Electrocoalescence of a pair of conducting drops in an insulating oil

Abstract

The effect of an electric field on the coalescence of two water drops suspended in an insulating oil is investigated. We report four new results. (i) The cone angle for the non-coalescence of drops can be significantly smaller (as small as $19^{\circ }$) than the value of $30.8^{\circ }$ reported by Bird et al. (Phys. Rev. Lett., vol. 103 (16), 2009, 164502). (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 mode for castor oil. (iii) The critical capillary number for non-coalescence decreases with increase in the conductivity of the droplet phase. (iv) Systematic experiments prove that the apparent bridge during non-coalescence is indeed transitory and not permanent, as reported elsewhere. Theoretical calculations using analytical theory and the boundary integral method explain the formation of the cone–dimple mode as well as the transitory bridge length. The numerical calculation and thereby the physical mechanism 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.

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

“…As shown in figure 2(i) the slightly asymmetric droplets approach each other and at contact, exhibit cone-dimple formation (Anand et al. 2019). The resultant bridge then starts to grow in diameter, indicating tendency to coalesce.…”

“…The critical electrocapillary number above which DI water droplets suspended in castor oil exhibit non-coalescence is 0.08 (Anand et al. 2019). Figure 1( b ) shows how at the two droplets approach each other, form a bridge and finally coalesce.…”

“…On the contrary, when the strength of the electric field is increased, beyond a critical capillary number of 0.08 (Anand et al. 2019), non-coalescence is observed. Figure 5( a ) shows how the bridge evolves with time, reaching the peak value of in the experiments and predicted as in numerical simulation ( , ), and finally starts to decrease, resulting in non-coalescence.…”

“…Non-coalescence was also observed for two free asymmetric droplets under electric field in both castor and silicone oils (Anand et al. 2019). However, the mechanism of the resulting approach, contact and coalescence/non-coalescence is fundamentally different in these two different non-conducting media.…”

Optimum Operating Frequency for Electrocoalescence Induced by Pulsed Corona Discharge

“… 12 − 14 Providing a reliable and efficient design while meeting the compact dimension requirement in some industries is a difficult task, regardless of the target demulsification efficiency. 15 , 16 Most of the current electrocoalescer designs utilize alternating current electric fields [alternating current (ac) electric fields] with frequencies in the range of 50–60 Hz. However, using ac electric field has shown a limited response in later stages of the coalescence because of the increasing distance between water droplets and decreasing electrocoalescence forces.…”