Self-propelling Leidenfrost ratchet boats

A small liquid droplet placed on an asymmetric ratchet surface heated above the Leidenfrost temperature is known to self-propel. Here, we examine an alternative configuration: a heated macroscopic alumina boat (14 cm long) decorated with asymmetric ratchets and floated in volatile perfluorocarbon liquid. Boats with 1 mm pitch ratchets, heated to 280 °C, showed robust self-propulsion in the expected direction, reaching several centimeters per second and traveling more than one meter. An impulse–momentum balance scaling, derived from Leidenfrost droplet propulsion, predicts boat velocities of the same order but somewhat below experiments, suggesting enhanced efficiency of vapor flow in contact with bulk liquid. Propulsion efficiency depends strongly on ratchet geometry: for small pitches, bottom and sidewall ratchets act synergistically, while at larger pitches, sidewall ratchets reduce thrust by redirecting vapor. Herringbone-patterned ratchets further enhanced propulsion. Towing tests confirmed reduced drag when boats moved with their ratchet orientation, with ∼10% lower drag coefficients compared to smooth boats.