An experimental study on the effect of conductivity, frequency and droplets separation on the coalescence of two aqueous drops under an electric field

[Display omitted] •Brine conductivity can change the critical capillary number for non-coalescence.•Frequency has no effect on the coalescence pattern of two suspended drops.•Critical field of coalescence sensitively depend upon the local water holdup.•Critical cone angles decrease with an increase in the conductivity of the droplet phase. The coalescence of water drops, dispersed in oil, is critical to the destabilization of a water-in-oil emulsion under an electric field. In this work, we determine the conditions for coalescence or non-coalescence of two aqueous water drops that are suspended in an insulating oil and subjected to a uniform electric field. Specifically, we investigate the effect of the conductivity of the droplet phase (σ), the type of oils (silicone oil and castor oil), the effect of waveform AC and DC and frequency, and the separation between the droplets on the electrocoalescence/non-coalescence of two suspended aqueous drops. The main highlights are, (i) we provide expressions for critical electrocapillary number (Cac) vs. σ for silicone oil (Cac=0.010×σ−0.20) that shows cone-cone and castor oil (Cac=0.022×σ−0.15) that shows cone-dimple mode of contact. (ii) Two closely spaced drops can coalesce at electrocapillary numbers much higher than the critical electrocapillary number for widely spaced drops due to a smaller cone angle at the point of contact. These results should be important in the design of industrial electrocoalescers.