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Electrocoalescence of a pair of conducting drops in an insulating oil

Published online by Cambridge University Press:  26 November 2018

Vikky Anand Open the ORCID record for Vikky Anand [Opens in a new window] ,
Subhankar Roy ,
Vijay M. Naik ,
Vinay A. Juvekar  and
Rochish M. Thaokar Open the ORCID record for Rochish M. Thaokar [Opens in a new window]
Vikky Anand
Affiliation:
Department of Chemical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India
Subhankar Roy
Affiliation:
Department of Chemical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India
Vijay M. Naik
Affiliation:
Department of Chemical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India
Vinay A. Juvekar
Affiliation:
Department of Chemical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India
Rochish M. Thaokar*
Affiliation:
Department of Chemical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India
*
†Email address for correspondence: [email protected]
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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.

JFM classification

Drops and Bubbles: Drops Drops and Bubbles: Drops and Bubbles Drops and Bubbles: Electrohydrodynamic effects
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 859 , 25 January 2019 , pp. 839 - 850
Copyright
© 2018 Cambridge University Press 

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References

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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.

Download Anand et al. supplementary movie 1(Video)
Video 5.5 MB

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.

Download Anand et al. supplementary movie 2(Video)
Video 1.5 MB

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.

Download Anand et al. supplementary movie 3(Video)
Video 5.5 MB

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.

Download Anand et al. supplementary movie 4(Video)
Video 2.8 MB

Anand et al. supplementary movie 5

Interaction and transient bridge formation between two Milli-Q water drops in silicone oil. Capillary number (Ca) = 0.066, frequency = 5000 Hz. The video was captured at 41000 frames/sec and slowed down to 5 frames/sec.

Download Anand et al. supplementary movie 5(Video)
Video 133.2 KB
Supplementary material: PDF

Anand et al. supplementary material

Supplementary information

Download Anand et al. supplementary material(PDF)
PDF 149.8 KB