Experimental studies of deformation and break-up of aqueous drops in high electric fields

Abstract

High electric fields have been used to separate dispersed aqueous drops from an oil phase in the chemical and oil industries. Nevertheless, very high electrostatic fields can disintegrate aqueous drops which is detrimental to the overall efficiency of the electrocoalescence process. The limit above which an electrostatic field can deform and break-up the drops, rather than coalescing the drops, is of great practical importance. The outcomes of an experimental investigation of drop deformation and break-up in several liquid–liquid systems are presented in this article. The onset of drop instability corresponds to the situation when the ratio of the length of the major axis to that of the minor axis of the drop is about 1.9. This corresponds to an electrostatic field strength of between 350 and 380 kV m−1 for a 1.2-mm diameter aqueous drop, corresponding to a critical electrostatic Weber number of about 0.49. The magnitude of the critical electric field depends on the initial drop size. The investigations were performed using a single aqueous drop in a rectangular Perspex cell. The drop adopted most commonly a prolate shape. However, if the drop was not in the centre of the electric field, then it could take on different shapes altogether. A filament penetrating into the continuous phase could disintegrate and produce a droplet at a time from its tip, and the detached droplets move towards the counter electrode. Movement of a charged drop between a pair of electrodes had also been investigated. To predict the average translational velocity of a charged drop, several aspects need to be considered, such as the non-spherical drop shape, especially as it contacting and leaving the electrodes. Therefore, the mechanism of drop deformation and break-up under high electric field is complex, and warrants more experimental investigations.