Oblique impacts of water drops onto hydrophobic and superhydrophobic surfaces: outcomes, timing, and rebound maps

This paper presents an experimental study on water drop oblique impacts onto hydrophobic and superhydrophobic tilted surfaces, with the objective of understanding drop impact dynamics and the conditions for drop rebound on low wetting surfaces. Drop impact experiments were performed with millimetric water drops with Weber numbers in the range 25 < We < 585, using different surfaces with advancing contact angles 111 degrees < theta(A) < 160 degrees and receding contact angles 104 degrees < theta(R) < 155 degrees. The analysis of oblique impacts onto tilted surfaces led to the definition of six different impact regimes: deposition, rivulet, sliding, rolling, partial rebound, and rebound. For superhydrophobic surfaces, surface tilting generally enhanced drop rebound and shedding from the surface, either by reducing drop rebound time up to 40 % or by allowing drop rebound even when impalement occurred in the vicinity of the impact region. On hydrophobic surfaces, rebound was never observed for tilt angles higher than 45 degrees.