Soap film

One of the fascinations of blowing soap bubbles is the beautiful swirls of iridescent color that flow across their surface when the bubbles catch the light. But physicists have another reason for playing with soap films. These mystical whorls may provide the key to understanding important phenomena from the Great Red Spot on Jupiter to the intricacies of deep ocean currents.

Film schematic
WET SANDWICH. A soap film consists of a layer of water trapped between two layers of soap molecules. The water accounts for 98 percent of its volume.
The image shown above was made by Michael Rivera of the University of Pittsburgh along with Peter Vorobieff and Robert E. Ecke of the Center for Nonlinear Studies at Los Alamos National Laboratory when they conducted the first quantitative measurements of the properties through an entire soap film; previous measurements were made on a point-by-point basis. The team reported its results in the August 17, 1998 issue of Physical Review Letters.

Vortex map
CURRENTS. Computer image shows swirling motion in a soap film; red indicates counterclockwise motion; blue, clockwise.

Lately, physicists have become interested in soap films because they provide a unique window into turbulence in two dimensions. The films consist of a thin slab of water sandwiched between two layers of soap molecules. The films range from a few microns (millionths of a meter) to several tens of microns in thickness. Yet they can have a surface area that is, by comparison, immense--several meters. So the fluid trapped between the layers is constrained to movement in two spatial dimensions.

To study these flow patterns, the Los Alamos researchers added highly reflective titanium dioxide particles to the fluid, and snapped pictures with a digital camera. The technique allowed them to obtain detailed images of thousands of points of the fluid every 160 to 300 microseconds. By tilting the film so it slid through a comb-like structure, they created turbulence and were able to measure the formation of swirling vortexes. The results indicated that the flow of energy was from larger to smaller eddies, where it gradually dissipated downstream.

There may be more truth than we thought to the old saw, "the world in a soap bubble."

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