Effect of different fluids on rectified motion of Leidenfrost droplets on micro/sub-micron ratchets

Leidenfrost droplets, liquid droplets placed on a hot flat surface above the Leidenfrost temperature of the liquid, are an interesting model system to understand and achieve frictionless motion of droplets on a surface. Controlled unidirectional motion of otherwise random Leidenfrost droplets can be achieved by replacing the flat surface by a surface with topological ratchets. In this study, it is shown that an increase in the vapor layer thickness below the Leidenfrost droplet influences the droplet motion for underlying ratchets with various periods ranging from 1.5 mm down to 800 nm. This was exploited by systematically studying the Leidenfrost droplet motion of various liquids with low boiling points including acetone, isopropanol, and R134a on the aforementioned various ratchets. For all liquids with boiling points lower than water, no unidirectional motion was observed for 800 nm. This indicates that the asymmetric vapor flow beneath the Leidenfrost droplet becomes negligible due to the large vapor layer thickness relative to the ratchet depth. However, unidirectional droplet motion was still observed for the micron and millimeter scale ratchets even when the ratchet surface temperature was increased up to 360 °C and 230 °C for acetone and isopropanol, respectively. This can be attributed to the insulating property of the thick vapor layer which prevents the droplet from producing more vapor with increasing temperature. Also reported, is the effect of the ratchet period on the droplet motion at room temperature using R134a droplets.

Leidenfrost droplets on ratchet surfaces show rectified droplet motion without friction at the droplet/ratchet interface. Here we investigated the Leidenfrost droplet motion on miniaturized ratchets with the ratchet periods ranging from 800 nm to 1.5 mm using different fluids. We learn that the vapor layer thickness beneath the droplet with respect to the ratchet depth presents the limitation in obtaining the rectified motion.Figure optionsDownload as PowerPoint slide