What is this thermal phenomenon?

I was making fried chicken tonight, and I took the frying oil out of the freezer, where I always store it. I never remove the oil from the fry vat—I remove the vat from the fryer when I am done with it, let it cool to room temperature, cover it, and put it in the freezer until I am ready to use it again. When I need it, I melt it on the range top, put it in the fryer, and insert the heating element into the melted oil. Tonight, the fry vat started rocking back and forth while the oil was melting. You can see it here. What is causing this?

I’ve noticed the same thing when I’m melting butter in the microwave. I’ll put some butter in a measuring cup. As part of it begins to boil I can hear the sound of glass on glass, indicating that the measuring cup is moving. I haven’t paid much attention though. I assumed it was that the bursting bubbles in the boiling butter (which is obviously more viscous than water) exert enough force to cause movement of the vessel.

I didn’t see boiling in your video. Maybe the oil is boiling at the bottom, but cools a bit before sloshing over the edges of the non-melted oil? Maybe there are just strong convection currents at the bottom of the pan?

The cause is a convection current with a massive solid weight above the heat source. Once the liquid heats, it tries to rise. The solid is lifted a bit as a result, the mass shifts slightly and the hot liquid rises off to the side. The solid in the pan shifts to one side, and the center of balance for the pan shifts too. The solid settles to the bottom again, and the lifting cycle starts all over. The pan you have is bulged out on the bottom, so a shift in the center of gravity causes the pan to shift on the pivot point. The cycle in the movie is occurring at a regular interval because of the harmonics of the system being in phase.

Melting grease in a pan with a flat bottom has the same cycle, but the pan is stable and doesn’t move. The solid grease in the pan still rocks around in the pan once it is able to float on some oil.

I think what you’ve created there is something a bit like a Stirling cycle engine - agreed, the harmonics of the thing are making the effect very apparent.