FRAGMENTATION OF PRIMARY FLOCS IN EMULSIONS AND THE SUBSEQUENT REDUCTION OF COALESCENCE

In existing theories emulsion desiabilization is considered as the combined processes of irreversible flocculation and coalescence of dispersed droplets. This approach can be justified when the potential pit characterizing the energy of droplet interaction is sufficiently deep, i.e. excluding small droplet dimensions, strong electrosiatic repulsion and low electrolyte concentrations. For smaller droplet dimensions and stronger electrostatic repulsions the emulsion instability must be considered as a combined process of reversible flocculation and coalescence. In this paper a mathematical model that couples the kinetics of flocculation, coalescence and floe fragmentation is developed in order to quantify the kinetic instability of emulsions with charged submicron droplets. The characteristic limes for flocculation (Smoluchowski's time τ_c) for coalescence (coalescence time τ_c) and for disaggregation (doublet lifetimeτ_d) are considered model parameters. The mathematical model applies to the case when and τ_d<< τ_c, which corresponds to a situation with a small multiplet concentration compared to the concentration of doublets and a singlet-doublet quasi-equilibrium. It is established that at singlet-doublet quasi-equilibrium the rate of the decline in the total droplet concentration is described by second order kinetics in distinction to the exponential time dependence valid for coalescence at irreversible flocculation. The double disintegration reduces the entire coalescence rate, expressed as τ_sm/ τ_d. This reduction is very large at small values of Td. The mathematical model presented can hased on the spontaneous disintegration of doublets predict changes in emulsion stability for model systems and also for technologically important emulsions.     

Atomic force microscopy of emulsion droplets: probing droplet-droplet interactions

A method has been developed for attaching oil (tetradecane) droplets to the end of an atomic force microscopy (AFM) cantilever and for immobilizing droplets on a glass substrate. This approach has permitted the monitoring of droplet-droplet interactions in aqueous solution as a function of interdroplet separation. Coating the droplet surfaces with added proteins or surfactants has allowed the production of model emulsions. We demonstrate that AFM measurements of droplet deformability are sensitive to interfacial rheology by modifying the interfacial film on a pair of droplets in situ. For droplets coated with the anionic surfactant sodium dodecyl sulfate, screening of the double layer has been found to facilitate coalescence. Direct imaging of the droplets has revealed the presence of regularly spaced concentric rings on the droplet surfaces. Careful experimental studies suggest that these structures may be imaging artifacts and are not perturbations of the droplet surface determined by the composition of the interface.     

Formation and Release of Cobalt(II) Sorption and Precipitation Products in Aging Kaolinite-Water Slurries

A comprehensive simulation of the coarsening mechanism coalescence-induced coalescence (CIC) is developed to predictthe growth rate of inviscid droplets in a viscous matrix fluid. In CIC, the shape relaxations of coalescing droplets establish flow fields that drive other droplets into contact, thus creating a cascade of coalescence events. It is believed that CIC is responsible for droplet growth in some demixed polymer solutions, such as isotactic polypropylene (iPP) and diphenyl ether (DPE). A cascade of coalescence events is simulated using a three-dimensional molecular dynamics-like simulation of a dispersed two-phase isopycnic fluid system. The coalescence-induced flow is driven mostly by the strong gradients in curvature at the neck of a coalescing pair of droplets, and the flow is modeled analytically by approximating it as due to a ring of point forces. The resultant velocity of each droplet in the suspension is calculated by superimposing all of the coalescence-induced flow fields and applying Faxen's Law. The mean droplet size grows like t(xi), where t is the coarsening time and xi a growth exponent that increases with increasing minority phase volume fraction varphi. Good agreement with experimental values of xi (0.22/=0.23. It is also shown that the droplet size distribution broadens for semidilute suspensions (varphi/=0.54). A phenomenological kinetic theory of coalescence is proposed. It is believed that in nondilute emulsions, CIC can account for coarsening that has been attributed previously to more traditional coalescence mechanisms. Copyright 2000 Academic Press.     

