In a twist on a classic physics experiment, researchers in the Netherlands and France have worked out why room-temperature alcohol droplets will levitate and propel themselves across a pool of liquid nitrogen for long periods of time. The team, led by Anaïs Gauthier at the University of Twente, have studied the propulsion associated with the “inverse Leidenfrost effect” and their work could lead to more efficient ways to transport small amounts of biological material.
The Leidenfrost effect arises when a liquid droplet is deposited onto a surface hotter than its boiling point, causing the bottom of the droplet to evaporate continuously. This creates a repulsive cushion of vapour, which both prevents the droplet from quickly boiling away, and causes it to hover above the hot surface. There is virtually no friction between droplet and surface, and changes in surface texture can cause the droplet to accelerate, climb small hills and even negotiate a maze.
Formally identified in 1756 by the German scientist Johann Leidenfrost, the effect has probably entertained people for millennia and has become a popular classroom demonstration.
Hot objects
The inverse Leidenfrost effect was first described in 1969 and involves a hot object such as a droplet levitating above a cold liquid. In this case, heat from the droplet causes some of the cold liquid to evaporate, creating the repulsive cushion of vapour.
Leidenfrost drops race through a maze
Gauthier’s team did this by depositing a room-temperature droplet of alcohol on top of a pool of liquid nitrogen at −196 °C. Within just a few seconds of deposition, the droplets propelled themselves from rest to reach speeds of several centimetres per second; gliding in straight lines before ricocheting off the container walls (see video). This continued for tens of minutes before the droplets slowed down as they cooled to the temperature of the nitrogen bath. Gauthier and colleagues propose that this self-propelling behaviour arises from subtle symmetry-breaking in the vapour film, which cause vapours to flow out from under the droplet, inducing drag.
Based on their observations, the physicists constructed simulations to model the inverse Leidenfrost effect. By modelling variations in vapour film thickness and the cooling dynamics of the drops, they could accurately recreate the variations in their observed droplet velocities.
Gauthier’s team believe the effect could be used to develop efficient techniques for freezing and transporting biological materials including cells and proteins. With the help of simulations, they hope that this transport could occur with no risk of contamination or heat degradation to the materials.
