The influence of surfactants on the hydrodynamical interaction in emulsion systems (original) (raw)
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Role of surfactants on the approaching velocity of two small emulsion drops
Journal of colloid and interface science, 2012
Here we present the exact solution of two approaching spherical droplets problem, at small Reynolds and Peclet numbers, taking into account surface shear and dilatational viscosities, Gibbs elasticity, surface and bulk diffusivities due to the presence of surfactant in both disperse and continuous phases. For large interparticle distances, the drag force coefficient, f, increases only about 50% from fully mobile to tangentially immobile interfaces, while at small distances, f can differ several orders of magnitude. There is significant influence of the degree of surface coverage, θ, on hydrodynamic resistance β for insoluble surfactant monolayers. When the surfactant is soluble only in the continuous phase the bulk diffusion suppresses the Marangoni effect only for very low values of θ, while in reverse situation, the bulk diffusion from the drop phase is more efficient and the hydrodynamic resistance is lower. Surfactants with low value of the critical micelle concentration (CMC) m...
Hydrodynamical interaction of two emulsion droplets at small separations
Colloid & Polymer Science, 1985
The fluid's motion inside emulsion droplets is analysed when they mutually approach along their common axis and a thin liquid film is formed outside. A qualitative flow pattern is presented. Two particular cases are treated -a creeping motion and a boundary layer flow inside the droplets. Estimates are made for the tangential velocity at the droplet/film interface, for the drag force and for the energy dissipated in the respective phases.
Wall migration and shear-induced diffusion of fluid droplets in emulsions
Physics of Fluids, 2003
The spatial distribution of drops in multiphase Stokes flow is derived theoretically as a function of two dimensionless parameters, accounting for wall migration, buoyancy, and shear-induced diffusion. The wall migration effect, which drives drops away from the walls and toward the center of the gap, is often significant even when droplets are 100 times smaller than the gap. By comparison with the experimental drop concentration profile, the shear-induced down-gradient diffusivity is measured and found to be approximately four to five times larger than the prediction for drop self-diffusivity. These are the first such measurements of the diffusivity of drops with clean interfaces and contrast markedly with previous measurements on surfactant-laden drops. Nonuniform concentration along the vorticity axis is also investigated briefly.
Industrial & Engineering Chemistry Research, 2006
As surfactant-oil-water systems approach Winsor III phase behavior, at the so-called optimum formulation, the interfacial tension decreases, thus allowing the generation of smaller drops upon stirring. On the other hand, the coalescence rate increases, thus favoring the formation of larger drops. The two opposite effects do not alter the drop size in the same way and result in a minimum drop size. Such a minimum drop size is found on both sides (for O/W and W/O emulsions) of optimum formulation, whatever the variable used to produce the scan. The location of these minima, which correspond to the most efficient use of the stirring energy to make small droplets, is found to be slightly shifted by a change in stirring energy.
Dynamics of molecular transport by surfactants in emulsions
Soft Matter, 2012
We consider the dynamics of equilibration of the chemical potential of a fluorophore in a monodisperse emulsion containing droplets with two initially different concentrations of the fluorophore. Although the exchange mechanism involves a single timescale at the droplet (microscopic) level, the organisation of the droplets determines the exchange dynamics at the population (macroscopic) level. The micelle concentration in the continuous phase and the chemistry of the fluorophore control the microscopic exchange rate while the disorder of the initial condition determines the power-law of the long timescale, recovered in a minimal analytical model. We also show here that an additive in the droplet such as Bovine Serum Albumin (BSA) acts on the microscopic exchange rate and slows down the exchange process by increasing the solubility of the fluorophore in the dispersed phase rather than by creating a viscoelastic layer at the droplet interface.
Rheology of a dilute emulsion of surfactant-covered spherical drops
Physica A: Statistical Mechanics and its Applications, 2000
The rheology of a diluted emulsion of surfactant-covered spherical drops has been investigated. A diluted ÿlm of insoluble surfactant is assumed. A matrix formulation of the problem is derived and analyzed by perturbation expansions for low-and high-shear rates, and for high-viscosity drops; the high-viscosity expansion converges rapidly for a wide range of parameters. Our theory provides a quantitative description of shear thinning and normal stress di erences that occur as a result of surfactant redistribution.
