Guillaume Dupeux

Viscous mechanism for Leidenfrost propulsion on a ratchet

An evaporating drop placed on a ratchet self-propels, as discovered by Linke et al. in 2006. Sublimating platelets do the same, and we discuss here a possible viscous mechanism for these motions. We report that the flow of vapor below the levitating material is rectified by the asymmetric teeth of the ratchet, in the direction of descending slopes along each tooth. As a consequence, the resulting viscous stress can entrain the material in the same direction, and we discuss the resulting self-propelling force.

Propulsion mechanisms for Leidenfrost solids on ratchets

We propose a model for the propulsion of Leidenfrost solids on ratchets based on viscous drag due to the flow of evaporating vapor. The model assumes pressure-driven flow described by the Navier-Stokes equations and is mainly studied in lubrication approximation. A scaling expression is derived for the dependence of the propulsive force on geometric parameters of the ratchet surface and properties of the sublimating solid. We show that the model results as well as the scaling law compare favorably with experiments and are able to reproduce the experimentally observed scaling with the size of the solid.

The spinning ball spiral

We discuss the trajectory of a fast revolving solid ball moving in a fluid of comparable density. As the ball slows down owing to drag, its trajectory follows an exponential spiral as long as the rotation speed remains constant: at the characteristic distance L where the ball speed is significantly affected by the drag, the bending of the trajectory increases, surprisingly. Later, the rotation speed decreases, which makes the ball follow a second kind of spiral, also described in the paper. Finally, the use of these highly curved trajectories is shown to be relevant to sports.

Self-propelling uneven Leidenfrost solids

Placed on a hot surface, a solid that sublimates at atmospheric pressure can levitate on a cushion of its own vapor. Discovered by Leidenfrost, this effect has mostly been studied with liquids. Whereas the shape of a droplet is determined by a competition between gravity, surface tension, and stress in the vapor layer, a solid does not deform. In this paper, we show experimentally and theoretically that asymmetric mass distributions in a Leidenfrost solid can lead to a non homogenous vapor layer in which the lubrication flow generates a lateral force able to propel the body.

The aerodynamic wall

We study the trajectory of dense projectiles subjected to gravity and drag at large Reynolds number. We show that two types of trajectories can be observed: if the initial velocity is smaller than the terminal velocity of free fall, we observe the classical Galilean parabola: if it is larger, the projectile decelerates with an asymmetric trajectory first drawn by Tartaglia, which ends with a nearly vertical fall, as if a wall impeded the movement. This regime is often observed in sports, fireworks, watering, etc. and we study its main characteristics.

Successive instabilities of confined Leidenfrost puddles

A Leidenfrost drop confined between two hot plates is unstable, if large enough. After a short delay to build a central vapor pocket, it forms a ring which rapidly expands and eventually bursts. We analyze this sequence of instabilities theoretically, and show that the ring size increases in a non-linear manner as a function of time, in agreement with experiments.