Question

The usual explanation for warm fluids rising past cooler ones is that the warmer fluid has a lower density. I'm trying to understand what this looks like at a molecular scale. The density seems to be a large-scale phenomenon, and I don't understand how it can affect whether a particular molecule rises or falls.

Consider a cylinder of fluid that is being heated at the bottom. The molecules at the bottom have a higher average energy. How does this result in the tendency of the warmer molecules at the bottom to move upwards past the cooler ones?

Answer 1

The molecules are all moving, quite rapidly, all the time, and constantly colliding against each other. The warmer ones are moving even more rapidly, thus "winning out" in their collisions with the cooler ones, pushing them away. (That's what lower density is.)

Then if there's some gravity field pulling all of them downwards against a surface (they're not in free fall) the cooler ones have less velocity to "get away" from the ones underneath or the surface, therefore they congregate below.

Even in something as dramatic as a rocket engine, the thermal velocity of the molecules is much higher than the exhaust velocity. This is seen in videos of rocket engines in space, where the exhaust plume is very wide, rather than narrow as near the ground.

Answer 2

this is about buoyan force. In a fluid if a body is denser than the fluid it will sink, and if it is less dense than the fluid will float.