Most of the time, I don’t stir the coffee in the French Press when brewing. Sometimes, I push the plunger a little bit, just to force some of the water to pass through the floating coffee bed at the beginning. I imagine it somehow assists the dissolution.
I did that one time, and left the French Press a bit too long. I was preparing for a #CoffeeMonday set up. Then I noticed that the liquid at the top of the brewer got separated from the bottom. Most of the coffee grounds sank to the bottom already. There were a few left adhering to the metal filter. And there is an air gap between the metal filter and the surface of the bottom liquid!
I looked at the brewer curiously and noticed that the volume of liquid above is a bit bigger than before. So I tried to think of an explanation to it. And there is: Osmosis.
In the video below, I replicated the phenomenon and tried to explain after the separation occurred.