Interparticle interactions in concentrate water–oil emulsions
Advances in Colloid and Interface Science, 2004
The present investigation is based on the description of electrostatic interaction in concentrated disperse systems proposed 45 years ago by Albers and Overbeek. Starting from their model, we developed a stability theory of concentrated Brownian W/O emulsions in which nondeformed droplets undergo electrostatic and Van der Waals interactions. While the droplets in dilute emulsion may be described by pair interaction, in dense emulsions, every droplet is closely surrounded by other droplets, and when two of them come together, not only the energy of their pair interaction, but also their interaction with surrounding droplets change. Unlike in dilute emulsion, for which the reference energy of the pair is the energy at infinity (taken equal to zero), in concentrate emulsion, the reference energy is not zero but is the energy of interaction with averaged ensemble of nearest droplets. The larger the volume fraction, the higher the reference energy and, thus, the lower the energy barrier between two coagulating droplets, which enhances the coagulation. In dense packing of drops, the energy of interaction and the reference energy coincide, therefore, the height of energy barrier vanishes. In contrast with dense emulsion, at medium volume fraction, when two coagulating droplets interact only with a few nearest neighbors, our analysis shows that the energy barrier may also increase, which extends thus the domain of stability. Because in W/O emulsion, the thickness of the electric double layer is of the same order or larger than the size of droplets, the electrostatic energy was calculated with a correction factor b that accounts for the deviation of double layers from sphericity. A more complete van der Waals interaction with account of screening of interaction by electrolyte has been used. Both factors promote the decrease of energy barrier between coagulating droplets and enhance the coagulation. Our model introduces two critical volume fractions. The first one, u c1 , is the volume fraction depending on the characteristics of system (size of drops, thickness of double layer, surface potential, dielectric permittivity of medium) that limits the validity of the pair interaction model. The second one, u c2 , is a volume fraction that limits the applicability of the simplified model of interaction of three or more double layers. By comparing the energies of barrier height and of Brownian motion, a critical volume fraction u c3 is defined, which determines the starting point of rapid coagulation. Finally, the influence of drop interaction on gravitational coagulation is also briefly presented. It is shown that the probability of coagulation between fixed in space and sedimenting droplets is larger than with only Brownian coagulation. Unlike at free sedimentation of two identical drops, the gravitation cannot accelerate their aggregation. The surface potential, which leads to the equilibration of surface forces, gravitational and Archimedes forces for a given volume fraction, is then obtained.
Journal of Dispersion Science and Technology, 2002
Surfactant-oil-water systems with a phase behavior insensitive to temperature and composition can be achieved by anionic-nonionic mixing. By using of a linear mixing rule and a linear temperature dependency, it is possible to interpret most of the features exhibited by the experimental phase behavior data obtained with sulfonate and ethoxylated alkylphenol mixtures. Deviation from the theoretical model are probably due to anionic and nonionic groups association which reduces the overall hydrophilic character.
Effect of Surfactants on Drop Stability and Thin Film Drainage
Fluid Mechanics of Surfactant and Polymer Solutions, 2004
The stability of suspensions/emulsions is under consideration. Traditionally consideration of colloidal systems is based on inclusion only Van-der-Waals (or dispersion) and electrostatic components, which is refereed to as DLVO (Derjaguin-Landau-Verwey-Overbeek) theory. It is shown that not only DLVO components but also other types of the inter-particle forces may play an important role in the stability and colloidal systems. Those contributions are due to hydrodynamic interactions, hydration and hydrophobic forces, steric and depletion forced, oscillatory structural forces. The hydrodynamic and colloidal interactions between drops and bubbles emulsions and foams are even more complex (as compared to that of suspensions of solid particles) due to the fluidity and deformability of those colloidal objects. The latter two features and thin film formation between the colliding particles have a great impact on the hydrodynamic interactions, the magnitude of the disjoining pressure and on the dynamic and thermodynamic stability of such colloidal systems.
Flocculation and coalescence of micron-size emulsion droplets
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1999
We analyze the relative importance of droplet deformation, surfactant transfer and interfacial rheology for the properties and stability of emulsions. The appearance of deformation (flattening or film) in the zone of contact of two interacting droplets has the following consequences. It enhances the importance of the surface forces of intermolecular origin and gives rise to contributions from the interfacial dilatation and the bending energy. The flattening increases the viscous dissipation in the gap between two colliding drops and thus prolongs the lifetime of the doublet of two such drops. The critical thickness of the gap also depends on whether the drops are deformed or non-deformed. The factors which facilitate the flattening in the zone of contact between two emulsion drops are the increase in droplet size, the decrease in interfacial tension, the bending energy for water-in-oil emulsions, the increase in droplet-droplet attraction and the suppression of droplet-droplet repulsion. The presence of surfactant strongly affects the interfacial tension, the bending moment, and influences all kinds of DLVO and non-DLVO surface forces operative in the gap between two droplets. The rheological and dynamic properties of the surfactant adsorption monolayers (Gibbs elasticity, surface diffusivity, surface viscosity, and adsorption relaxation time) are major factors for the stability of emulsions under dynamic conditions. The solubility of the surfactant in one of the two phases can determine whether oil-in-water or water-in-oil emulsion will be formed. A criterion for emulsion stability accounting for the interplay of all thermodynamic and hydrodynamic factors mentioned above is obtained. It provides an interpretation and generalization of the Bancroft rule.