(Gregory L. Hansen) wrote in message ...
When I pull the hose from the nitrogen exhaust on my cryostat, chunks of
ice will fall on to the aluminum stand and as they melt they'll move side
to side, sometimes dancing back and forth quite rapidly until they're very
small compared to the puddle they're floating in.

Any ideas why they would dance?

I'm thinking as one side floats to the edge of the puddle it must be
warmer (thinner water layer due to surface tension?), which makes it melt
faster and rockets it off to the other side. But that explanation seems
kind of weak.

If I understand your question right, you're on the right track. The
"ice" could be melting so fast it boils at the same time. More likely,
whatever the substance is, the atmosphere is probably below the
critical pressure, the same reason dry ice doesn't melt, it goes
straight from solid to gas. So you have this little chunck of "ice"
boiling rapidly, and that creates a blanket of gas around it, allowing
it to behave like a little hovercraft. Any little "pop" in the boiling
of the ice would make it dance.