A ratchet trap for Leidenfrost drops

Abstract

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.

Leidenfrost effect: Accurate drop shape modeling and refined scaling laws

“…Although discovered in 1756 and widely studied in connection with heat transfer technologies, this so-called Leidenfrost effect is the subject of a renewed interest nowadays [2], particularly in view of new perspectives in the field of microfluidics. Indeed, as the relatively small thermal conductivity of the vapor layer slows down the phase change process, while its low viscosity confers an extreme mobility to the drop, the control and manipulation of Leidenfrost drops turns out to be possible using ratchets or other surface structures [3][4][5][6], magnetic fields [7], or electric fields [8].…”

Leidenfrost Dynamics

“…It turns out that this pattern induces self-propulsion: The levitating liquid moves in the direction toward the steep side of the teeth and quickly reaches a final velocity of the order of 10 cm s −1 (Fenga et al 2012, Lagubeau et al 2011, Linke et al 2006, Ok et al 2011) (see Supplemental Video 12). This effect was exploited by Cousins et al (2012), who built concentric ridges (each asymmetric in cross section) and thus achieved a modern version of Leidenfrost's spoon: Regardless of the place where a drop is deposited, after a few oscillations, it ends up at the center of the ridges.…”

“…The flow is isotropic if the Leidenfrost material levitates above a flat solid, but it can be rectified on asymmetric patterns, as shown in the context of flows against ratchets ( Jiang et al 1998, Yang et al 2004). For thin films (h < a), it was shown experimentally (using tracers) and numerically that the vapor flow is generally a lubrication flow (Cousins et al 2012, Dupeux et al 2011b, in which the gas is directed toward the deepest part of each tooth (Supplemental Video 13), then hits the step, and escapes laterally along it. Hence the pattern makes the vapor flow cellular (there is nearly no gas exchange between two successive teeth), and rectified, in the direction chosen by the levitating body: Owing to the vapor viscosity, this flow can drag the material above (Cousins et al 2012, Dupeux et al 2011b, Linke et al 2006, which suggests a traction force F scaling as (η v U/d )R 2 , where d is a typical vapor thickness (between h and a).…”

Take Off of Small Leidenfrost Droplets

“…Over the years, the Leidenfrost phenomenon has attracted the attention of many investigators, see for instance [2][3][4][5][6][7][8], but seemingly little interest has been paid to the observation by Leidenfrost of the final stage of the droplet evaporation, in particular, what happens when the droplets become very small before finally disappearing. We investigate in this Letter theoretically and experimentally the final regime of the Leidenfrost droplets.…”