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Interfacial Phenomena and Heat Transfer
ESCI

ISSN Print: 2169-2785
ISSN Online: 2167-857X

Open Access

Interfacial Phenomena and Heat Transfer

DOI: 10.1615/InterfacPhenomHeatTransfer.2015014454
pages 203-220

RECOIL OF DROPS DURING COALESCENCE ON SUPERHYDROPHOBIC SURFACES

Madhu R. Gunjan
Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India
Praveen Somwanshi
Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India
Ayush Agrawal
Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India
Sameer Khandekar
Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur, India
Krishnamurthy Muralidhar
Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur, India

ABSTRACT

A numerical and experimental study has been carried out with a pair of water drops coalescing over a superhydrophobic surface. Initially, a stationary isolated drop (of given radius R) in the sessile configuration is just touched by another drop (of radius R2), vertically in line with it, at zero speed. The solid substrate is a chemically treated, polished superhydrophobic surface made up of copper, for which the equilibrium contact angle for water was measured to be 150°. The Bond number, considering the total volume of the combined drop, falls in the range of 0.3−0.5. For these conditions, experiments clearly reveal that for a given range of R1/R2, the merged drop bounces off once before it starts to eventually spread and attain an equilibrium configuration. The above experiment has been numerically simulated using COMSOL©, in an axisymmetric coordinate system. The simulations have been carried out for different combinations of droplet volumes, while keeping their combined volume to be fixed. The predictions of the simulation are compared with the experiment in terms of the interface shapes attained, distinctive timescales, and recoil height. The comparison between the two sets of data is seen to be favorable. Factors leading to recoil are delineated according to the available energy budget.