Time dependence of the dominant mechanisms of self-propelled droplets by Leidenfrost phenomenon on Zn plate surfaces with and without ZnO nanorods

For an earlier study, using Zn plates with a specific pitch and height, we analyzed the self-propulsion of water droplets caused by the Leidenfrost phenomenon. The results demonstrated the existence of two modes: volume expansion (partial contact mode), which accompanies nucleate boiling; and gravity (gravity mode), which water droplets receive. The dominant mode depends on the temperature. The findings also suggested that proper arrangement of ZnO nanorods onto the ratchet structure might provide synergistic effects between the two modes. Continuing from that earlier study, we analyzed water droplets that were self-propelled by the Leidenfrost phenomenon on Zn plates with double the height and double the pitch of the ratchet structures. Similarly to the structure studied earlier, the self-propulsive behavior of water droplets on these samples was produced by partial contact mode and gravity mode. Furthermore, when the time change of the dominant propulsive force was examined along with data of the earlier report, the high propulsive force found mainly in the partial contact mode at the initial stage of self-propulsion tended to shift to the gravity mode over time. The initial high propulsion duration was also confirmed as becoming shorter as the sample temperature rises. This temperature dependence is regarded as related to the time dependence of the vapor film formation process with the temperature rise of water droplets. The time change of the dominant propulsive force was clarified as dependent on the ratchet structure and surface modification by ZnO nanorods. These facts and related findings demon-strated that the self-propulsion of water droplets attributable to the Leidenfrost phenomenon is controllable by the solid surface design.