Science makes rainbows dance to elevator music

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This video combines the fascination of watching colors shift on the surface of a soap bubble with the old-fashioned enjoyment of watching laundry spin in a tumble dryer. But it's more complicated than both.

These are smectic films — although they look like soap, they are tiny liquid crystals along a thin plane. The film is held in a ring shape between two electrodes. A difference in electrical potential is applied between the inner and the outer electrode, which causes the crystals inside to engage in convection. The term "convection" generally applies to heat, and describes the movement made as hot material rises, letting cold material rush in below. Often the rising material cools down and sinks, in order to start the cycle over again. In the videos, it is electricity that drives the convection, as electron-rich materials rush to the positive electrode, discharge, and sink back towards the negative electrode again. The colors are caused by the different thickness of the film, the same way they are in soap bubbles.

There is one difference between the upper and lower films, though. The upper film is unsheared, and the lower one is sheared. Shear refers to pressure put on a substance. (You may have heard the term before in regards to shear-thickening fluids like cornstarch in water — which thicken when pressure is applied to them.) In the lower video, the central electrode is rotated to put pressure, or shear, on the fluid. The result is finer rainbow swirls than the upper one. By looking at the two, we can see how these films act differently in slightly different conditions.


Also, it's cool.

Via Stephen Morris, University of Toronto.