Bouncing dynamics of a droplet impacting onto a superhydrophobic surface with pillar arrays

Abstract

A superhydrophobic surface (SHS) patterned with pillar arrays has been demonstrated to achieve excellent water repellency and is highly effective for self-cleaning, anti-icing/frosting, etc. However, the droplet impact dynamics and the related mechanism for contact time (tc*) reduction remain elusive, especially when different arrangements of pillar arrays are considered. This study aims to bridge this gap by exploring a droplet impinging on an SHS with square pillar arrays in a cuboid domain. This fluid dynamics problem is numerically simulated by applying the lattice Boltzmann method. The influences of the droplet diameter (D*), the Weber number (Wew), and the pillar spacing and height (s* and h*) on the droplet dynamics and tc* are investigated. The numerical results show that the droplet can exhibit different bouncing patterns, normal or pancake bouncing, depending on Wew, s*, and h*. Pancake bouncing usually occurs when Wew ≥1.28, h*≥1, and s* ≈ 1, yielding a small tc*. Among all cases, a small tc* can be attained when the conversion rate of kinetic energy to surface energy (ΔĖsur*) right after the impacting exceeds a critical value around 0.038. This relation broadens that given in A. M. Moqaddam et al. [J. Fluid Mech. 824, 866–885 (2017)], which reported that the large total change of surface area renders small tc*. Furthermore, the maximum impacting force remains nearly the same in all cases, regardless of the bouncing patterns.