Phase-field simulations of droplet impact on superhydrophobic surfaces

Although the impingement of droplets on superhydrophobic surfaces has been extensively studied, the critical role of the air is typically ignored, leading to some paradoxical conclusions. In this paper, a phase-field model with dynamic contact angles is implemented to simulate the impingement of droplets on a superhydrophobic surface. The simulation results agree well with the experimental data. The droplet's impact on the surface observed and divided into four categories: contactless bouncing, wet bouncing, dry-out bouncing and bouncing. The role of air was emphasised by analysing pressure distribution and velocity field corresponding to the droplet evolution, revealing the mechanism behind the droplet impact. A non-monotonically varying trend of the non-dimensional maximum wetting diameter is observed by increasing Ohnesorge number (Oh). The value of dimensionless viscous extension first increased and then decreased with increasing impact number, and counter-intuitively outcome has not been reported previously. Non-dimensional contact time of a bouncing droplet decreases rapidly with increasing Weber number (We), but approaches a plateau value if We is greater than 1. Additionally, for liquids with different viscosities, the non-dimensional contact time of bouncing droplets is independent of Oh. Moreover, regime maps of the dynamic behaviours of a droplet impact are developed present an intuitive interpretation of different droplet fates.