Viscous dissipation mechanism of roll/sliding droplets

Droplets on an incline can experience rolling and/or sliding depending on their interaction with the substrate and the fluid properties. Here, we investigate the dissipative frictional forces imposed on these droplets for Newtonian and non-Newtonian fluids. We find that the dissipative forces vary depending on the inclination angle, fluid type, substrate, and droplet sizes. High-speed PIV imaging and interferometric visualization techniques are used to quantify the vapour profile beneath the droplet and viscous dissipative forces as the droplet transitions from pure sliding motion to roll-slip motion. On superheated surfaces Newtonian droplets with higher viscosity exhibit significant droplet deformation compared to non-Newtonian droplets, specifically at higher inclination angles. Further, the increased stiffness of non-Newtonian droplets substantially increases the total viscous dissipative forces. These findings unravel previously unexplored droplet dynamics and provide new insights into the dissipative mechanism involved when droplets levitate on superhydrophobic and superheated surfaces.