Interplay of substrate inclination and wettability on droplet impact dynamics

Abstract

Experimental investigations were carried out to elucidate the role of surface wettability and inclination on the post impact dynamics of droplets. Maximum spreading diameter and spreading time were found to decrease with increasing inclination angle and normal Weber number for superhydrophobic surfaces. The experiments on SH surfaces were found to be in excellent agreement with an existing analytical model, incorporated with the modifications for the oblique impact conditions. Energy ratios and elongation factor were also measured for different inclination angles. On inclined SH surfaces, different features like arrest of secondary droplet formation, reduced pinch off at the contact line and inclination dependent elongation mechanism were observed. Contrary to SH surfaces, hydrophilic surfaces show opposite trends of maximum spreading factor and spreading time with inclination angle and normal Weber number respectively. This was due to the dominance of tangential kinetic energy over adhesion energy and gravitational potential at higher inclination angles. Finally, colloidal solutions of nanoparticles were used to elucidate slip and disjoining pressure on SH and hydrophilic surfaces, respectively. Overall, the article provides a comprehensive picture of post impact dynamics of droplets on inclined surfaces encompassing a broad spectrum of governing parameters like Reynolds number, Weber number, degree of inclination and surface wettability.