Stratified fluid behavior. “stratified flow”.
When I was little, I read an old (then) sci-fi story where time travelers who was otherwise extremely careful accidentally brought with them a single butterfly which ended up wreaking havoc with the ecosystem of that timeframe and destroying it all.
The way the butterfly was purely accidental: there was a stagnation layer near one of the exhausts and the butterfly just happened to be fluttering in that zone when the time travel ship left and was safely transported in an otherwise impossibly hostile travel condition.
So, this kind of phenomenon has fascinated me since although I never really stopped to look at it like this.