Effect of Surface Wetting of Micro/Nano Ratchets on Leidenfrost Liquid Drop Motion

A micro- and nanoscale asymmetric potential enhances propulsion of liquid drop motion in the Leidenfrost (film boiling) regime. However, the requirement of high operation temperature to levitate a liquid droplet is a critical drawback for practical application of micro- and nanosystems. In this study, we show that the threshold temperature for the droplet motion can be reduced by controlling the wetting properties of ratchet surfaces. Various sizes of metallic ratchets down to sub-micrometer period were fabricated using micro- and nanoscale machining tools. Then, the wettability of the ratchet surfaces was modified by coating hydrophobic fluorinated silane molecules after activating the ratchet surfaces with oxygen plasma. Room temperature contact angle induced by the silane coating increased significantly (∼ 20° for brass ratchets and ∼ 70° for nickel ratchets). We found that the hydrophobic coating of ratcheted surface significantly affected the levitating droplet motion, both decreasing the threshold temperature and increasing the average droplet velocity.