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Electrocoalescence of a pair of conducting drops in an insulating oil
Published online by Cambridge University Press: 26 November 2018
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.
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- © 2018 Cambridge University Press
References
Anand et al. supplementary movie 1
Coalescence of Milli-Q water drops in silicone oil. Capillary number (Ca) = 0.033, frequency = 50 Hz, Cone angle = 20o. The video was captured at 10000 frames/sec and slowed down to 30 frames/sec.
Anand et al. supplementary movie 2
Non-coalescence of Milli-Q water drops in silicone oil. Capillary number (Ca) = 0. 0.09, frequency = 50 Hz, Cone angle = 22o. The video was captured at 10000 frames/sec and slowed down to 10 frames/sec.
Anand et al. supplementary movie 3
Coalescence of Milli-Q water drops in castor oil. Capillary number (Ca) = 0.063, frequency = 50 Hz. The video was captured at 10000 frames/sec and slowed down to 30 frames/sec.
Anand et al. supplementary movie 4
Non-coalescence of Milli-Q water drops in castor oil. Capillary number (Ca) = 0.10, frequency = 50 Hz. The video was captured at 10000 frames/sec and slowed down to 30 frames/sec.
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