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Understanding vertical coalescence dynamics of liquid drops over a superhydrophobic surface using high-speed orthographic visualization

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Abstract

Experiments on coalescence are carried out with two water drops over a chemically textured superhydrophobic surface (equilibrium contact angle ~ 150° and contact angle hysteresis ~ 10°), with one drop initially above the other, under atmospheric conditions. Drops are of equal volumes with a combined Bond number of 0.11. The coalescence events are imaged using two synchronized high-speed cameras in orthogonal directions. As the drops coalesce, a great variety of interface shapes appear as the merged drop approaches equilibrium. The velocity components of the combined drop, and hence the wall shear stress, are estimated by curve fitting the centroidal position data. Two distinct timescales are clearly revealed. These are related to the balance of inertia forces and surface tension on one hand and a balance of inertia and viscous forces on the other. The interface shapes in the two orthogonal views are seen to be similar, indicating the process to be practically axisymmetric during the entire time period. The drop shape gradually evolves over the shorter timescale with the appearance of a positive as well as negative curvature and leads to recoil. Wall shear stresses are estimated to be quite large during the early time period. At later times, it is characterized by small amplitude damped oscillations of a merged drop dominated by viscous dissipation. The scaling law for curvature seen during neck formation corroborates the one reported for drops coalescing in free space. Further, the bulge and neck curvatures as well as the drop shapes seen in experiments are in good agreement with numerical simulation carried out in an axisymmetric coordinate system.

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Acknowledgements

Financial support for colleagues from India was provided by the Department of Science & Technology, India from an Indo-Russian Project (DST/ME/2018193) and is gratefully acknowledged. Financial support for colleagues from Russia was received from the Russian Science Foundation (Project No. 21-79-10373). Measurement of the wetting angle of the substrate is performed under state contract with IT SB RAS.

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Authors and Affiliations

  1. Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur, 208016, India

    K. Muralidhar & S. Khandekar

  2. Kutateladze Institute of Thermophysics SB RAS, Novosibirsk, 630090, Russia

    Praveen M. Somwanshi, V. V. Cheverda & O. A. Kabov

Authors
  1. Praveen M. Somwanshi
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  2. V. V. Cheverda
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  3. K. Muralidhar
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  4. S. Khandekar
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  5. O. A. Kabov
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Correspondence to Praveen M. Somwanshi.

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The video of coalescence events of equal volume water drops in a vertical alignment is shown for a duration of 300 ms when imaged at 20,000 fps. The Bond number of the merged drop is 0.11, and the equilibrium contact angle is 150 ± 4°.

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Somwanshi, P.M., Cheverda, V.V., Muralidhar, K. et al. Understanding vertical coalescence dynamics of liquid drops over a superhydrophobic surface using high-speed orthographic visualization. Exp Fluids 63, 47 (2022). https://doi.org/10.1007/s00348-022-03394-w

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  • DOI: https://doi.org/10.1007/s00348-022-03394-w

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