David Erickson, Virginia Polytechnic

Gary Williams, UCLA

This movie shows the motion of the drop over the liquid nitrogen surface.  It initially accelerates to a speed of about 4 cm/s, and then moves with nearly constant speed, “bouncing” off the meniscus of the liquid nitrogen at the side walls.  A plume of water vapor trailing behind the drop is visible for the first few seconds, indicating that the evaporating nitrogen gas is coming off preferentially in one direction.  This jet of gas may be the source of the self-propelled drop motion.  The plume is even more evident in the movie below, where a drop of ethanol alcohol is used instead.

Near the end the drop slows down as the boiling is not as strong, sometimes spinning and moving erratically, as shown here for a small water drop.

Very small drops of some liquids never fall through the surface even when they come to equilibrium with the liquid nitrogen.  A chemical repulsion prevents complete wetting by the nitrogen, and the frozen drops can float due to surface tension forces for hours.  A small drop of hydrogen peroxide is seen in this movie to stop boiling the nitrogen after only a few seconds of motion; it then just drifts around the surface on stray currents at the liquid nitrogen surface.

The same type of Leidenfrost effect can also be seen for small solid objects dropped onto the nitrogen surface.  Here a small glass bead levitates and moves across the surface much like the liquid drops, though it quickly cools down and drops through the surface.

For more details about the experiment and data on the Leidenfrost effect, the REU report on this project can be seen here.

This work was supported by the NSF REU program hosted by the UCLA Department of Physics and Astronomy.