On the interfacial properties of micrometre-sized water droplets in crude oil

The remarkable stability of water-in-crude oil emulsions is due to the pres ence of a complex adsorbed layer at the surfaces of the dispersed droplets. Except for its role as a steric barrier, little is known about the in situ properties of this interfacial structure. In this study, new insights into the adsorbed layer are provided by direct micrometre-scale measurements. A t low crude content in the bulk where, according to interfacial tension iso therms, there should be little or no surfactants on the droplet surface, th e adsorbed layer displays pronounced rigidity and is capable of preventing coalescence and coagulation of the droplets. Such interfaces are highly dis sipative and can be well described by the Boussinesq-Scriven model. As the supply of surface active materials in the bulk (i.e. the crude content) inc reases, the adsorbed layer transforms from a rigid structure to a fluid int erface. This fluid layer continues to inhibit coalescence, although signs o f weak interdroplet adhesion begin to appear. Under area compression, the f luid interface will discharge micrometre-sized emulsion droplets into the o il phase via a 'budding' mechanism.