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Hi Paul,
I tend to think that mercury would flow without any stiction. It also has very high surface tension so it would tend to stay close to ball form except on the side next to the glass. There wouldn’t be any problem with “bubble” size change.

Oil - I’m not sure about. I tried baby oil in a bubble. Low surface tension so it leaked and was messy. Water would be better except for freezing and supporting bug growth.

I may try some mercury in an A-10 bubble with the glasses reversed so the radius is on the bottom. First try might be mercury in air unless someone can think of an ideal high surface tension surrounding fluid. The A-10 uses a reflected bubble image.
Regards, Noell