knowing that the liquid coacervate droplets are pre-formed and are available in the vicinity of the oil or solid core, this coating could, in principle, occur via different routes: (i) the polymer-rich phase spreads at the oil/ water interface, thereby forming a primary thin coacervate film, and a shell forms by further growth via coalescence of additional coacervate droplets; (ii) individual coacervate droplets wet the oil core, without spreading spontaneously, and the shell is formed by simultaneous or sequential wetting with additional coacervate droplets until a contiguous shell is obtained. At first sight, the question seems to be whether the coacervate ‘spontaneously’ spreads at the oil/solvent interface to form a shell around to oil core, and the feasibility of encapsulation would simply be predicted by the capacity of the coacervate to spread at the interface. However, Fig. 4 shows that this is clearly not the case: a droplet of oil – here: eugenol (2-Methoxy-4-(2-propenyl)phenol, a fragrance oil with a relative density higher than that of water) – is dispensed onto a gelled coacervate surface immersed in the aqueous coexisting solvent consistently assumes a contact angle far greater than 90°. In combination with the aqueous coexisting solvent, no spontaneous spreading occurs. Fig. 4 also shows the events observed for the inverted configuration: a spherical, gelled bead of a coacervate phase formed by gelatin as the protein and Acacia gum as the polyanion, is placed on a hydrophobic substrate (PTFE surface) immersed in the coexisting aqueous solvent at 25°C. Initially, the apparent ‘contact angle’ of the spherical coacervate bead is θapp = 180°, a value which is of course fully determined by the gel elasticity and integrity of the bead. As the temperature rises (at a rate of 1°C per minute), the bead melts, and the apparent advancing contact angles are now determined by a balance of viscoelasticity in the bead and surface energy. Gravitational sagging of the coacervate bead pushes the liquid front in the outward direction, and the contact angle attains a minimum value at θapp ≈110° at a temperature of 31°C. Notice that even at this point, θapp N 90°. Increasing the temperature further again increases the angle to θapp ≈ 135–140° above T = 31 °C; the rheology data shown above strongly suggest that elastic stresses no longer play a significant role in this regime, and that wetting is now controlled entirely by interfacial energy. Indeed, the values of the contact

angles above the melting point of the protein gel are similar to those found above for the inverse, oil drop-on-gel configuration, where the wetting drop was purely Newtonian. 4.2. Interfacial energy balance in three-phase compound drops Generally, two immiscible liquid drops surrounded by a third immiscible liquid can form a range of equilibrium configurations determined by the relative interfacial tensions σij and spreading coefficients Si = σjk − (σij + aik). This topic has been addressed in 1970 in a seminal article by Torza and Mason. Following their definitions, coacervate compound drops with complete encapsulation of the oil drops correspond to 2-singlet (2 s) structures (shown in the right-hand side in Fig. 7). Torza and Mason's work is based entirely on equilibrium interfacial energy considerations (Fig. 8). They already pointed out that in many cases additional factors arise, and specifically mention increased viscosities as one of them. However, in view of our discussion of the role of non-Newtonian fluid rheology for three-phase wetting, high viscosity alone is not expected to be the sole reason for non-engulfment: a high shear viscosity alone merely slows down a process. In contract, the presence of shear elasticity in the shell material will actually resist the spreading flow. Waters [92] and van Zyl et al. [93] described the role of surface thermodynamics for the synthesis of polymer particles with a variety of wetting/dewetting-controlled morphologies. This same approach was more recently also used for microfluidic particle generation [7,94,95]. Torza and Mason's study is the basis for the understanding of the additional effects of fluid flow, interparticle forces and the actual volumes of the two droplets (or the shell and the core droplet). These effects were later discussed in several articles in the 1980s by Johnson, Sadhal, and coworkers [96]. These authors additionally discussed the role of the volumes present for each phase (shell and core drop). In additional to fluid rheology effects, the role of the actual flow process also influences the formation of compound drops. To illustrate this effect, Fig. 6 shows two different flow configurations, with experiments performed using identical fluids (fluids: (i) an oil drop, (+)-carvene, rising from a stainless steel capillary, (ii) a complex coacervate phase as shown in the previous figures formed from gelatin and Acacia gum, and (iii) the aqueous coexisting solvent that has been kept in equilibrium with the coacervate phase; in all experiments shown in Fig. 6, the optical setup is placed in a transparent thermostated glass cell kept at T = 50°C (cell inner diameter 9.5 mm). In the first experiment (top row of images), the oil drop is retained at the rising steel capillary and is in contact with the solvent. The coacervate phase, also kept at 50 °C, is injected from the top at varying flow rates. If the flow rate of the coacervate thread is increased above a critical volumetric flow rate, the flowing coacervate thread begins to fold, rather than to increase its thread diameter. The folding patterns [97,98] are similar to some of those observed with fluid threads formed in miscible, but highly viscous fluid pairs by Cubaud and coworkers [99–102]. Due to the low viscosity of the continuous phase the timescales are shorter and folded morphologies only last for times of the order of a few tenths of a second. Folding of the coacervate thread flowing downwards onto the oil drop is favored over wetting/coating of the coacervate phase. This behavior is determined by the overall balance of interfacial energy between the three fluids, the viscosities of the coacervate phase and the solvent, and the flow rate of the coacervate thread. No core/shell structure is formed at any of the flow rates. Although the coacervate establishes contact and wets the oil drop, it simply flows downwards and easily de-wets the oil/solvent interface. In the second set of experiments, shown in the bottom row of images in Fig. 6, the optical cell is filled with the molten coacervate phase to a level just above the oil drop. In this experiment, the (+)-carvene oil is injected and made to detach from the capillary. As shown in the figure, the detached oil rises upwards through the molten coacervate phase,

