Monday, September 24, 2012

Emulsion separation time lapse

Fluid turbulence in a bottle of hairspray. Johanna bought a bottle of hair spray, consisting of two fluids, a clear and a white one. The product. The spray should be shaken before using, so that the two fluids mix. Above are some snapshots of turbulence achieved by gently shaking the bottle.

After a while, if left alone, the two fluids separate again. We've been quite fascinated by the mixing and re-separation processes.

This is a time lapse video of the hair spray, taken after shaking the bottle. The video starts with the emulsion completely white, that is, with the two solutions mixed. The whole separation process in the video takes about twenty hours.

From each frame in the video I cut a narrow vertical slice, always at the same position in the frame - this is apparently known as 'time slicing'. When these slices are placed beside each other a picture new is formed, where the horizontal direction represents time.

Three regions with different behavior can be seen in the picture. Here exponential curves have been fitted to the interface between the two fluids, in order to find the time constants. I expected to find a simple exponential time dependence, based on a vaguely remembered chemistry demonstration, but the result turned out to be more complex. When I searched for some explanation, I could only find complicated theories for how foams drain (pdf), and no mention of the simple exponential behavior.

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