# Self-propulsion of inverse Leidenfrost drops on a cryogenic bath

## TL;DR

This study investigates the self-propulsion of room-temperature drops on a cryogenic liquid nitrogen bath, revealing how vapor film dynamics induce sustained motion and proposing a model to explain the symmetry breaking responsible.

## Contribution

It introduces a new understanding of inverse Leidenfrost drops on cryogenic baths and develops a model linking vapor film behavior to droplet propulsion.

## Key findings

- Drops reach velocities of a few cm/s before slowing down.
- Self-propulsion persists even after the drops freeze.
- A model accurately reproduces velocity variations based on vapor film and cooling dynamics.

## Abstract

When deposited on a hot bath, volatile drops are observed to stay in levitation: the so-called Leidenfrost effect. Here, we discuss drop dynamics in an inverse Leidenfrost situation where room-temperature drops are deposited on a liquid nitrogen pool, and levitate on a vapor film generated by evaporation of the bath. In the seconds following deposition, we observe that the droplets start to glide on the bath along a straight path, only disrupted by elastic bouncing close to the edges of the container. Initially at rest, these self-propelled drops accelerate within a few seconds and reach velocities on the order of a few cm/s before slowing down on a longer time scale. They remain self-propelled as long as they are sitting on the bath, even after freezing and cooling down to liquid nitrogen temperature. We experimentally investigate the parameters that affect liquid motion, and propose a model, based on the experimentally and numerically observed (stable) symmetry breaking within the vapor film that supports the drop. When also the film thickness and the cooling dynamics of the drops are modeled, the variations of the drop velocities can be accurately reproduced.