Please cite this article as: Dardelle G, Erni P, Three-phase interactions and interfacial transport phenomena in coacervate/oil/water systems, Adv Colloid Interface Sci (2013), http://dx.doi.org/10.1016/j.cis.2013.10.001

G. Dardelle, P. Erni / Advances in Colloid and Interface Science xxx (2013) xxx–xxx

a

7

b

c

θapp (o)

180

135

90

intermediate regime: - fluid viscoelasticity - interfacial energy - flow (e.g. gravitational sagging)

gel elasticity dominates 25

30

interfacial energy dominates 35

40

T (oC) Fig. 4. Three-phase wetting behavior in coacervate/water/oil systems. (a) Typical macroscopic appearance of a liquid oil drop in contact with the interface between a coacervate substrate below its gelation temperature and the aqueous coexisting phase. Apparent contact angles are typically far above 90° (here: 135°). The oil used here (eugenol) has a higher specific density than the aqueous phase and forms a sessile drop. (b) Evolution of the apparent contact angle of an initially spherical, gelled bead of coacervate resting on a PTFE support immersed in the aqueous coexisting solvent during a slow change in temperature. (c) Apparent contact angle θapp obtained from the same images. The evolution of θapp with increasing T is in parallel to the disappearance of elasticity revealed by the rheology experiments. Scale bar: 1 mm.

4.10–4

d d

(tc - t)· /

0

d (m)

3.10–4 2.10–4 1.10–4

0 0.0

0.5

1.0

1.5

2.0

t (s) Fig. 5. Capillary thinning dynamics of a coacervate thread in contact with its coexisting aqueous solvent; the polymer-rich phase has previously been formed by complex coacervation with gelatin and Acacia gum, separated by sedimentation and reinjected into the polymer-depleted water phase. The temperature is T = 60 °C, which is above the gelation temperature of the protein. In first approximation, the rate of thinning of the coacervate filament is controlled by the radius of curvature, the coacervate/water interfacial tension, and the viscosities. The linear decrease of the radius of the coacervate filament with time suggests that a Newtonian fluid model can be used to describe the behavior [89]; since an apparent shear viscosity η0 can easily be obtained from independent measurements, the data shown here can be used to extract an estimate for the coacervate/solvent interfacial tension σ from the viscocapillary time scale tvc = η0R0/σ. Image width for each frame: 8 mm. Scale bar: 0.5 mm, time step between images shown: 0.25 s.

Please cite this article as: Dardelle G, Erni P, Three-phase interactions and interfacial transport phenomena in coacervate/oil/water systems, Adv Colloid Interface Sci (2013), http://dx.doi.org/10.1016/j.cis.2013.10.001

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a

b

c Fig. 6. Hydrodynamics in three-phase coacervate/oil/water systems. Oil drop brought into forced contact with a liquid coacervate phase. The images show a drop of limonene oil rising from a steel capillary immersed into a salt solution. The heated coacervate phase is injected from a second capillary on top of the measuring cell. (a) An initial large drop of liquid coacervate phase is formed. The large coacervate drop sediments, widens, and comes into contact with the oil drop, rolls past the oil drop, leaving behind an extended thread of the coacervate phase. (b) As the flow rate of the coacervate phase is increased, the stream does not spontaneously coat the oil/water interface but flows past the oil drop. A coacervate stream flowing at a faster rate starts to fold but still does not spread on the oil. (c) Oil drop pinching off from a capillary into a heated coacervate phase, penetrating the coacervate/solvent interface, dragging behind a coacervate stream in its wake. Coacervate phase in all examples: gelatin/Acacia gum.

and pushes through the coacervate/solvent interface while retaining a gradually thinning coacervate film at its front. After penetration of the coacervate/solvent interface, a compound drop remains, leaving behind a thinning coacervate thread in its wake. The comparison of these two examples demonstrates that three-phase wetting in coacervate compound drops strongly depends not only on interfacial thermodynamics, but also on the character of the flow process. We note that this second type of flow has also been described in the literature for encapsulation [103]. 5. Fluid dynamics of compound drops The core/shell structures of coacervate/oil or coacervate/solid drops have already been discussed since the first half of the 20th century from

the technological [104], colloidal [19,20,66], and biophysical [105] perspectives. In contrast, interest in the fluid dynamical aspects of core/ shell liquid drops has only developed later; an initial wave of studies was initiated by Mason and coworkers in the late 1960s and 1970s. They investigated equilibrium configurations, as well as spreading, deformation and coalescence of immiscible compound drops [106–110]. Only later on, in the 1980s, full fluid mechanical analyses were published by Johnson, Sadhal and coworkers [96,111–113]. In the vein of historic analyses on translating simple drops [114–116], these studies first focused on spherical geometries. Stone and Leal [117] presented an analytical solution using a perturbation method [118] for the small deformation behavior of core/shell drops, and applied the boundary integral method to larger deformations. More recent studies have mostly focused on computational approaches [119–121].