Numerical simulation of droplet coalescence behavior in gas phase under the coupling of electric field and flow field

The coalescence behavior of droplets in an electric field belongs to the important research contents of electrohydrodynamics. Based on the phase field method of the Cahn–Hilliard equation, the electric field and the flow field are coupled to establish the numerical model of twin droplet coalescence in a coupled field. The effects of flow rate, electric field strength, droplet diameter, and interfacial tension on the coalescence behavior of droplets during the coalescence process were investigated. The results show that the dynamic behavior of the droplets is divided into coalescence, after coalescence rupture, and no coalescence under the coupling of electric field and flow field. The proper increase of the electric field strength will accelerate the coalescence of the droplets, and the high electric field strength causes the droplets to burst after coalescence. Excessive flow rates make droplets less prone to coalescence. Under the coupling field, the larger the droplet interface tension, the smaller the droplet diameter, the smaller the flow rate, and the shorter the droplet coalescence time. The results provide a theoretical basis for the application of electrostatic coalescence in gas–liquid separation technology.     

Steric repulsion between internal aqueous droplets and the external aqueous phase in double emulsions

A theoretical model for analyzing the steric repulsion energy between internal aqueous droplets and the external aqueous phase in double emulsions, which results from the steric interaction between the surfactant molecules adsorbed at the two interfaces, has been established. The steric interaction is dependent on the separation distance between the internal aqueous droplets and the external aqueous phase, the thicknesses of the two adsorbed surfactant layers, and the size of the internal aqueous droplets and the oil globules, all of which determine the extent of the compression of the adsorbed surfactant molecules. The thickness of each of the two surfactant layers have the same effect on the steric repulsion, and stronger steric interaction can be achieved with thicker adsorbed layers, which can effectively prevent coalescence between the internal aqueous droplets and the external aqueous phase. Increasing the internal aqueous droplet size can produce stronger steric repulsion; however, larger oil globules will weaken the steric repulsion, indicating that a more stable double-emulsion system can be achieved by preparing the system with smaller oil globules and larger internal aqueous droplets.     

Controlled production of monodisperse double emulsions by two-step droplet breakup in microfluidic devices

A microfluidic device having both hydrophobic and hydrophilic components is exploited for production of multiple-phase emulsions. For producing water-in-oil-in-water (W/O/W) dispersions, aqueous droplets ruptured at the upstream hydrophobic junction are enclosed within organic droplets formed at the downstream hydrophilic junction. Droplets produced at each junction could have narrow size distributions with coefficients of variation in diameter of less than 3%. Control of the flow conditions produces variations in internal/external droplet sizes and in the internal droplet number. Both W/O/W emulsions (with two types of internal droplets) and oil-in-water-in-oil emulsions were prepared by varying geometry and wettability in microchannels.     

Induction of instability in water-in-oil-in-water double emulsions by freeze-thaw cycling

Individual water-in-oil-in-water (W1/O/W2) double-emulsion globules loaded with fluorescently labeled bovine serum albumin (FITC-BSA) were optically monitored within cylindrical capillaries during freeze-thaw cycling. Coalescence of internal aqueous droplets (W1) and external aqueous phase (W2), termed external coalescence, was not observed before or during freezing of the oil phase (O). On the other hand, this instability mechanism was readily promoted during thawing. This realization confirms the previously suggested potential of W1/O/W2 double emulsions to trigger release upon oil thawing and demonstrates that it is a direct result of globule breakage through external coalescence. The presented results also identified a threshold in the relative W1 droplet size above which instability occurred, while smaller droplets remained unperturbed and therefore indicate that optimization of the delivery can be achieved by tuning the size of W1 droplets. In addition, we propose a possible explanation for the occurrence of instability during oil thawing and its dependence on the size of W1 droplets. Because this alternative globule-breakage mechanism simply uses temperature increase (solid-to-liquid-phase transition) as external stimulus, W1/O/W2 double-emulsion delivery systems can be easily tailored by choosing an oil phase with the appropriate phase-transition temperature.     

Dielectric behavior of double emulsions with "core-shell droplet" morphology

The dielectric behavior of double emulsions with "core-shell droplet" morphology is investigated over a broad range of frequency. A new modified Pauly-Schwan model is proposed for the complex permittivity of double emulsions. The proposed model takes into consideration the morphology and packing limit of droplets. The dielectric behaviors of oil-in-water-in-oil (O/W/O) and water-in-oil-in-water (W/O/W) types of double emulsions, as predicted by the proposed model, are discussed.     

