Study of oblique impingement of water droplets on superhydrophobic surfaces patterned with micropillars: A lattice Boltzmann approach

Normal impingement of droplets on superhydrophobic surfaces patterned with micropillars exhibits pancake bouncing, significantly reducing the droplet-surface contact time under certain conditions. However, after pancake bouncing, the droplets retract, leading to secondary contact with the substrate, which is undesirable in some engineering applications. To inhibit such undesired secondary impacts, oblique impingement on superhydrophobic surfaces can be employed, inducing asymmetric dynamics that are not well understood. With over 80 sets of three-dimensional lattice Boltzmann simulations of oblique impingement of droplets on superhydrophobic surfaces patterned with micropillars, a regime diagram that encompasses a broad range of vertical (Wev) and horizontal (Weh) Weber numbers is constructed to describe the impact outcomes of droplets. We explain the impingement mechanisms from both dynamics and energy evolution perspectives. A theoretical model is built to predict the penetration depth (Δhmin) of the droplet in superhydrophobic micropillar surfaces under different Wev. We discover that droplets experience pancake bouncing without secondary impact when Wev exceeds 35 and Weh is higher than 8 at the same time. The total contact time of the droplet oblique impingement is reduced by an order of magnitude compared to that of the normal impingement. According to our dynamics and energy evolution analysis, with an increase in Wev, the droplet rebounds to a greater height during pancake bouncing, which prevents the contact of the recoiling droplet cusp with the surface. Furthermore, a higher Weh amplifies the droplet’s central viscous dissipation, thereby mitigating the central recoil of the droplet.