Effect of Superhydrophobic/Hydrophilic Surfaces on Dynamic Radius of Water Droplet Impact

In this research, we aim to investigate the dynamic contact angle between water droplets and three different surfaces from the hydrophilic (<90 degrees) to the superhydrophobic region (congruent to 150 degrees in ambient air. The dynamic contact angle and the triple line velocity were experimentally determined from a two-dimensional projection recorded with a high-speed camera in a backlighting setup. The findings from the experiment were used to determine the model parameters of an empirical dynamic contact angle model, which was subsequently used in three-dimensional (3D) numerical simulations. The focus of this work is on the advancing and receding contact angles over the triple line velocity, which is approximated by the model used. The behaviors on the different surfaces were analyzed in detail using the capillary number, and different regimes were identified. It was demonstrated that with the adjusted model, the simulation results regarding the droplet diameter were improved compared with the simulations using static (equilibrium) contact angle models. Thus, it was clarified that the contact angle at the reversal point should be taken and not the static contact angle. Additionally, it was concluded that only one contact angle is needed for a super-hydrophobic system.