Thursday, May 15, 2014

Strange patterns

For more than a year, I am trying to understand that

Or more dynamically, this

This is no leopard or fish skin pattern, nor salad dressing-like demixing. It appears when I make a yogurt in a very thin container, actually in a gap of a tenth of a millimetre thick, between two glass plates. Important detail: the glass is made extremely slippery, nothing appears with usually sticky glass.

The pattern is random but there is a well defined size. I can change the thickness of the cell (from 50 microns to one millimetre) and the pattern will scale up accordingly.

By the way, there is nothing living in my yogurt, no bacteria. The gelation is induced by slow and homogeneous acidification by chemicals. You can see the process of gel formation at high magnification in the video below. At first, things move around, this is Brownian motion. Then the gel forms and everything stops. Then the pattern appears.

It took me a couple of month to understand what these patterns are. I let you guess until next time.