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

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 (t(c)*) 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 (We(w)), and the pillar spacing and height (s* and h*) on the droplet dynamics and t(c)* are investigated. The numerical results show that the droplet can exhibit different bouncing patterns, normal or pancake bouncing, depending on We(w), s*, and h*. Pancake bouncing usually occurs when We(w) >= 1.28, h*>= 1, and s* approximate to 1, yielding a small t(c)*. Among all cases, a small t(c)* can be attained when the conversion rate of kinetic energy to surface energy (Delta & Edot;(sur)*) 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 t(c)*. Furthermore, the maximum impacting force remains nearly the same in all cases, regardless of the bouncing patterns.