Flocculation and coalescence of micron-size emulsion droplets

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. © 1999 Elsevier Science B.V. All rights reserved.