Ellipsoidal drop impact on a single-ridge superhydrophobic surface

Abstract

A water drop impacting on a superhydrophobic surface exhibits a complete bounce after a certain residence time bounded by a theoretical time scale. Recent studies have demonstrated that the residence time is modified below the time scale through an asymmetric bouncing on a ridge surface. However, previous studies still assumed initially spherical shapes of drops, focusing on a high impact velocity for a desirable residence time. Herein, we propose asymmetric bouncing dynamics of ellipsoidal drops on a single-ridge superhydrophobic surface to create synergistic cooperation between the initial drop shapes and the ridge structures. Numerical simulations demonstrate the feasibility of reducing the residence time by approximately 55% using a volume-of-fluid method, compared with a conventional drop impact on flat surfaces. We examine how the drop's ellipticity and impact velocity affect the spreading, retraction, and splitting and interpret the underlying mechanism behind the residence time reduction by quantifying the evolutions of the shape and axial momenta of the drops. We also predict the bouncing directions of the drops and the effect of the off-centred distances and ridge sizes in the height and width on the residence time. These findings are expected to illuminate the fundamentals of asymmetric bouncing on a ridge and provide beneficial guidance to strategies such as self-cleaning and anti-icing.