Droplet motion by Leidenfrost phenomenon on Zn plate surfaces with and without ZnO nanorods

Abstract Zinc oxide nanorods (ZnO-NR) were prepared on Zn plates with a ratchet structure by hydrothermal processing. Then the Leidenfrost phenomenon of the sample was evaluated after coating with fluoroalkyl silane. After formation of the ZnO-NR, the Leidenfrost temperature of Zn plates decreased by approximately 20 °C. The terminal velocity of self-propelled droplets on the plates depended on the temperature. The dominant mechanism for the self-propulsion of water droplets on Zn plates without ZnO-NR at 160–200 °C was nucleate boiling and resultant volume expansion, but it switched to gravity at temperatures higher than 200 °C. Because of suppressed adhesion of droplets onto the ratchet wall and because gravity appeared to be the dominant mechanism for droplet self-propulsion on the sample at all temperatures, both the temperature for self-propulsion and the self-propelled droplet velocity were lower after ZnO-NR modification onto the Zn plate surface. However, when ZnO-NR was formed onto every other ratchet, the self-propelled droplet velocity increased with lowered temperatures for self-propulsion. The estimated terminal velocity for the sample at 160 °C exceeded that for the Zn plate without ZnO-NR.