Temperature-induced protein release from water-in-oil-in-water double emulsions

A model water-in-oil-in-water (W1/O/W2) double emulsion was prepared by a two-step emulsification procedure and subsequently subjected to temperature changes that caused the oil phase to freeze and thaw while the two aqueous phases remained liquid. Our previous work on individual double-emulsion globules1 demonstrated that crystallizing the oil phase (O) preserves stability, while subsequent thawing triggers coalescence of the droplets of the internal aqueous phase (W1) with the external aqueous phase (W2), termed external coalescence. Activation of this instability mechanism led to instant release of fluorescently tagged bovine serum albumin (fluorescein isothiocyanate (FITC)-BSA) from the W 1 droplets and into W2. These results motivated us to apply the proposed temperature-induced globule-breakage mechanism to bulk double emulsions. As expected, no phase separation of the emulsion occurred if stored at temperatures below 18 degrees C (freezing point of the model oil n-hexadecane), whereas oil thawing readily caused instability. Crucial variables were identified during experimentation, and found to greatly influence the behavior of bulk double emulsions following freeze-thaw cycling. Adjustment of these variables accounted for a more efficient release of the encapsulated protein.     

Preparation characteristics of water-in-oil-in-water multiple emulsions using microchannel emulsification

Microchannel (MC) emulsification is a novel technique for preparing monodispersed emulsions. This study demonstrates preparing water-in-oil-in-water (W/O/W) emulsions using MC emulsification. The W/O/W emulsions were prepared by a two-step emulsification process employing MC emulsification as the second step. We investigated the behavior of internal water droplets penetrating the MCs. Using decane, ethyl oleate, and medium-chain triglyceride (MCT) as oil phases, we observed successful MC emulsification and prepared monodispersed oil droplets that contained small water droplets. MC emulsification was possible using triolein as the oil phase, but polydispersed oil droplets were formed from some of the channels. No leakage of the internal water phase was observed during the MC emulsification process. The internal water droplets penetrated the MC without disruption, even though the internal water droplets were larger than the resulting W/O/W emulsion droplets. The W/O/W emulsion entrapment yield was measured fluorometrically and found to be 91%. The mild action of droplet formation based on spontaneous transformation led to a high entrapment yield during MC emulsification.     

Prediction of emulsion particle sizes using a computational fluid dynamics approach

For many food products emulsification processes play an important role. Examples are ice cream, spreads, sauces, etc. As is well known, droplet break-up and coalescence phenomena are the local processes underlying the control of particle size in an emulsion process. Quite a number of studies have generated scaling laws which can be easily applied and which are useful in the design of a process. However, the prediction of particle sizes in an inhomogeneous flow, where the flow velocity is changing spatially in strength and direction and with time, is not yet well established. For one-phase flows computational fluid dynamics (CFD) methodologies are in use to predict details on the flow with quite some success. This methodology has been extended to capture the dispersed phase in an efficient way. The essence is that break-up and coalescence processes determine source terms in a transport equation for the moments of the particle size distribution, while velocity vectors as obtained in the one-phase CFD simulation determine the convective term. This method allows particle size prediction in any equipment. The approach is illustrated for the particle size evolution of an oil-in-water emulsion, for a phase-separated biopolymeric mixture (a so-called water-in-water emulsion) and for the escape of the included oil droplets from a double emulsion of the type oil-in-water-in-oil. In all cases experimental results are compared with simulation results, which match very well. This shows the strength of the method.     

Mastering a double emulsion in a simple co-flow microfluidic to generate complex polymersomes

We show that the production and the geometrical shape of complex polymersomes can be predicted by varying the flow rates of a simple microdevice using an empirical law which predicts the droplet size. This device is constituted of fused silica capillaries associated with adjusted tubing sleeves and T-junctions. Studying the effect of several experimental parameters, double emulsions containing a controlled number of droplets were fabricated. First, this study examines the stability of a jet in a simple confined microfluidic system, probing the conditions required for droplets production. Then, multicompartmental polymersomes were formed, controlling flow velocities. In this work, poly(dimethylsiloxane)-graft-poly(ethylene oxide) (PDMS-g-PEO) and poly(butadiene)-block-poly(ethyleneoxide) (PBut-b-PEO) amphiphilic copolymers were used and dissolved in chloroform/cyclohexane mixture. The ratio of these two solvents was adjusted in order to stabilize the double emulsion formation. The aqueous suspension contained poly(vinyl alcohol) (PVA), limiting the coalescence of the droplets. This work constitutes major progress in the control of double emulsion formation in microfluidic devices and shows that complex structures can be obtained using such a process.     