Fig. 7. Scheme and examples of interacting three-phase systems droplet 1/droplet 2/solvent, following the classical paper by Torza & Mason [110].

Please cite this article as: Dardelle G, Erni P, Three-phase interactions and interfacial transport phenomena in coacervate/oil/water systems, Adv Colloid Interface Sci (2013), http://dx.doi.org/10.1016/j.cis.2013.10.001

G. Dardelle, P. Erni / Advances in Colloid and Interface Science xxx (2013) xxx–xxx

9

Fig. 8. Theoretical wetting state diagrams for a coacervate/oil/water system. Each subplot shows the feasibility of encapsulation based on interfacial equilibrium thermodynamics as a function of the interfacial tensions oil/coacervate and oil/water (σoil/coa,σoil/w, shown on the axes of the plots) for a given value of the coacervate/water interfacial tension (σcoa/w, constant for each subplot). Boundaries between the different colored regions are made to appear diffuse to emphasize experimental uncertainty in measuring ultralow interfacial tensions. Note that realistic values of coacervate/water interfacial tensions are below 1 mN/m.

5.1. Partial coverage — drops with ‘caps’

5.2. Fluid dynamics of fully encapsulated compound drops

Sadhal and Johnson [112] provided an exact solution for Stokes flow past bubbles and drops that are partially coated with a thin, stagnant film in the limit of spherical drop configurations. In particular, those authors provided results for the drag force as a function of the size of the stagnant cap of immobile film and of the capillary number. Additionally, the authors extended their study to the corresponding problem of drops covered with larger fluid caps with internal circulation [96,111]. One interesting result in the context of encapsulation processes is that the thickness of a macroscopic fluid cap on a primary drop under flow depends on the viscosity of the surrounding medium, rather than on the viscosity of the primary drop. To describe the flow of ‘capped droplets’ (assuming the shell is still liquid-like and not yet a gel), the Navier–Stokes equations need to be solved for each of the three phases i = o,w,c, and appropriate boundary conditions at the interfaces oil/coacervate, coacervate/water, and – in case not all the oil droplets are covered with coacervate – oil/water must be specified. Since the structure is axisymmetric, the drag force follows directly from the stream function [122]. For the full expressions of the stream functions for the different drop geometries we refer to the literature for the cases of clean drops [114,115,118], drops with a rigid cap Sadhal and Johnson [112], or fully encapsulated compound drops [113,123,124]. Curves for the normalized drag force Fd on compound drops with a rigid cap are shown in Fig. 10. Fd is shown for a range of drop-to-continuous phase viscosity ratios ηd/ηc and for varying angles of coverage with the cap φ, measured in radians in the forward apex direction of the drop. Two limiting values found: at small coverage angles, the drag force approaches that of a clean drop, identical to the Hadamard– Rybczynski solution [114,115]. For fully covered drops (φ → π), or for extremely high relative drop viscosities, Fd approaches the Stokes solution for the drag force on a solid sphere.

We now move from the ‘capped droplet’ geometry to fully encapsulated core/shell droplets. Although this geometry appears simpler at first sight, a different coordinate system is necessary to solve the Stokes equations: following the classical approach used for two interacting spheres in creeping flow by Stimson and Jeffery [125] in 1926, Sadhal and Oguz [113] showed that the flow field in and around compound droplets can be described analytically using a bipolar coordinate system. The translation of spherical core/shell drops under creeping flow conditions was studied by Rushton and Davies [124], motivated by applications in membrane separation processes, where the selective extraction of one liquid into a second immiscible liquid is of interest. Sadhal and Oguz [113] found analytical solutions for the flow field of core/shell drops in the limit of spherical shapes at small capillary numbers. They provided a parametrization of the geometry using bipolar coordinates following the classic study by Stimson and Jeffery [125] on the flow of two interacting spheres, and gave expressions for the stream functions, translational velocity and the drag force on the compound drop. In particular, these authors studied the effect of the eccentricity of the core liquid on the flow behavior, including the streamlines of the flow in all three phases, and also provided a stability analysis. We note that in the examples discussed herein, the shell of the liquid capsules is relatively thick; for thin shells in combination with relatively large core diameters, hydrodynamic instability of thin films becomes important [126,127]. When the oil/water interface is coated with the polymer-rich phase, it is typically already covered with a rigid polymer adsorption layer [128,129]. Such interfaces behave differently from those stabilized with small-molecular weight surfactants because the bulk stresses are no longer balanced by the interfacial tension alone, but interfacial shear and dilatational stresses [70,130–135].

Please cite this article as: Dardelle G, Erni P, Three-phase interactions and interfacial transport phenomena in coacervate/oil/water systems, Adv Colloid Interface Sci (2013), http://dx.doi.org/10.1016/j.cis.2013.10.001

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5.3. Deformation and fragmentation of compound drops Most analytical solutions for the flow of compound drops require the drop shape(s) to be spherical. In addition to the analytical solutions summarized above, most studies on the flow of core/shell drops now involve one of the available methods of computational fluid dynamics for free surface flows, typically focusing on the low Reynolds number regime (Stokes flow). Numerical simulation has become popular because the analytical treatment and interface parametrization of two concentric liquid interfaces that move and deform simultaneously are quite challenging already at small deformations (Fig. 9). From the analytical side, description of the small-deformation behavior has been achieved using perturbation methods [117,136]. Stone and Leal [117] provided analytical solutions for the behavior of core/shell droplets in linear flows for the limiting case of small deformations; for finite interface deformations, they performed numerical simulations using the boundary integral method. A combined Eulerian–Lagrangian technique was used in a computational study by Kan et al. [137] focusing on the deformation and recovery of a core/shell droplet in an extensional flow field. These authors used the ratio of the two time scales defined from the ratio of viscosity and capillary pressure both for the inner core and the outer drop to assess the deformation behavior of the drop. They found that sufficiently deformed core/shell drops behave like simple, homogeneous drops if these two time scales are comparable, but found different relaxation stages defined by the core and the shell if the two time scales are different.

