Soap-film shots tell more about swirls

Science News, August 22, 1998 by Peter Ulrich Weiss

In the iridescent eddies of a soap film, scientists see a parallel to the whirling flows of Earth's thin coating of atmosphere and oceans. The analogy also extends to sister planets, whose turbulent phenomena befuddle fluid-flow experts.

Past laboratory studies of soap-film turbulence have generated wishy-washy results because experimenters have been limited to recording the velocity of flowing films at a single point. Now, a research team at Los Alamos National Laboratory has leaped that hurdle with an apparatus that simultaneously measures velocities at thousands of points in a patch of cascading film.

"They're pushing the instrumentation about as hard as you could," says John Sommerer at Johns Hopkins University.

In the latest version of the experiment, a continuously replenished film of soapy water races more than a meter down two nylon wires strung 6 centimeters apart. A comb inserted through the film induces swirling eddies downstream, which a digital camera records through a 5-centimeter-square window. Mixed into the water, microscopic spheres of titanium dioxide, the reflective substance in white paint, render the eddies visible.

By flashing strobe lights twice per camera exposure, the researchers create images that show the change in position of each of the thousands of marker particles. Computer analysis transforms the stop-action frames into velocities yielding, for the first time, a quantitative portrait of a soap film's swirls, report Michael Rivera of the University of Pittsburgh and Peter Vorobieff and Robert E. Ecke of Los Alamos in the August 17 Physical Review Letters.

The velocity data is sufficiently detailed for unprecedented mathematical examination of the turbulence, says Vorobieff. "This analysis allows us to explicitly compare our data to existing theories and provide benchmarks for people who do numerical calculations."

From their investigation, the researchers conclude that the observed turbulence is "roughly consistent" with two-dimensional turbulence theory. That model, however, assumes a uniform thickness of the film, which the researchers did not find. The camera detected plumping-up at the fringes of eddies, where more light was scattered by the increased number of marker particles.

Fluid dynamics researchers have had some doubts about soap films as a model for two-dimensional turbulence, Sommerer says, because they suspected that such films might have significant flow in the third dimension, evident as thickness variations. The finding that the film has ripples but still can be described by existing theory suggests that the flow in the third dimension is suitably small. That should inspire "increased confidence" in the results derived from soap-film turbulence studies, he says.

Given the striking images possible from their easy-to-build, inexpensive apparatus, Vorobieff and Ecke, are promoting it as a classroom tool for physics teachers.