Convection Pattern In a "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 between two wires about 2 mm apart.

The film is made of a pure material and contains no water as solvent as in an ordinary soap bubble. This gives the film a low electrical conductivity. When a small voltage is applied across the two wires, the electric current is carried partly by convection, that is, by the flow of charged fluid. This flow stirs up the film's inhomogeneous thickness in spectacular swirling vortices. Convection is an important hydrodynamic instability in nature and leads to cellular flow patterns found 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)


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