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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.

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I repeated it many times and got a similar pattern every time. Here's a diagramatic view of the pattern.

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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?

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Separating the layers induces drag forces, organising the semifluid into a crystal at the molecular level. It is easiest for the process to continue where the crystal has already started to form, hence the process takes place along branches. One may compare the patterns to snowflakes (although in snowflakes the crystal forms from condensation under conditions which preserve symmetries).

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  • $\begingroup$ When you say "crystal at the molecular level", do you mean the bulk still remains a fluid? I was thinking along the lines of air flow, this crystal approach seems new to me. $\endgroup$
    – Bipasha
    May 6, 2020 at 0:37
  • $\begingroup$ Yes, it is my understanding that what makes a semifluid is that there is a very loose binding so that, in effect, the bulk remains liquid, but solid structures form within the liquid. $\endgroup$
    – Charles Francis
    May 6, 2020 at 6:39

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