Impact states and energy dissipation in bouncing and non-bouncing droplets

Superhydrophobic surfaces made up of an array of micron-scale posts are important candidates for a wide range of applications in microfluidics. The feasibility of such surfaces is, however, limited by the fact that the more useful partially wetting Cassie-Baxter state is often metastable compared to the complete wetting Wenzel state. We study the transitions between these two states in droplet impact by means of computer simulations using a two-phase lattice Boltzmann model. Depending on the impact velocity, the droplet may bounce off the surface or land on the surface in different wetting states. In order to better understand the factors affecting these different behaviours, we analyse the energy dissipation in the droplet impact and find that, when liquid is withdrawn from in between the posts, strong dissipation occurs. Thus, the number of posts impaling the droplet has a significant effect on the observed impact regime.