Racing droplets: hydrodynamics of large droplets on inclined surfaces.

We perform experiments to understand the behavior and speed of droplets larger than the capillary length on inclined surfaces: (i) superhydrophobic and (ii) superheated surface. We consider Newtonian and non-Newtonian fluids to produce droplets with three different viscosities. The kinematics of the droplets when they are gently released down an inclined plane of varying tilt angles are characterized. We find that the non-Newtonian droplets travel at higher velocity compared to the Newtonian at lower tilt angles. Crucially, the velocity of Leidenfrost droplets becomes weakly dependent on viscosity at higher tilt angles, which is in stark contrast with droplets placed on the superhydrophobic surfaces. Using time-resolved imaging and interferometric visualization, we show that at a critical tilt angle, the internal flows of the non-Newtonian drops transition from axisymmetric to asymmetric profiles. This consequently induces sufficiently strong asymmetry in the vapor profile beneath the drop to induce self-propulsion. These findings provide new insights into the kinematics of Leidenfrost droplets, more specifically the rolling and slipping dynamics of droplets, and the mechanism driving the symmetry-breaking phenomenon which gives rise to self-propulsion.