The role of liquid viscosity and of air entrapped on the splashing of drops impacting over superhydrophobic substrates*

Here we analyze the splash transition of drops of liquids with different viscosities impacting normally over different types of rough superhydrophobic substrates, finding that the velocity for the splash transition, V^*, increases for increasing values of the liquid viscosity and also when the proportion of air entrapped at the substrate corrugations decreases. Our experimental results also reveal that the amount of air entrapped between the substrate and the wall increases with the value of the relative roughness ϵ = ε/R, with ε and R indicating the amplitude of the corrugations and the radius of the drop, respectively. We show that our experimental values of V^*, as well as those reported in similar contributions in the literature, can be predicted using the splashing model presented in Quintero, Riboux & Gordillo (J. Fluid Mech., vol. 870, 2019, 175−188) once the liquid shear stress at the wall is expressed as a decreasing function of ϵ namely, of the proportion of air entrapped at the substrate.