Great concept, it really appeals to me as an energy engineer :)
The gameplay itself is a bit hard to grasp initially though, especially if you're not familiar with Maxwell's thought experiment and did not read the intro on the itch page. An instructions screen before the game start would be nice. Even then it's a lot of sheer luck in my experience, I got higher deltas (12K) before understanding the intended gameplay (at that time I was trying to get all atoms in the lower container, regardless of their internal heat) than afterwards when I was trying to only get hotter atoms in the bottom gas.
Great work on the Brownian movement too :) I think the atom colors could be emphasized so that it's easier to differentiate hot and cold atoms - and maybe the velocity factor should be increased too.