Gliding Leidenfrost droplets: shear-thinning vs Newtonian

It is well-known that the droplet sliding velocity along an inclined hydrophobic surface would depend on the droplet rheological properties. For instance, the larger the viscosity, the slower a droplet would slide under the influence of gravity. Yet, if we mimic this experiment by substituting the hydrophobic surface with a Leidenfrost one where the surface temperature exceeds the so-called Leidenfrost point, we observe that the viscous influence is nullified. Here, we conduct a controlled experimental study where we investigate the droplet sliding velocity on a heated copper plate (length L ~ 60 cm, temperature T > 300 C, beyond the Leidenfrost point for copper to water) inclined to the ground at varying angles (2 to 8 degrees). We use a needle to produce different-sized droplets of a shear-thinning Xanthan gum aqueous solution with varying viscosity and show that the viscosity of the liquid doesn’t affect the final terminal velocity of the droplet. This non-intuitive result hints at deeper physics involving droplet motion on Leidenfrost surfaces.