Fat retention at the tongue and the role of saliva: adhesion and spreading of 'protein-poor' versus 'protein-rich' emulsions

Fat perception of food emulsions has been found to relate to in-mouth friction. Previously, we have shown that friction under mouth-like conditions strongly depends on the sensitivity of protein-stabilized emulsion droplets to coalescence. Here, we investigated whether this also implies that oral fat retention depends in a similar manner on the stability of the emulsion droplets against coalescence. We investigate the separate contributions of droplet adhesion and droplet spreading to fat retention at the tongue, as well as the role of saliva. We perform ex vivo (Confocal Raman Spectroscopy; Confocal Scanning Laser Microscopy) experiments using pig's tongue surfaces in combination with human in vivo experiments. These reveal that protein-poor (unstable) emulsions are retained more at the tongue than protein-rich (stable) emulsions. Furthermore, the layer formed by adhering protein-poor droplets is more stable against rinsing. Saliva is found to be very efficient in removing fat and emulsion droplets from the oral surface but its role in fat retention needs further research. We relate our results to the colloidal forces governing droplet adhesion and spreading.     

Effect of Ionic Strength of Dispersed Phase on Ostwald Ripening in Water-in-Oil Emulsions

Water-in-oil emulsions with a low electrolyte content in the internal phase are unstable with respect to Ostwald ripening. The main components of the total pressure acting on the surface of internal phase droplets are considered. The equilibrium values of the diameters of dispersed phase droplets are calculated. The dependences of the difference in the osmotic and Laplace pressures on the droplet size and electrolyte concentration in the droplets are obtained. It is shown that, at the electrolyte concentration below the critical value, the number of droplets in emulsion decreases. If the concentration is above the critical value, water diffuses from small to large droplets, but their number remains unchanged. The change in NaCl concentration in the droplets of internal phase of polydisperse emulsion during the Ostwald ripening is calculated. The results of calculations correlate with the experimental data on the stability of emulsions with respect to coalescence and sedimentation.     

Langevin-dynamics simulation of flocculation in water-in-oil emulsions

Aggregation of internal phase droplets in water-in-oil emulsions has been simulated by the Langevin-dynamics method. At initial stages, the process rate obeys the Smoluchowski equations. The concentration ranges have been determined in which percolation clusters are formed. It has been shown that the examined emulsions may be divided into three groups, which enable one to predict their sedimentation stability. Percolation clusters are not formed in emulsions with internal-phase fractions of ≤0.1; therefore, their sedimentation rate is high. In the range of internal-phase fractions from 0.1 to 0.3–0.4, the percolation clusters are formed within a few fractions of a second; however, the rate of subsequent droplet addition to the clusters is low. The flocculation of internal-phase droplets results in the formation of a weak network structure followed by its densification and the separation of a dispersion medium with time. In emulsions with internalphase fractions of >0.3–0.4, almost all droplets unite into percolation clusters within a few fractions of a second. This structure is slowly densified; therefore, the rate of dispersion medium separation is low. The results obtained have been confirmed by experimental data on the sedimentation stability of the water-in-oil emulsions.     

Coalescence in semiconcentrated emulsions in simple shear flow

The coalescence frequency in emulsions containing droplets with a low viscosity (viscosity ratio approximately 0.005) in simple shear flow has been investigated experimentally at several volume fractions of the dispersed phase (2%-14%) and several values of the shear rate (0.1-10 s(-1)). The evolution of the size distribution was monitored to determine the average coalescence probability from the decay of the total number of droplets. Theoretically models for two-droplet coalescence are considered, where the probability is given by P(c)=exp(-tau(dr)tau(int)). Since the drainage time tau(dr) depends on the size of the two colliding droplets, and the collision time tau(int) depends on the initial orientation of the colliding droplets, the calculated coalescence probability was averaged over the initial orientation distribution and the experimental size distribution. This averaged probability was compared to the experimentally obtained coalescence frequency. The experimental results indicate that (1) to predict the average coalescence probability one has to take into account the full size distribution of the droplets; (2) the coalescence process is best described by the "partially mobile deformable interface" model or the "fully immobile deformable interface" model of Chesters [A. K. Chesters, Chem. Eng. Res. Des. 69, 259 (1991)]; and (3) independent of the models used it was concluded that the ratio tau(dr)tau(int) scales with the coalescence radius to a power (2+/-1) and with the rate of shear to a power (1.5+/-1). The critical coalescence radius R(o), above which hardly any coalescence occurs is about 10 microm.     

