I was playing around with some thick ointment and I set a thin film of it (any semi fluid works fine too) between 2 layers of plastic (it works with thick papers as well). When I released the upper sheet slowly, a dendritic fractal pattern emerged.
I repeated it many times and got a similar pattern every time. Here's a diagramatic view of the pattern.
Why does this happen? I was thinking the path of air flow rushing out could influence the fluid, but I can't come up with the reason- why those dendritic fractals, and not just straight lines?