Propulsion of droplets on micro- and sub-micron ratchet surfaces in the Leidenfrost temperature regime

Spatially periodic systems with localized asymmetric surface structures (ratchets) can induce directed transport of matter (liquid/particles) in the absence of net force. Here, we show that propulsion for the directed motion of water droplets levitating on heated ratchet surfaces in the Leidenfrost (film boiling) regime is significantly enhanced as the ratchet period decreases down to micro- and sub-micrometers. At the temperature range slightly above the threshold temperature of droplet motion, sub-micron ratchets yield water droplet velocities reaching similar to 40 cm/s, a speed that has never been achieved with any chemical and topological gradient surfaces. This dramatic increase in the droplet velocity is attributed to an enhanced heat transfer through the local contacts between ratchet peaks and bottom of the droplet. A hydrophobic coating on the ratchet surfaces is found to further increase the droplet velocity and decrease the threshold temperature of the droplet motion. The results suggest that miniaturized ratchet surfaces can potentially be used in diverse applications requiring control over fluid transport and heat transfer such as two phase cooling systems for microprocessors and fuel injection for combustion technology and that for those applications the design of ratchet dimensions and surface chemistry are critically important.