APS 1999 Centennial meeting giant bubble.


Dear Participants,
Many thanks for volunteering, for what should prove to be a fun job. Back in April 1998 the APS was looking for an eye catching way to attract people toward the APS historical exhibit at the upcoming centennial meeting. They asked me to make a giant soap film exhibit for the occasion. In the end, the bubble will not be close to the APS historical exhibit, but, moreso close to the Division of Condensed Matter Physics exhibit. It still promises to be the most noticeable and spectacular thing in the entire football field sized exhibit hall.

The main purpose of this document is for you to learn what a giant soap film is, where to find it in the GWCC, and how to operate it. You don't have to read all of this if you don't want to. As long as you find your way there, I'll make sure I am, or my grad student Brent Daniel is there to start things out and explain everything in person.

What is a giant soap film?

To study 2D turbulence I refined a flowing soap film apparatus a few years back. The refinement worked so well that we did some demonstrations for fun at a Pittsburgh science museum. We made soap sheets 4 stories high and up to 4 meters wide.

Principles of the bubble operation

In the rest of this page I describe how to run the soap bubble once it is set up.


I wanted to explain a little of "how it is done" so that you can have an idea of the art of making big films. If you don't have time to read, the whole process in summarized in the last paragraph.

DEMONSTRATION SETUP:


This picture was for the soap film in Pittsburgh. There we had the luxury of being able to walk up a spiral walkway and make adjustments to all parts of the film. At the GWCC, we will not have any access to things high up, but the basic operation will remain the same. Refer to the Pittsburgh museum figure during the rest of this explanation.

Two nylon fishing lines (primary lines, black) hang straight down from an eight foot wooden boom on the third floor (from the roof at the GWCC). They are held taut by a lead weight at the ground floor. A Bottle of soapy water sits at the top of the film, and feeds the flow through a nozzle. The primary vertical guide wires go into this nozzle. A tube goes from the bottle to the nozzle, and through a pinch valve on the way. The valve is operated by a stepper motor, which you control through a small control box on the floor.

A secondary set of thinner wires (red) are tied to the primary lines at four points above the floor. These lines are fed through eyelets attached to the railing of the ramp (vertical steel cables at the GWCC) and then all are fed up to the second floor (down to the floor at the GWCC) By pulling on these secondary lines you can make the primary lines separate. The figure shows the primary lines separated. At this point you should know how the whole thing works. At the gwcc, all four secondary lines will be fed into a fancy fishing reel, so you can operate the wires with one hand, and the remote valve control with the other. As the operator, you will sit behind a small table to which this equipment is attached.

Further equipment, like the pump which brings the soap up to the bottle will operate automatically. A level sensor will turn the pump on intermittently when required. If the soap solution should run out, you want to fill the glass carboy half way with water, add about half a bottle of Dawn dish soap, swirl the bottle around to mix, and then fill up carefully with water. We'll try to have two carboys there, so one can always sit ready. We found that very cold water reduces performance.

HOW TO MAKE THE FILM:

One person can operate the film independently. Once you try it a few times you will get the hang of it.

The starting position is to have the primary lines together, which means you must release all tension on the lines. You do this by pushing the large button at the bottom of the reel. If it does not go all the way, pull the last bit of wire out with your hands. At this time, it often pays to close the valve enough to have only a little solution dribbling down the line. If you open the valve too far, all the fluid will not travel down the wire and "rain down" around the wire too. This rain is to be avoided, since it will break the film as it is first created. Open the valve just enough to let the solution wet the wires. We have tried to program the stepper motor controller so you can go to this starting valve position by pushing a single button.

It takes some time for the solution to make it down to the bottom, but be patient. Reel in a little now. A soap film will appear between the two primary lines. Simply keep reeling until the film is as wide as you like, which can be several meters. The soap will now be perpetually flowing down at a few meters per second. It will last practically indefinitely (or at least tens of seconds).

While you keep making the film wider, you also have to give it more liquid to make up for the extra film area. You can either push the automatic button (this will be clear once you see the control box) or use the continuous open/close buttons to keep up. Since the film is nominally a few microns thick, every square meter of film requires a few cubic centimeters of fluid. When the film is large, that can be a total of 10-30 cc! You will have clues as to what flow is too slow or too fast. In the beginning, you must avoid the rain phenomenon, which translates to keeping the flow to a minimum. As the you widen the film, the thickness will change for a constant flow, and the color of the film will change. Very thin films have the most brilliant colors. The film will probably pop soon if you don't open the valve some. There is also an upper limit the amount of flow the film can sustain, due to a supersonic instability of the film at point A. Supersonic in the sense that the speed of the film exceeds the speed of waves with a displacement out of the plane of the film.

[Physics interlude] Remember how the speed of waves in a string is sqrt(T/m) where T is the tension of the wire, and m the mass per unit length. So, higher tension means faster waves, but heavier line means slower waves. For a soap film you just write the 2D analog: v=sqrt(2s/m2) where s is the surface tension of soapy water and m2 is the mass per unit area of film. The factor of two arises because the film has two sides. In soap films, the surface tension is basically independent of the film thickness, but mass per unit area is directly proportional to the thickness. So, the thicker the film, the slower and more sluggish the waves you can excite.

Now you understand what happens at the top of the film (point A). As you crank open the valve, fluid rushes out between the wires. The film is very thick at the top and hence it's wave speed is very slow, so slow in fact that the film moves down faster than the wave speed. You see violent ripples, which are actually supersonic instabilities. If these ripples get too large they start spitting droplets from their cusps, which fall down and can break the film downstream, or, the ripples are so violent that the film pops right at that point.

Once the film does break, you must nearly close the valve, and bring the lines back together, and repeat the whole thing.

SUMMARY.

To summarize the procedure: Once the liquid has reached point C, start reeling in. As the wires separate, increase the flow, avoiding a brightly colored film on the low flow end and supersonic ripples on the high flow end.

By all means experiment with everything mentioned above, see what it takes to break the film. See how big or how colorful you can make the film. Remember you too are there to have fun!