Electrocoalescence of a pair of conducting drops in an insulating oil

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.

[1]  M. H. Davis.  TWO CHARGED SPHERICAL CONDUCTORS IN A UNIFORM ELECTRIC FIELD: FORCES AND FIELD STRENGTH , 1964 .

[2]  Rochish Thaokar,et al.  Electrocoalescence in non-uniform electric fields: An experimental study , 2015 .

[3]  D. Saville.  ELECTROHYDRODYNAMICS:The Taylor-Melcher Leaky Dielectric Model , 1997 .

[4]  C. Pearce.  The mechanism of the resolution of water-in-oil emulsions by electrical treatment , 1954 .

[5]  H. Stone,et al.  Critical angle for electrically driven coalescence of two conical droplets. , 2009, Physical review letters.

[6]  Rochish Thaokar,et al.  Interrelationship between electrocoalescence and interfacial tension in a high acidity crude: Effect of pH and nature of alkalinity , 2018, Colloids and Surfaces A: Physicochemical and Engineering Aspects.

[7]  Regis Vilagines,et al.  The electrocoalescers' technology: Advances, strengths and limitations for crude oil separation , 2012 .

[8]  Limin He,et al.  Coalescence behaviour of two large water-drops in viscous oil under a DC electric field , 2014 .

[9]  Bouncing of charged droplets: An explanation using mean curvature flow , 2013, 1302.4862.

[10]  Nicolas Bremond,et al.  Breaking of an emulsion under an ac electric field. , 2009, Physical review letters.

[11]  Howard A. Stone,et al.  Electrohydrodynamic deformation and interaction of drop pairs , 1998, Journal of Fluid Mechanics.

[12]  P. Atten,et al.  Electrocoalescence of water droplets in an insulating liquid , 1993 .

[13]  J. Bird,et al.  Coalescence and break-up of nearly inviscid conical droplets , 2014, Journal of Fluid Mechanics.

[14]  H. A. Stone,et al.  Non-coalescence of oppositely charged drops , 2009, Nature.

[16]  Rochish Thaokar,et al.  Electrocoalescence of a drop pair , 2015 .

[17]  P. Atten.  On electrocoalescence of water droplets in an insulating liquid , 1992, Conference Record of the 1992 IEEE Industry Applications Society Annual Meeting.

[18]  W. Ristenpart,et al.  Electrically tunable partial coalescence of oppositely charged drops. , 2012, Physical review letters.

[19]  J. Sherwood,et al.  Breakup of fluid droplets in electric and magnetic fields , 1988, Journal of Fluid Mechanics.

[20]  C. Pozrikidis,et al.  Interfacial dynamics for Stokes flow , 2001 .

[21]  S. G. Mason,et al.  Effects of electric fields on coalescence in liquid+liquid systems , 1961 .