A qualitative analysis of the various outcomes of a drop impact on solid surfaces with different roughness and wettability is carried out. Water, ethanol, different mixtures of glycerin and water, liquid alloys, and silicone oil were used to provide a wide range of material properties such as surface tension, viscosity, and density. The impact velocity was varied by moving the drop generator vertically with respect to tie plate. Also two drop diameter classes were considered. A variation of roughness amplitude and wavelength was achieved using a laser ablation process on polyvinyl chloride and glass substrates, creating a deterministic microstructure. A highly nonwettable rough surface was prepared with alkylketene dimer (AKD) [1]. A first classification of the different outcomes, in terms of splash, rebound, partial rebound, deposition, and other features, is presented.

Experiments to understand the effect of surface wettability on impact characteristics of water drops onto solid dry surfaces were conducted. Various surfaces were used to cover a wide range of contact angles (advancing contact angle from 48° to 166°, and contact angle hysteresis from 5° to 56°). Several different impact conditions were analyzed (12 impact velocities on 9 different surfaces, among which 2 were superhydrophobic). Results from impact tests with millimetric drops show that two different regimes can be identified: a moderate Weber number regime (30 < We < 200), in which wettability affects both drop maximum spreading and spreading characteristic time; and a high Weber number regime (We > 200), in which wettability effect is secondary, because capillary forces are overcome by inertial effects. In particular, results show the role of advancing contact angle and contact angle hysteresis as fundamental wetting parameters to allow understanding of different phases of drop spreading and beginning of recoiling. It is also shown that drop spreading on hydrophilic and superhydrophobic surfaces occurs with different time scales. Finally, if the surface is superhydrophobic, eventual impalement, i.e., transition from Cassie to Wenzel wetting state, which might occur in the vicinity of the drop impact area, does not influence drop maximum spreading.