Effect of Emulsifier Type on the Characterization of O/W Emulsions Using a Spectroscopy Technique

The droplet size distribution (DSD) of emulsions is the result of two competitive effects that take place during emulsification process, i.e., drop breakup and drop coalescence, and it is influenced by the formulation and composition variables, i.e., nature and amount of emulsifier, mixing characteristics, and emulsion preparation, all of which affect the emulsion stability. The aim of this study is to characterize oil-in-water (O/W) emulsions (droplet size and stability) in terms of surfactant concentration and surfactant composition (sodium dodecyl benzene sulphonate (SDBS)/Tween 80 mixture). Ultraviolet-visible (UV-vis) transmission spectroscopy has been applied to obtain droplet size and stability of the emulsions and the verification of emulsion stability with the relative cleared volume technique (time required for a certain amount of emulsion to separate as a cleared phase). It is demonstrated that the DSD of the emulsions is a function of the oil concentration and the surfactant composition with higher stability for emulsions prepared with higher SDBS ratio and lower relative cleared volume with the time. Results also show that smaller oil droplets are generated with increasing Tween 80 ratio and emulsifier concentration.     

Influence of oil polarity on droplet growth in oil-in-water emulsions stabilized by a weakly adsorbing biopolymer or a nonionic surfactant

The influence of oil type (n-hexadecane, 1-decanol, n-decane), droplet composition (hexadecane:decanol), and emulsifier type (Tween 20, gum arabic) on droplet growth in oil-in-water emulsions was studied. Droplet size distributions of emulsions were measured over time (0-120 h) by laser diffraction and ultrasonic spectroscopy. Emulsions containing oil molecules of low polarity and low water solubility (hexadecane) were stable to droplet growth, irrespective of the emulsifier used to stabilize the droplets. Emulsions containing oil molecules of low polarity and relatively high water solubility (decane) were stable to coalescence, but unstable to Ostwald ripening, irrespective of emulsifier. Droplet growth in emulsions containing oil molecules of relatively high polarity and high water solubility (decanol) depended on emulsifier type. Decanol droplets stabilized by Tween 20 were stable to droplet growth in concentrated emulsions but unstable when the emulsions were diluted. Decanol droplets stabilized by gum arabic exhibited rapid and extensive droplet growth, probably due to a combination of Ostwald ripening and coalescence. We proposed that coalescence was caused by the relatively low interfacial tension at the decanol-water boundary, which meant that the gum arabic did not absorb strongly to the droplet surfaces and therefore did not prevent the droplets from coming into close proximity.     

Versatile Preparation of Nonspherical Multiple Hydrogel Core PAM/PEG Emulsions and Hierarchical Hydrogel Microarchitectures

The preparation of nonspherical materials composed of separated multicomponents by droplet‐based microfluidics remains a challenge. Based on polymerization‐induced phase separation and droplet coalescence in microfluidics, we prepared emulsions of variously shaped PAM/PEG core/shell droplets and hydrogels composed of two separated components, which show flexible and transformable hierarchical structures and microarchitectures. We find that AM/PEG aqueous droplets form a core/shell structure after polymerization resulting from phase separation. Thus multicore/shell droplets are easily produced by coalescence of core/shell structures. By changing the polymerization temperature and the flow rate, the morphology of the multicore droplets and the hydrogel can be easily adjusted. The hydrogels exhibit apparent anisotropy and different protein release rates depending on their structures. The preparation technique is simple and versatile and the resulting hydrogels have potential applications in many fields.     

Generation of Water-In-Oil-In-Water (W/O/W) Double Emulsions by Microfluidics

Novel compartment microparticles prepared with double emulsion droplets as templates provide a protected internal space for material encapsulation. The effect of three-phase flow rate on the micro-droplet generation of double emulsion mechanism is available for reference to produce precise size and highly monodisperse particles. The influence of three-phase flow rate on the formation mode and size of the emulsion droplets is investigated by combination of experiment and numerical simulation. The size of compound droplets decreases and frequency increases with the increasing outer fluid flow rate. The monodispersity of the double emulsion reduces due to transition from dripping to narrowing jetting regime. Outer droplet size increases with the increasing flow rate of the middle fluid, whereas inner droplet size is the opposite. The frequency increases and then stabilizes, which leads to a widening regime. When Q₂/Q₁ > 6, the multi-core type double emulsion droplets are produced. Droplet coalescence occurs when surfactants is not involved. As Q₁ increases, there is an increasing tendency for inner drop size. The outer drop size is proportional to the sum of the inner and middle flow rate, and that is irrelevant to Q₁/Q₂. For drop size, the ratio of core-shell and internal structure is precisely controlled by adjusting three-phase flow rate respectively.