Toose et al. [138] simulated the deformation of non-Newtonian compound drops in axisymmetric flow fields using a 2D boundary integral method; an Oldroyd-B fluid model was used and the effect of the non-Newtonian character of the outer membrane layer of the drop on the breakup behavior as studied. Further extending the range of capillary numbers to include the breakup regime, Smith et al. [119] numerically solved the equations of motion for core/shell drops in shear flow with a level set method [139] to describe the interface trajectories. These authors assembled a phase diagram for the different resulting droplet morphologies following breakup using the Capillary number and the ratio σi/σo, where σi and σo are the interfacial tensions of the inner and outer interfaces. The breakup of compound liquid threads (which ultimately results in the formation of core/shell droplets) was investigated in a comprehensive study by Craster et al. [140] using asymptotic analysis, linear stability analysis and numerical simulation. Feng's group uses a two phase, diffuse-interface approach [120,121,141] with a variational framework to model droplet flows (in this two-phase model, the core and the outer phase are the same fluid). In it, the classical, ‘sharp’ interfacial stress boundary condition is replaced by a diffuse interface term G∇ϕ, where G is the chemical potential expressed as a function of the three parameters (i) interfacial energy density, (ii) capillary width, and (iii) the phase field parameter ϕ. These authors successfully applied the finite element method with adaptive meshing to simulate the deformation of core/shell drops flowing through a contraction geometry [142]. Using this approach,

Analytical:

Small Deformation (First Order)

Zero Deformation (Spherical Compound Drops) Stream function

Shape of interfaces

Drag force Kinematic condition Rushton & Davies [124] Johnson et al. [111-113, 123]

Stone & Leal [117]

Numerical: Boundary Integral Method (BIM) BIM solution to quasi-steady Stokes equation:

Level Set Methods

Cahn-Hilliard/Diffuse Interface Approach

Momentum balance

Momentum balance incl. diffuse interface term

Kinematic condition

Equation for level set function L

Stone & Leal [117] Toose et al. [138]

Smith et al. [119] Chang et al. [139]

Phase field

(or: concentration field c)

Chemical potential

Park & Anderson [144] Feng & coworkers [120, 141-143] Fig. 9. Overview of the main methods used in the literature to investigate the fluid dynamics of core/shell drops, organized into analytical (top row) and numerical (bottom row) approaches along with key references. All methods start with the continuity equation ∇ ⋅ v = 0 and momentum balance; for the analytical approaches the Stokes equation is shown, whereas for the numerical methods we show the full Navier–Stokes equations as in the original publications, but with slightly modified notation for consistency of presentation. Zero deformation theories [111,112,124] provide information on streamlines, translation velocity, and drag force. Ψi is an exemplary stream function [124] in phase i, A–D are coefficients determined from the boundary conditions. Asymptotic analysis for small deformations involves solution of the shape for the interfaces along with the kinematic condition dxs/dt. The drop shape coefficients A12 and A23 for the inner and outer interfaces are a result of the analysis [117]. The Boundary Integral Method (BIM) allows to solve the Stokes equation numerically for larger deformation via a set of coupled integral equations [117,138]. The Level Set [119,139] and Diffuse Interface methods [144–146] both allow to predict deformation and breakup of compound drops in significant detail. Symbols used: v: velocity, v∞: far field velocity, p: pressure; η: viscosity, x, y: position vectors, n: interface unit normal vector, r: radial coordinate, κ: ratio inner/outer undeformed drop radius, E: rate-of-strain tensor, Ti: shear stress in phase i, J, K: BIM kernels [117], Re: Reynolds number, F: body force, Li: level set function, ϕ: phase field variable [142], μ: chemical potential, M: mobility parameter [144], f: specific free energy, : gradient energy parameter [144], c: concentration, N: number of fluids (here: 3). The index i identifies one of the three fluid phases (i = 1,2,3).

Please cite this article as: Dardelle G, Erni P, Three-phase interactions and interfacial transport phenomena in coacervate/oil/water systems, Adv Colloid Interface Sci (2013), http://dx.doi.org/10.1016/j.cis.2013.10.001

G. Dardelle, P. Erni / Advances in Colloid and Interface Science xxx (2013) xxx–xxx

a

11

b Fd / (4 v R)

1.4 d/ c

1.2

1.0

0

1

2

d/ c

= 10

d/ c

=1

d/ c

= 0.1

d/ c

= 0.001

3

(-)

c

d

Ca (-)

2.2

0.4

0

0.5

/ core/shell

shell/continuous

(-)

Fig. 10. (a) Illustration of flow patterns in the absence of deformation (shape is postulated to be spherical); from ‘clean’ drops to core/shell drops in a homogeneous flow field. Left and middle: drop with a sta\gnant interfacial region; right: fully enclosed compound drop surrounded by a macroscopic fluid layer. Streamlines plotted following the analytical work by Johnson and Sadhal [96] for spherical drops covered with a cap and compound drops. Flow direction of outer fluid is upwards. (b) Drag force Fd on a compound drop [112] as a function of the cap angle for varying drop/continuous phase viscosity ratios ηd/ηc. v∞: far field velocity, R: drop radius, φ: cap angle (in radians). (c,d) Sample results from the literature for the simulation of deformation and breakup of compound drops, as investigated by Smith et al. [119] (data replotted, with permission). Ca: capillary number; σi: interfacial tensions.

