A poor's man Rayleigh-Taylor instability
This simulation employs periodic boundary conditions on a square. Since the code cannot currently deal with different densities, I have created a color field: 0 (bottom) or 1 (top), then a "gravity" that pulls upwards for the 0 value, downwards for the 1. There are two inferfaces, actually, but only the one in the middle is unstable. This is a particle code similar to pFEM