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Turbulence in Soap Films
False-color image of turbulence in a 30-micron-thick soap film. A soap film is really nothing more than a thin slab of water sandwiched between two layers of soap molecules (surfactants). These films can display fluid patterns that are turbulent--that is, they have a complex, multiscale structure varying randomly in space and in time. Studies of the turbulent patterns in soap films can provide insights into turbulence on faraway worlds (the Great Red Spot of Jupiter) at majestic scales (the world's oceans), and in any other situation in which the flow takes place principally in two dimensions. (Image courtesy Los Alamos National Laboratory.)
A soap film is 98% water and 2% surfactant (soap). Each surfactant molecule contains a water-loving (hydrophilic) head and water-repelling (hydrophobic) tail. The surfactants form two single-molecule layers that surround a slab of water. The water region also contains loose surfactant molecules that flow along with the water. Soap films are very thin -- typically they are from a few microns (millionths of a meter) to several tens of microns in thickness. As a point of comparison, a typical human hair has a thickness of about 75 microns. Even though they are so thin, soap films can have lengths and widths that are several meters. Flow in soap films takes place principally along these two dimensions. (Figure courtesy Los Alamos National Laboratory.)
Some data from an experiment on soap turbulence undertaken by Michael Rivera (University of Pittsburgh), Peter Vorobieff and Robert E. Ecke at the Center for Nonlinear Studies at Los Alamos National Laboratory. A soap film is sent through a comblike obstacle which gives it turbulent flow patterns. The lefthand image shows the turbulent eddies of fluid which result. The image on the upper right hand corner shows variations in thickness for a 12 mm-by-12 mm portion of film. The bright regions correspond to regions of of higher thickness. As it turns out, these thickness variations play an active role in influencing the film's flow pattern. The bottom-right-hand corner shows measurements of vorticity, essentially the measurement of a fluid's swirling motion. The brighter the region, the higher the vorticity. Red indicates counterclockwise swirling; blue, clockwise. (Figure courtesy Los Alamos National Laboratory.)