Zhou et al. [142,143] also performed detailed numerical studies on the formation of compound drops formed from concentric channels in flow-focusing type geometries. In particular, these authors used the adaptive meshing/diffuse interface technique to construct a morphology diagram for compound droplet formation, predicting morphologies as a function of the Capillary number, flow rate ratio and viscosity ratios. Full three-phase simulations with a diffuse interface model were used by Anderson's group [144] to predict the formation of core/shell drops in axisymmetric flow-focussing geometries. They solved the Navier–Stokes/Cahn–Hilliard model [145,146] for three immiscible, Newtonian fluids using a finite element method. The four governing equations are the mass and momentum balances, a composition equation, and a difference equation for the chemical potential. For the finite element solution using a velocity/pressure formulation, the latter two equations are first solved at each time step and are then used to solve the flow problem. For the Cahn–Hilliard approach, the interfacial thermodynamics as described above for coacervates follows from the model's free energy parameters; these derive (in principle) from the phase diagrams of the systems studied, but for simulations they are typically postulated such that core/shell configurations ensue. 6. Summary and conclusions The core/shell structure of complex coacervate compound drops or capsules is not formed by direct, spontaneous spreading of the coacervate phase on the oil drops, but rather by successive buildup of a shell layer by deposition and fusion of individual coacervate nodules.

Complex coacervation forms two liquid phases: a polymer-rich coacervate phase and a dilute continuous solvent phase. If a third liquid or solid phase in the form of disperse oil droplets is present, the coacervate phase tends to wet the oil/water interface. Here, we discussed the connection between wetting phenomena and fluid viscoelasticity in coacervate/oil/water systems from the perspective of both colloid chemistry and fluid engineering. An important point to take into account is the non-Newtonian rheology of typical protein-based coacervates used in practically relevant delivery systems. It influences not only the fluid dynamics of the encapsulation process, but also the physicochemical measurements that depend on the viscosity. In particular, the interfacial tension between coacervate phases and their coexisting solvent is known to be orders of magnitude lower than typical oil/water gas/liquid interfacial tensions. Recent new methods for nanomechanical capillary force measurements have led to new insight on the interfacial tension of coacervates. In this article, we also outlined how quantitative analysis of the capillary thinning dynamics of liquid threads can potentially be used to estimate the interfacial tension in coacervate/water systems. For the capillary thinning method, the fluid rheology must be known. This point is also a conceptual advantage: it forces the user to directly address the question of coacervate viscoelasticity and gel formation, whereas in nanomechanical methods (AFM, surface force apparatus) these important effects are obscured since they are not included in the force balance used for data analysis. For typical coacervates formed by gelatin as the protein and Acacia gum as the weak polyanion, the temperature-dependent rheology appears to be dominated by the protein component above, around, and below the gel point. A wetting state

Please cite this article as: Dardelle G, Erni P, Three-phase interactions and interfacial transport phenomena in coacervate/oil/water systems, Adv Colloid Interface Sci (2013), http://dx.doi.org/10.1016/j.cis.2013.10.001

