Rayleigh -Taylor instability
Consider the folloween experience : we put in a recipient two fluids with a different weight. The light weighted one at the bottom and the heavy one at the top. It's clear that such situation is instable. but an equilibrium point existe. little perturbations in the neiberhood of this state can lead to an unsteady flow.
The main forces witch control the stability of this syctem are : the gravity and the superficiel tension.
At the begining the interface between the fluids is horizontale, but after little perturbation the interface is deformed. the superficiel tension try to keep a horizontal interface (this surface have the less area, so it's the optimal surface). in the other hand gravity try to destabilise the systeme. so the system can bascule to an unsteady flowing out. amplification or not of the perturbations is controled by the rate between the gravity forces and the superficiel tension forces.