Consider an apparatus in which air is trapped in a small insulated container with an open bottom, and weighted with weights so that the air is slightly above neutral buoyancy.
This apparatus is placed in a closed bottle of water, shown below in State 1 with the floating apparatus.
Now we slightly squeeze the bottle, thus causing hydrostatic compression of the air. The slightly buoyant State 1 air then sinks to the bottom, shown in State 2 above.
I've had some arguments about whether this process is reversible or not, and I think it depends on 2 cases:
- If we consider the increase in hydrostatic pressure, the air gets more compressed as it sinks to the bottom. Releasing the squeeze would therefore not cause the air to spontaneously rise again. This case is irreversible.
- If we ignore hydrostatic pressure, then we can simply release our squeeze on the bottle and the air will expand again, thus regaining its buoyancy to float back to the top. This case is reversible.
In the case of including hydrostatic pressure, I can't seem to reconcile the irreversibility with entropy generation, however, so I doubt my intuition on this. Is there a way to show that the process is reversible or irreversible if we include hydrostatic pressure?