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analysis for compound drops based on equilibrium surface thermodynamics is the basis to understand wetting and coating of coacervate/ oil/water systems, but those approaches do not yet account for rheology and fluid dynamical effects. Deformation and breakup of core/shell compound drops are governed by the three viscosities and their ratios, the two interfacial tensions, and the relative and absolute sizes. In the simplest approach for experiments and analytical or computational modeling, these properties can be written as a Capillary number for either the inner or outer interface (i.e. for one fluid pair) and a ratio of interfacial tensions or viscosities, while keeping the ratios of the other fluid pair constant. Predictions of the deformation and breakup behavior using analytical approaches (asymptotic analysis and perturbation methods) are more challenging for compound drops than for simple drops (geometrical parametrization of the two concentric or eccentric interfaces can become tedious even for the seemingly simply case of near-spherical drops). Therefore, recent work on the multiphase fluid dynamics of compound drops has focused on computational methods, including boundary integral or diffuse interface approaches. References [1] Yeo Y, Bellas E, Firestone W, Langer R, Kohane DS. J Agric Food Chem 2005;53: 7518. [2] Erni P, Dardelle G, Sillick M, Wong K, Beaussoubre P, Fieber W. Angew Chem Int Ed 2013;125:10524. [3] Martinez CJ, Kim JW, Ye C, Ortiz I, Rowat AC, Marquez M, et al. Macromol Biosci 2012;12:946. [4] Pretzl M, Neubauer M, Tekaat M, Kunert C, Kuttner C, Leon G, et al. ACS Appl Mater Interfaces 2012;4:2940. [5] Bouquerand P, Dardelle G, Erni P, Normand V. In: Garti N, McClements DJ, editors. Encapsulation technologies and delivery systems for food ingredients and nutraceuticals. Cambridge UK: Woodhead Publishing Ltd.; 2012. [6] Shum HC, Kim J-W, Weitz DA. J Am Chem Soc 2008;130:9543. [7] Kim S-H, Abbaspourrad A, Weitz DA. J Am Chem Soc 2011;133:5516. [8] Park JI, Saffari A, Kumar S, Gunther A, Kumacheva E. Ann Rev Mater Res 2010;40: 415. [9] Cramer C, Fischer P, Windhab EJ. Chem Eng Sci 2004;59:3045. [10] Erni P, Cramer C, Marti I, Windhab EJ, Fischer P. Adv Colloid Interface Sci 2009;150: 16. [11] Whitesides GM. Nature 2006;442:368. [12] Whitesides GM. Lab Chip 2013;13:11. [13] Matalanis A, Lesmes U, Decker EA, McClements DJ. Food Hydrocoll 2010;24: 689. [14] Matalanis A, McClements D. Food Biophys 2012;7:72. [15] Matalanis A, McClements DJ. Food Hydrocoll 2013;31:15. [16] Lemetter CYG, Meeuse FM, Zuidam NJ. AIChE J 2009;55:1487. [17] Weinbreck F, Minor M, De Kruif CG. J Microencapsul 2004;21:667. [18] Veith SR, Perren M, Pratsinis SE. J Colloid Interface Sci 2005;283:495. [19] Bungenberg de Jong HG, Kruyt HR. Proc K Ned Akad Wet 1929;32:849. [20] Bungenberg de Jong HG. In: Kruyt HR, editor. Colloid science, vol. II. Amsterdam: Elsevier; 1949. p. 232–58 [chap. VIII]. [21] Veis A, Aranyi C. J Phys Chem 1960;64:1203. [22] de Kruif CG, Weinbreck F, de Vries R. Curr Opin Colloid Interface Sci 2004;9: 340. [23] Veis A. Adv Colloid Interface Sci 2011;167:2. [24] Schmitt C, Sanchez C, Desobry-Banon S, Hardy J. Crit Rev Food Sci Nutr 1998;38: 689. [25] Bohidar H, Dubin PL, Majhi PR, Tribet C, Jaeger W. Biomacromolecules 2005;6: 1573. [26] Kayitmazer AB, Bohidar HB, Mattison KW, Bose A, Sarkar J, Hashidzume A, et al. Soft Matter 2007;3:1064. [27] Kaur S, Weerasekare GM, Stewart RJ. ACS Appl Mater Interface 2011;3:941. [28] Hwang DS, Waite JH, Tirrell M. Biomaterials 2010;31:1080. [29] Hwang DS, Zeng H, Srivastava A, Krogstad DV, Tirrell M, Israelachvili JN, et al. Soft Matter 2010;6:3232. [30] Chollakup R, Smitthipong W, Eisenbach CD, Tirrell M. Macromolecules 2010;43: 2518. [31] Schmitt C, Turgeon SL. Adv Colloid Interface Sci 2011;167:63. [32] Turgeon SL, Beaulieu M, Schmitt C, Sanchez C. Curr Opin Colloid Interface Sci 2003;8:401. [33] Turgeon SL, Schmitt C, Sanchez C. Curr Opin Colloid Interface Sci 2007;12:166. [34] Kizilay E, Kayitmazer AB, Dubin PL. Adv Colloid Interface Sci 2011;167:24. [35] Doublier JL, Garnier C, Renard D, Sanchez C. Curr Opin Colloid Interface Sci 2000;5: 202. [36] Mekhloufi G, Sanchez C, Renard D, Guillemin S, Hardy J. Langmuir 2005;21:386. [37] Sanchez C, Mekhloufi G, Schmitt C, Renard D, Robert P, Lehr CM, et al. Langmuir 2002;18:10323. [38] Sanchez C, Mekhloufi G, Renard D. J Colloid Interface Sci 2006;299:867.

