Maarten A. Rutgers makes soap films three stories tall. And not just for fun. Some of today's most intractable physics conundrums involve turbulent fluids. Physicists often look for ways to experiment with simpler, two-dimensional systems to check their ideas before they tackle real-world, three-dimensional problems. Rutgers, a professor at Ohio State University, has mastered 2-D fluids. So I was quite honored when he recently came to my home laboratory to teach me his secrets for making a lab-size version.
Rutgers's method is as ingenious as it is simple. He poises a reservoir of soap solution above two stretched monofilaments (single-strand string, such as 20- or 30-pound fishing line) and allows gravity to draw the solution over them. Once the filaments are thoroughly wet, he slowly separates them with four fine threads. A fast-flowing and extremely uniform soap film forms between the filaments. When he wets a small object, such as a toothpick, and pokes it into the flow, vortices form in the object's wake. The result is an awe-inspiring display of turbulent motion in two dimensions. I have now spent hours ruminating the complex interactions between patterns of vortices that I have created with toothpicks, hair combs and knife edges.
A plastic two-liter soda bottle, hanging upside-down, makes an ideal reservoir for the soap [see inset diagram at above right]. In the bottom, cut a large hole (for refilling the solution) and two small holes (for stringing the monofilaments). The solution will drain out the bottle cap through a five-centimeter length of soft silicone tubing, which you can buy at an aquarium supply store. Drill a hole in the bottle cap with a diameter slightly greater than the inner diameter of the tube. Then cut the tip at a diagonal and thread the tube through the hole. This should create a watertight seal. If not, a dollop of aquarium cement will.
The bottle hangs from a short piece of monofilament, perhaps 30 centimeters long, strung through the small holes. A second piece of monofilament, four meters long, will guide the soap solution. Fold this piece in half, thread the two ends through the two small holes and pull them out through the tubing. At this point, you can screw the cap back on the bottle. To control the flow of solution through the tubing, attach an adjustable hose clamp, which you can find at any hardware store. To smooth out the flow as it moves onto the filament, Rutgers slides a disposable plastic pipette tip over the end of the tube, but you can also use the neck of a plastic eyedropper. You may have to trim the tip to keep the fluid flowing fast enough to maintain the film.
We hung the apparatus from a beam in my workshop, but Rutgers recommends a wooden frame that can be easily set aside for later use. The monofilaments are kept taut by a large rubber band at the bottom [see illustration above]. Tie the two strands together, pass them through a small plastic funnel and tie them to the rubber band. A plastic tube from the funnel drains the solution into a container. Or you could just let it drip onto a large towel or into a pan.
Last, take four thin threads about 50 centimeters long and attach a paper clip to one end of each. Tie the other ends to the monofilament. To create the film, bring the filaments together and open the hose clamp to wet them thoroughly. Then gently separate the filaments by pulling on the threads and secure the paper clips on hooks screwed into the wooden frame. This forms a film with a long straight section in which the film flows with a nearly constant speed. Opening the hose clamp increases the speed. The film will break often during your experiments, so just repeat this procedure to regenerate it