Motion of Leidenfrost self-propelled droplets on ratchet in low- and high-temperature regimes

Leidenfrost droplet experiments were conducted to investigate the movement of droplets on a ratchet in low- and high-temperature regimes (L and H regimes). Slightly above the threshold temperature, the terminal velocity increased with increasing temperature until it reached the highest value. After achieving the highest value, the terminal velocity gradually decreased with temperature until there was a significant decrease in the terminal velocity. Leidenfrost regimes were identified based on not only the droplet velocity but also the droplet shape and motion. In the H regime, there was a complete thin vapor film underneath the droplets, which caused them to levitate from the ratchet. However, in the L regime, there was no a complete vapor film underneath the droplets, causing direct contact between the droplets and ratchet, resulting in drastic nucleate boiling. This resulted in a faster vapor flow and generated a stronger rotational motion than that in the H regime. A stronger rotational motion results in a faster velocity in the translational direction.