[39] Girard M, Sanchez C, Laneuville SI, Turgeon SL, Gauthier SE. Colloid Surf B 2004;35: 15. [40] Espinosa-Andrews H, Baez-Gonzalez JG, Cruz-Sosa F, Vernon-Carter EJ. Biomacromolecules 2007;8:1313. [41] Espinosa-Andrews H, Sandoval-Castilla O, Vazquez-Torres H, Vernon-Carter EJ, Lobato-Calleros C. Carbohydr Polym 2010;79:541. [42] Mohanty B, Gupta A, Bohidar HB, Bandyopadhyay S. J Polym Sci B Polym Phys 2007;45:1511. [43] Spruijt E, Sprakel J, Lemmers M, Stuart MAC, van der Gucht J. Phys Rev Lett 2010;105:208301. [44] Weinbreck F, Wientjes RHW, Nieuwnhujise H, Robijn GW, De Kruif CG. J Rheol 2004;48:1215. [45] Thomasin C, Merkle HP, Gander BA. Int J Pharm 1997;147:173. [46] Joly-Duhamel C, Hellio D, Ajdari A, Djabourov M. Langmuir 2002;18:7158. [47] Gurov AN, Nuss PV. Food Nahrung 1986;30:349. [48] Burgess DJ. In: Dubin PL, editor. Macromolecular complexes in chemistry and biology. Berlin: Springer; 1994. [49] Thomassin C, Merkle HP, Gander BA. Int J Pharm 1997;147:173. [50] Liberatore MW, Wyatt NB, Henry M, Dubin PL, Foun E. Langmuir 2009;25:13376. [51] Priftis D, Megley K, Laugel N, Tirrell M. J Colloid Interface Sci 2013;398:39. [52] Sanchez C, Despond S, Schmitt C, Hardy J. In: Dickinson E, Miller R, editors. Food colloids: fundamentals of formulation. London: Royal Society of Chemistry; 2001. p. 332. [53] Minale M, Mewis J, Moldenaers P. AIChE J 1998;44:943. [54] Antonov YA, Van Puyvelde P, Moldenaers P. Food Hydrocoll 2012;27:264. [55] Wolf B, Frith W. J Rheol 2003;47:1151. [56] Spyropoulos F, Ding P, Frith W, Norton I, Wolf B. J Colloid Interface Sci 2008;317:604. [57] Norton I, Frith W. Food Hydrocoll 2001;15:543. [58] Wolf B, Frith W, Singleton S, Tassieri M, Norton I. Rheol Acta 2001;40:238. [59] Wolf B, Scirocco R, Frith W, Norton I. Food Hydrocoll 2000;14:217. [60] van Puyvelde P, Antonov YA, Moldenaers P. Food Hydrocoll 2003;17:327. [61] Antonov YA, van Puyvelde P, Moldenaers P. Biomacromolecules 2004;5:276. [62] Priftis D, Farina R, Tirrell M. Langmuir 2012;28:8721. [63] Spruijt E, Sprakel J, Cohen Stuart MA, van der Gucht J. Soft Matter 2010;6:172. [64] Le XT, Turgeon SL. Soft Matter 2013;9:3063. [65] Parker A, Normand V. Soft Matter 2010;6:4916. [66] de Ruiter L, Bungenberg de Jong HG. Proc K Ned Akad Wet 1947;50:836. [67] van Puyvelde P, Antonov YA, Moldenaers P. Food Hydrocoll 2002;16:395. [68] Scholten E, Visser JE, Sagis LMC, van der Linden E. Langmuir 2004;20:2292. [69] Scholten E, Sprakel J, Sagis LMC, van der Linden E. Biomacromolecules 2006;7:339. [70] Erni P, Fischer P, Windhab EJ. Macromol Mater Eng 2011;296:249. [71] Tomotika S. Proc R Soc London Ser A 1935;150:322. [72] de Gennes PG, Brochard-Wyart F, Quere D. Capillarity and wetting phenomena. New York: Springer; 2004. [73] Demarquette NR. Int Mater Rev 2003;48:247. [74] Yu JH, Fridrikh SV, Rutledge GC. Polymer 2006;47:4789. [75] Clasen C, Eggers J, Fontelos MA, McKinley GH. J Fluid Mech 2006;556:283. [76] McKinley GH. Rheology reviews. Aberystwyth UK: The British Society of Rheology; 2005. p. 1–48. [77] Entov VM, Hinch EJ. J Non-Newtonian Fluid Mech 1997;72:31. [78] Anna SL, McKinley GH. J Rheol 2001;45:115. [79] Erni P, Varagnat M, Clasen C, Crest J, McKinley GH. Soft Matter 2011;7:10889. [80] Eggers J. Rev Mod Phys 1999;69:865. [81] Stelter M, Brenn G, Yarin AL, Singh RP, Durst F. J Rheol 2002;46:507. [82] Arratia PE, Gollub JP, Durian DJ. Phys Rev E 2008;77:036309. [83] Rodd LE, Scott TP, Cooper-White JJ, McKinley GH. Appl Rheol 2005;15:12. [84] von Ohnesorge W. Z Angew Math Mech 1936;16:355. [85] McCarthy MJ, Malloy NA. Chem Eng J 1974;7:1. [86] Lin SP, Reitz RD. Annu Rev Fluid Mech 1998;30:85. [87] Bhat BP, Appathurai S, Harris MT, Pasquali M, McKinley GH, Basaran OA. Nat Phys 2010;6:625. [88] Gramespacher H, Meissner J. J Rheol 1992;36:1127. [89] Papageorgiou DT. Phys Fluids 1995;7:1529. [90] Soga I, Dhinojwala A, Granick S. Langmuir 1998;14:1156. [91] Clasen C, McKinley GH. J Non-Newtonian Fluid Mech 2004;124:1. [92] Waters JA. Colloid Surf A 1994;83:167. [93] van Zyl AJP, Sanderson RD, deWet-Roos D, Klumperman B. Macromolecules 2003;36:8621. [94] Shum HC, Santanach-Carreras E, Kim J-W, Ehrlicher J, Bibette J, Weitz DA. J Am Chem Soc 2011;133:4420. [95] Jeong J, Um E, Park J-K, Kim MW. RSC Adv 2013;3:11801. [96] Johnson RE, Sadhal SS. Annu Rev Fluid Mech 1985;17:289. [97] Habibi M, Maleki M, Golestanian R, Ribe NM, Bonn D. Phys Rev E 2006;74:0066306. [98] Majmudar TS, Varagnat M, Hartt W, McKinley GH. arxiv.org p. 1012.2135v1 2010. [99] Cubaud T, Mason TG. Phys Rev Lett 2006;96:114501. [100] Cubaud T, Mason TG. New J Phys 2009;11:075029. [101] Cubaud T, Jose BM, Darvishi S. Phys Fluids 2011;23:042002. [102] Cubaud T, Mason TG. Soft Matter 2012;8:10573. [103] Abkarian M, Loiseau E, Massiera G. Soft Matter 2011;7:4610. [104] Green BK, Schleicher L. US Patent 2800457, The National Cash Register Co. 1957. [105] Kopac MJ, Chambers R. J Cell Comp Physiol 1937;9:345. [106] Mar A, Mason SG. Kolloid-Z 1968;224:161. [107] Mar A, Mason SG. Kolloid-Z 1968;225:55. [108] Huh C, Inoue M, Mason SG. Can J Chem Eng 1975;53:367. [109] Torza S, Mason SG. Science 1969;163:813. [110] Torza S, Mason SG. J Colloid Interface Sci 1970;33:67.

