Published online by Cambridge University Press: 27 February 2012
The Leidenfrost effect occurs when a drop of liquid (or a sublimating solid) is levitated above a sufficiently hot surface through the action of an insulating vapour layer flowing from its bottom surface. When such a drop is levitated above a surface with parallel, asymmetric sawtooth-shaped ridges it is known to be propelled in a unique direction, or ratcheted, by the interaction of the vapour layer with the surface. Here we exploit this effect to construct a ‘ratchet trap’ for Leidenfrost drops: a surface with concentric circular ridges, each asymmetric in cross-section. A combination of experiment and theory is used to study the dynamics of drops in these traps, whose centre is a stable fixed point. Numerical analysis of the evaporating flows over a ratchet surface suggests new insights into the mechanism of motion rectification that are incorporated into the simplest equations of motion for ratchet-driven motion of a Leidenfrost body; these resemble a central force problem in celestial mechanics with mass loss and drag. A phase-plane analysis of experimental trajectories is used to extract more detailed information about the ratcheting phenomenon. Orbiting drops are found to exhibit substantial deformations; those with large internal angular momentum can even undergo binary fission. Such ratchet traps may thus prove useful in the controlled study of many properties of Leidenfrost drops.
Supplemental movie 1: Quasi-one-dimensional orbit of a Leidenfrost drop in a ratchet trap. This high-speed movie shows a drop with very small initial orbital angular momentum executing back and forth motion around the trap centre. The drop exhibits noticeable shape deformations near the turning points of the orbit.
Supplemental movie 2: Fission of a Leidenfrost drop in a ratchet trap. A drop with large orbital angular momentum is observed to fission as it moves in a roughly circular orbit. The small satellite drop produced during breakup is visible.