Destabilization of water–oil emulsions by flowing them through constricted capillaries

Oil is usually produced in the form of water-in-oil (w/o) stable emulsions due to the presence of natural stabilizing components in the oil phase. Oil and water phases need to be separated by coalescence of the dispersed drops. Different strategies are used to destabilize the produced emulsions and promote coalescence. In this work, we propose and qualitatively evaluate a new method to enhance water drop coalescence in stable water–oil emulsions. First, stable water-in-oil emulsions were prepared using a nonionic surfactant (Span 80) dissolved in mineral oil (Primol 352). Visualization experiments showed that the emulsion stability comes from the formation of small droplets along the water drop interface by spontaneous emulsification, leading to a droplet-rich rough interface. Water drops that are brought into contact after the formation of the rough interface do not coalesce, leading to very stable emulsions. Later, the water drops with a droplet-rich interface are passed through a capillary tube with a constriction smaller than the drop diameter. As the drop flows through the capillary throat, its surface area is increased and a strong shear stress is imposed along its surface, removing the droplets from the interface, which favors coalescence. A free water phase formed by water drop coalescence can be observed after flowing the stable water-in-oil emulsion through the constricted capillary.