Please cite this article as: Dardelle G, Erni P, Three-phase interactions and interfacial transport phenomena in coacervate/oil/water systems, Adv Colloid Interface Sci (2013), http://dx.doi.org/10.1016/j.cis.2013.10.001

G. Dardelle, P. Erni / Advances in Colloid and Interface Science xxx (2013) xxx–xxx [111] [112] [113] [114] [115] [116] [117] [118] [119] [120] [121] [122] [123] [124] [125] [126] [127] [128]

Johnson RE, Sadhal SS. J Fluid Mech 1983;132:295. Sadhal SS, Johnson RE. J Fluid Mech 1983;126:237. Sadhal SS, Oguz HN. J Fluid Mech 1985;160:511. Hadamard JS. C R Acad Sci 1911;152:1735. Rybczynski W. Bull Int Acad Pol Sci Lett Cl Sci Math Nat Ser A 1911:41–6. Clift R, Grace J, Weber M. Bubbles, drops, and particles. Mineola NY: Dover; 2005. Stone HA, Leal LG. J Fluid Mech 1990;211:123. Leal LG. Advanced transport phenomena: fluid mechanics and convective transport processes. Cambridge, UK: Cambridge University Press; 2010. Smith KA, Ottino JM, Cruz MOdl. Phy Rev Lett 2004;93:204501. Yue P, Zhou C, Feng JJ, Ollivier-Gooch CF, Hu HH. J Comput Phys 2006;219:47. Gao P, Feng JJ. J Fluid Mech 2011;682:415. Payne LE, Pell WH. J Fluid Mech 1960;7:529. Oguz HN, Sadhal SS. J Fluid Mech 1987;179:105. Rushton E, Davies GA. Int J Multiphase Flow 1983;9:337. Stimson M, Jeffery GB. Proc R Soc London Ser A 1926;111. Patzer II JF, Homsy GM. J Colloid Interface Sci 1975;51:499. Craster RV, Matar OK. Rev Mod Phys 2009;81:1131. Erni P, Windhab EJ, Gunde R, Graber M, Pfister B, Parker A, et al. Biomacromolecules 2007;8:3458.

[129] [130] [131] [132] [133] [134] [135] [136] [137] [138] [139] [140] [141] [142] [143] [144] [145] [146]

13

Erni P, Parker A. Langmuir 2012;28:7757. Phillips WJ, Graves RW, Flumerfelt RJ. J Colloid Interface Sci 1980;76:350. Flumerfelt RW. J Colloid Interface Sci 1980;76:330. Sagis LMC. Rev Mod Phys 2011;83:1367. Edwards DA, Brenner H, Wasan DT. Interfacial transport processes and rheology. Stonheam, MA: Butterworth-Heinemann; 1991. Slattery JC, Sagis L, Oh E-S. Interfacial transport phenomena. 2nd ed. New York: Springer-Verlag; 2007. Fuller GG, Vermant J. Ann Rev Chem Biomol Eng 2012;3:519. Ha J-W, Yang S-M. Phys Fluids 1999;11:1029. Kan H-C, Udaykumar HS, Shyy W, Tran-Son-Tay R. Phys Fluids 1998;10:760. Toose EM, Geurts BJ, Kuerten JGM. Int J Numer Methods Fluids 1999;30:653. Chang YC, Hou TY, Merriman B, Osher S. J Comput Phys 1996;124:449. Craster RV, Matar OK, Papageorgiou DT. J Fluid Mech 2005;533:95. Yue P, Feng JJ, Liu C, Shen JIE. J Fluid Mech 2004;515:293. Zhou C, Yue P, Feng JJ. Int J Multiphase Flow 2008;34:102. Zhou C, Yue P, Feng JJ. Phys Fluids 2006;18:092105. Park JM, Hulsen MA, Anderson PD. Eur Phys J Spec Top 2013;222:199. Lowengrub J, Truskinovsky L. Proc R Soc London Ser A 1998;454:2617. Kim J, Lowengrub J. Interfaces Free Bound 2005;7:435.

Please cite this article as: Dardelle G, Erni P, Three-phase interactions and interfacial transport phenomena in coacervate/oil/water systems, Adv Colloid Interface Sci (2013), http://dx.doi.org/10.1016/j.cis.2013.10.001