Rayleigh-Taylor instability occurs between fluids of different densities. In the images below the green dyed fluid is less dense than the un-dyed fluid.
A video of stratified Rayleigh-Taylor instability:
Shadowgraphs of Rayleigh-Taylor instability between two stratified layers. A barrier is withdrawn from right to left and a turbulent mixing region develops. Images are shown at 5, 10, 20 and 80 seconds. At late-time stratifications remain above and below a mixed region.
The same experiment can also be viewed using dyed layers.
Shadowgraphs of a sphere made of hard candy, dissolving in water. Fluid with a higher concentration of sugar is more dense than the ambient, therefore a buoyancy-driven flow is set up. This affects the evolving shape of the sphere, smoothing the upper surface and roughening the underside.
A dissolving surface is roughened when the concentration boundary layer is gravitationally unstable. The instability results in covection of fluid away from the surface in the form of small plumes, which roughen the surface.
These images show the splash thrown out by a rotating wheel which is partially submereged in water. As the rotation speed of the wheel is increased more water is thrown from the wheel and the film becomes increasingly unstable.
These images show the splash thrown out by a rotating wheel which is partially submereged in either water or a non-Newtonian fluid with a finite yield stress, chosen to model mud. At low rotation speeds the flow looks very similar, but at higher speeds the film thrown from the non-Newtonian experiment breaks up, creating a great deal of splash.
Double diffusive instability occurs when there are two components to the density field, in this case, the temperature and salinity of the water. Overhead lamps preferentially heat the green-dyed fluid, making it less dense, such that it rises in the form of a thin plume. The heat quickly diffuses into the surrounding fluid, leaving fluid within the plume with a higher salinity than the surroundings.