Pillar height regulated droplet impact dynamics on pillared superhydrophobic surfaces

Abstract

Superhydrophobic surfaces that promote rapid droplet detachment have many significant engineering applications including self-cleaning, anti-icing, drag reduction. Adding macrotextures to superhydrophobic materials can markedly modify the dynamics of water impacting them, either by triggering a non-axisymmetric, center-assisted recoil or inducing pancake-like bouncing. However, the effect of pillar heights on superhydrophobic surfaces has received limited attention. Here, we have systematically investigated the role of the size ratio of pillar height-to-droplet radius (Q) on bouncing dynamics, including spreading, penetrating depth, contact time, bubble entrapment, impact pressure, and impact force, through numerical simulations. We show that droplet impact on superhydrophobic surfaces can generate two modes of bubble entrapment: one is an impinging cavity bubble, and the other is a recoiling cavity bubble, depending on Q. Furthermore, for low Weber number (We), droplet impacts may generate three peaks of impact force for superhydrophobic surfaces with high Q. For high We, only two peaks of impact force are generated during a droplet striking the superhydrophobic surface. However, the secondary peak in the impact force will weaken or may even disappear completely as Q increases, which was hitherto unknown. Interestingly, we also observe that despite lower impact force during droplet recoil compared to droplet impact, higher penetration depths are more easily achieved during the recoil stage. These findings shed light on the role of pillar heights in droplet impact dynamics, offering valuable insights for designing superhydrophobic surfaces.