Coalescence and non-coalescence of two aqueous drops in insulating oil under an electric field*

The coalescence of water drops, dispersed in oil, is critical to the destabilization of a water-in-oil emulsion. We report some new results, (i) The cone angle for the non-coalescence of drops can be significantly smaller (as small as 19^o) than the reported value of 30.8^o by Bird et al. (2009). (ii) A surprising observation of the dependence of the mode of coalescence/non-coalescence on the type of insulating oil is seen. A cone-cone mode for silicone oil is observed as against cone-dimple for castor oil (iii) The critical capillary number and the critical cone angle are found to be sensitive to the conductivity of the dispersed phase. (iv) The critical capillary number independent on the applied frequency, and (v) The apparent bridge during non-coalescence is indeed transitory and not permanent. The physical mechanism and thereby numerical calculations to explain the occurrence of very small non-coalescence angles, as well as the dependence of the phenomenon on the conductivity of the insulating oil and the water droplets, remain unexplained.