Breakup and non-coalescence mechanism of aqueous droplets suspended in castor oil under electric field

Abstract

The effect of an electric field on the coalescence of two water droplets suspended in an insulating oil (castor oil) in the non-coalescence regime is investigated. Unlike the immediate breakup of the bridge, as reported in earlier studies, e.g. Ristenpart et al. (Nature, vol. 461 (7262), 2009, pp. 377–380), the non-coalescence observed in our experiments indicate that at strong fields the droplets exhibit a tendency to coalesce, the intervening bridge thickens whereafter the bridge dramatically begins to thin, initiating non-coalescence. Numerical simulations using the boundary integral method are able to explain the physical mechanism of thickening of this bridge followed by thinning and non-coalescence. The underlying reason is the competing meridional and azimuthal curvatures which affect the pressure inside the bridge to become either positive or negative under the effect of electric field induced Maxwell stresses. Velocity and pressure profiles confirm this hypothesis and we are able to predict this behaviour of transitory coalescence followed by non-coalescence.