Droplet break-up in a stirred water-in-oil emulsion in the presence of emulsifiers

Scaling laws for droplet break-up in stirred emulsifier-containing water-in-oil emulsions with dispersed volume fractions up to 70% were evaluated. The emulsions were stirred with a standard turbine impeller and the droplet diameters were determined as a function of time both microscopically and via turbidity measurements. The droplet size was found to decrease very slowly: a steady state was reached after l-2 h. In emulsions without emulsifier the steady state droplet diameters were found to be in agreement with predicted values. Emulsions with emulsifier showed a bimodal size distribution with a very large number of very small droplets. The average diameter of the “coarse part” of the size distribution could be described reasonably well with the droplet break-up relationships provided that the equilibrium interfacial tension was applied, indicating that the transport of emulsifier was sufficiently fast to maintain (nearly) equilibrium adsorption during the deformation of the droplet. The very small droplets could possibly be formed by tip streaming: the flow pushes the emulsifier to the tips of the droplet, resulting in a very low interfacial tension locally which leads to a stream of very small droplets.