OPTICAL ANALYSIS OF A FLOWING SOAP FILM.
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.
Hilary Fleischer, Harold Metcalf and John Noé
Laser Teaching Center, Department of Physics and Astronomy, Stony Brook
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
This research was supported by the Simons Foundation.