Recoil of Drops During Coalescence on Superhydrophobic Surfaces

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 R1) 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 c ⃝ , 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.

Drop-on-Drop Impact Dynamics on a Superhydrophobic Surface

“…The droplet impact on the already deposited sessile droplet (drop-on-drop) is also analyzed in a few works for a range of Weber numbers, the configurations are shown in parts (ii) and (iii) of Figure 1a. Gunjan et al 21 and Somwanshi et al 22 investigated the coalescence of droplets with a low Weber number of vertically aligned oncoming droplet on a superhydrophobic substrate (We < 1) and concluded that the difference in the surface energy is the driving force for the lift-off of the combined droplet after coalescence. Wang et al 23 reported various outcomes during the drop-on-drop impact, namely: complete rebound, coalescence, and partial coalescence.…”