Electrostatic enhancement of coalescence of water droplets in oil: a review of the current understanding

This paper reviews the current understanding of electrocoalescence of water droplets in oil, highlighting particularly the mechanisms proposed for droplet-droplet and droplet-interface coalescence under the influence of an applied electric field, as well as various factors influencing the electrocoalescence phenomenon. Generally, the coalescence behaviour can be described in three stages: droplets approaching each other, the process of film thinning/drainage, and film rupture leading to droplet-droplet coalescence. Other possible mechanisms, such as droplet chain formation, dipole-dipole coalescence, electrophoresis, dielectrophoresis and random collisions, are also presented. Experimental work and mathematical modelling of the coalescence process are both reviewed, including various models, such as molecular dynamic simulation, random collision/coalescence modelling, and linear condensation polymerisation kinetics. The type of electric field, such as alternating, direct and pulsed direct current, plays a significant role, depending on the design and set-up of the system. The concept of an optimum frequency is also discussed here, relating to the electrode design and coating. Other factors, such as the average droplet size and the residence time of the liquid mixture exposed to the electric field, are highlighted relating to coalescence efficiency. The characteristics of the emulsion system itself determine the practicality of employing a high electric field to break the emulsion. Emulsions with high aqueous phase content tend to short-circuit the electrodes and collapse the electric field. Type and concentration of surface-active components have been shown to impart considerably stability and rheological property changes to the interfacial films, thus making the coalescence mechanism more complicated. More investigations, both experimental and by computer simulation, should be carried out to study the electrocoalescence phenomenon and to contribute to the design and operation of new electrocoalescers. (C) 2001 Elsevier Science B.V. All rights reserved.