The ideal film-making mixture is a 1 to 2 percent solution made by combining one or two parts of clear liquid dishwashing soap with 100 parts water. In English units, a 1 percent solution results from one teaspoon of soap in one pint of water. Whatever you do, avoid those bubble-blowing fluids. Gravity limits a bubble's lifetime by draining fluid away from the top until the bubble ruptures. To slow this down, toy companies add glycerin to increase the solution's viscosity. But viscosity is the last thing you need if you're studying turbulence, because high viscosity damps out turbulent motion.
The film flows between about 0.5 and 4 meters per second (1 to 9 mph)¿far too fast to study the millimeter-size vortices by eye. Moreover, the turbulence can be seen only under high-contrast lighting. You'll need a strobe light or video camera with a ¿sports shutter¿ option, which limits exposure on each video frame to between 1/1,000 and 1/10,000 of a second. Most new camcorders have this feature. You can also take fantastic still photographs using a fast shutter and high-speed film. Rutgers recommends ASA 3200 black-and-white film (which provides the best contrast) and a shutter speed of 1/2,000 of a second or faster.
The light source must be bright enough to compensate for the quick shutter, and the best way to create high contrast is to use monochromatic light. Rutgers uses a low-pressure sodium ¿sox¿ lamp, which generates an intense band of yellow light with a wavelength of around 585 nanometers. Because essentially all the energy goes into a single wavelength, you don't need a high-wattage bulb. An 18-watt sox bulb is plenty bright and retails for about $40. Unfortunately, it requires a fixture with an electric ballast, which costs about $100. You can find such fixtures at most industrial-lighting specialty stores. To save money, check a local industrial liquidator or an on-line auction site such as eBay. com. You'll want a floodlight with a trunnion fixture that can be mounted to a tripod. If you're in the U.S., make sure the fixture is compatible with a 120-volt outlet.
As an alternative to a special lamp, Mike Rivera of the University of Pittsburgh suggests adding powdered milk to the soap solution. Place a black cloth behind the milk film and light the setup with any bright lamp. The milk film scatters more light where it is thicker, and this can provide enough contrast to show what's happening. I can show turbulence using this technique, but the results have not been as good as with sodium light. You may have better luck with a spoonful of white paint. Let me know what you find.
Reflected energy reaches the camera from both the front and back surfaces of the soap film. These two light trains can interfere to create fringes¿widely spaced dark and light bands whose separation varies inversely with the thickness of the film. Rutgers reports that he can often get a uniformly bright surface in his laboratory, which means that the variation in film thickness is less than 100 nanometers. But the best I've been able to manage is six fringes. Turbulent flows will contort the fringes, making flow patterns visible much like smoke in a wind tunnel.
Try two simple experiments. A cylindrical obstruction, such as a toothpick, sheds pairs of oppositely rotating vortices. When two toothpicks are placed side by side, the leftmost vortex of the right toothpick interacts with the rightmost vortex of the left toothpick with a repulsive force. Can you decipher how the force between these objects changes with the separation between them? You can also experiment with shock waves. Just open the hose clamp until the flow speed is so fast that a bow shock, like the wake of a boat, appears around a toothpick. The shock wave moves toward the sides, but when it encounters fluid that is flowing too slowly, it reflects back to create a classic diamond shape