Trajectory of a self-propelled ethanol drop on a liquid nitrogen bath


A bead of alcohol stars in a playful variation on a centuries-old physics demonstration.

A drop of pure alcohol levitates above a pool of liquid nitrogen while propelling itself across the pool’s surface — a twist on a classic classroom experiment.

In demonstrations of the phenomenon known as the Leidenfrost effect, a spot of liquid nitrogen is dripped onto a smooth surface. The bottom part of the droplet evaporates so quickly that the remaining liquid is left sitting on a cushion of nitrogen vapour. As a result, the droplet can glide across the surface without friction.

Work led by Anaïs Gauthier at the University of Twente in the Netherlands demonstrated that room-temperature droplets of alcohol can hover above the surface of a liquid nitrogen bath by floating on the vapour the bath releases. Furthermore, these droplets propel themselves, accelerating from rest to speeds of several centimetres per second and ricocheting off container walls.

Simulations showed that the slightest disturbance causes vapour to rush from underneath the droplet, exerting drag on the droplet and whisking it across the surface.

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