Optical Analysis of a Flowing Soap Film


Introduction:

Soap films are sheets of water sandwiched between surfactant soap molecules. The surfactants provide stability and elasticity to the film. The molecules are amphipathic (both loving) ions. The films are very thin, approximately 1 - 10 microns. In this study, a flowing soap film is used. The movement of the soap can be used to study turbulent air movements that otherwise are not visible.











Device:

The device that I have built, from household materials, easily creates a flowing soap film. A 1.5% Dawn Soap solution is contained in a reservoir at the top of the apparatus and flows through a valve onto two guide wires, mono-filament fishing line. The soap solution is then collected in a reservoir. Pull wires separate the guide wires at a constant tension and allow the soap to flow in-between. Diagram

Thin Film Interference:

The two surfaces of the soap film allow for interference of light and the creation of visible interference fringes. These fringes can be used to measure the changes in thickness across a soap flow. The fringes are seen the best with monochromatic light, in this case a low pressure sodium lamp.

CCD Camera Pictures:

The surface plot shows the distribution of light intensity. The green peaks show the concentration of pixels at constructive fringes and the orange areas show low concentration at destructive fringes.

Vortex Shedding:

A von Kármán vortex street is a pair of vortex rows spinning in opposite directions. They can sometimes be seen in nature in the form of winds past a mountainous island. They can be seen in a soap film when an object is introduced into the soap flow. As long as an object is wet with the solution, it can be poked into the soap film. I have used the CCD camera to take pictures of these vortices and have again analyzed the intensity distribution to show how the film thickness is changing in a specific vortex.












Limitations and Plans:

The problem with my soap film apparatus is that it is sometimes unstable. I think the biggest problem was that I did not put enough soap into the solution. With that fixed, the film can last for tens of minutes without breaking. I also tried using a overflow mechanism, a pumping system, to regulate the pressure on the top of the device. However, the pump I used would cause the soap solution to foam and block the soap flow. Another problem is that I need a way to quantitatively analyze the flowing film. Some ideas have been suggested:
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