Convection pattern in an annular "soap" film

Smectic liquid crystals are thick organic liquids with a slimy feel to them; the word "smectic" just means "soapy". A smectic can form a suspended film, similar to a soap bubble. Just as in an ordinary soap bubble, when the thickness of the film is comparable to the wavelength of visible light, interference between light reflected from the film's front and back surfaces gives rise to brilliant colours. The contours of reflected colour map the thickness variations in the film. This photo shows a smectic film less than 1 śm thick suspended across an annular slot in a brass support. The outer diameter of the annulus is about 1 cm.

When a small voltage is applied between the inner and outer edges of the slot, the radial electric current through the film is carried partly by convection, that is, by the flow of charged fluid. The flow stirs up the film's inhomogeneous thickness in an array of left and right swirling vortices. This cellular flow pattern is a small scale electrical analogue of patterns due to convection driven by temperature gradients in the atmosphere and oceans, in the Earth's mantle, and the interior of stars.

Stephen Morris
University of California, Santa Barbara
Category 2 (Open) 1st prize

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