oil to show separation?
I've done the rain clouds before, putting water in a clear glass, top with a layer of shaving cream, then squeeze food coloring over the top and watch it filter down through.
Also, can't remember exactly how this one is done but you use milk, dishwashing det.(Dawn works best)...oh crap my mind just blew a gasket.
I can't remember if pepper or oil(maybe both) is used.
Not sure if that's what you had in mind.....