Hilary Fleischer, Harold Metcalf and John Noé

Laser Teaching Center, Department of Physics and Astronomy, Stony Brook University

Soap films are thin (1 - 10 microns) sheets of water sandwiched between surfactant soap molecules, which give the film elasticity and stability. Soap films are usually associated with static soap bubbles, but the less-familiar flowing soap films have many important contemporary applications, including studying airplane wake turbulence, weather patterns, and atmospheric flows.

In this study, a flowing soap film apparatus was constructed and optimized to create a stable soap flow. The aluminum support frame is 185 cm tall and 28 cm wide. Soap is contained in a reservoir at the top of the device, and the flow rate is controlled by a valve. The soap drips onto the guide wires, mono-filament fishing lines, which are separated by pull wires at two places on the frame at constant tension to allow the soap to flow in-between. The guide wires are fastened at the bottom of the apparatus in a container that collects the dripping soap. When the pull wires are fastened, the soap film is 155 cm tall and the width is at a maximum of 7 cm wide when the two guide wires are parallel. The soap solution used was 1 - 2% Proctor & Gamble Dawn Dishwashing Soap.

The thickness variations of the soap film were optically analyzed by means of thin film interference. A monochromatic light source, a low-pressure sodium lamp, illuminated the flow and allowed for clear viewing of constructive and destructive fringes. Photographs and movies were taken with a digital camcorder and a computerized CCD camera. The photographs were studied to observe the variations of intensity of the light and dark interference bands.

Any object can be inserted into the moving soap film if it is wet with the soap solution. Vortices form in the wake of this object and can be optically visualized through the interference fringes. A detailed investigation of the flow past a cylinder and the formation of a specific type of vortex is planned. A von Kármán vortex street is a pair of vortex rows swirling in opposite directions. This study will shed light on the development of these vortex streets in the natural environment including the visualization of wind flows past mountainous islands.

This research was supported by the Simons Foundation.