Making Giant Flowing Soap Films


As you can learn elsewhere in this web site we developed this technique for making 2D fluid dynamic measurements in the laboratory. Scientific publications on the subject are documented also, and some that we wrote are available on this site as pre prints in the Adobe Acrobat .pdf format.

For our own amusement we then tried to see how large we could make these soap films. When this turned out to be quite large, we decided to collaborate with the Carnegie Science Center in Pittsburgh to demonstrate it. It was evidently quite popular and we hope to have a permanent exhibit fashioned there somewhere during January 1997.

In response to the interest people showed for these giant soap films during our latest demo at the ASTC meeting (conference on science musea and such) in Pittsburgh on Oct. 28, 1996, I have composed this document. It explains how to make your own simple setup, either for fun, or to test if it would be possible as a museum exhibit.

If you do end up building an exhibit with the information below, please let me know. I make this information available to further public science education. Distribute it freely but do not alter it without my consent. Before asking questions, make sure you have read all the other HTML documents on soap on this web site. If you can't find it, contact me, and I can add it when I have time. We will know much more once we have built the permanent system at the Carnegie Science Center in Pittsburgh. At that point the museum should be able to take over as developer of giant bubbles for museum exhibits.

In any case, I encourage you to enjoy and to learn, just like I do in the laboratory.

Dr. Maarten A. Rutgers.

What you will need:


Setting it up.



Figure 1.


Cut the bottom out of the bottle. Be careful not to cut yourself. Young children trying this should get help from their parents! Attach the bottle so that you can somehow still reach it to adjust the flow later on, and to fill it with solution. The bottle should be upside down (A, Fig. 1), with the open end up, and the cap facing down (c, Fig. 1).

Drill a small hole in the center of the cap (d, Fig. 1). The hole must be slightly smaller than the tubing, since otherwise the water will be able to leak out past the tubing. Cut off several centimeters of tubing and put it through the hole in the cap (e, Fig. 1). If you cut the tubing off at an angle you can more easily insert it through the hole.

Now put the cap back onto the bottle. Take the heavy fishing line (e, Fig.1 ) and the washer (which must be so large it cannot fit through the neck of the bottle) and feed it through as shown in Fig. 1

Pull through enough line that both ends hang to the floor. Tie the ends together and to the weight (E, Fig. 1). The two lines should now hang down, originating from the tubing at the bottom of the bottle and being held taut by the weight.

Place some sort of bucket under the thing (H, Fig 1). Remember, we have made bubbles of over 4 meters wide, and when they break, all the fluid in the bubble will fall down. The bubble is typically 2 to 3 micrometers thick. For each square meter of bubble you get 1 cc per micron of thickness. So, for a bubble of 10 square meters and 3 microns thick, you get 30 cc of fluid falling down. So, count on an area getting wet which is at least the diameter of the bubble width, or maybe even twice that. We often use a large plastic tarp. It also helps to have a kiddy pool or large trash can in the middle, where most of the fluid will fall when you are trying to form the bubble.

Now you are ready to make the soap solution. Use about 1% or 2% by volume and the rest tap water. Experiment by using as little soap as possible. Higher soap percentages with glycerine additives make for less successful bubbles. Try it! Can you figure out why? I'll leave that as a question for you to examine.

Place the clamp on the tubing (f, Fig. 1) and squeeze the tubing completely shut. Now pour the fluid into the bottle. Nothing should leak out. If so, you may need to tighten the clamp, or get softer tubing.

Now you are ready to try it out. make sure the fishing lines are not twisted and open the valve a little. Once the wires are wet, pull the wires apart with your fingers. A bubble, which flows down, should form between them.

See how large you can get it. Instead of using your finger, tie the thinner fishing line to the ticker ones (C, Fig. 1) to pull the string apart. In the museum we had many strings tied to the main ones which all were fed through eyelets and to an operator who would control the width of the film. Wider bubbles will of course require more flow. Whereas too much flow is not good since the film will flutter. These are supersonic shock waves. You can try to figure out why they occur. You can read more about it in the scientific publications we have published on flow in soap films. They are available elsewhere on this web site as Pre-Prints. It takes some practice to tune it just right. Here it will be up to you to experiment with the best techniques. We learn something new every time we do a demonstration.

We have been able to get it more than four meters wide, lasting up to a minute. Narrower films will last longer. The height seems pretty much unlimited. We have done it in buildings of up to 20 meters tall. The taller the better. You probably cannot do this outside, or in a windy open space. The large bubbles are moved about by air currents. Again, you will have to test out your particular space.

The width of the film is limited to the height of your space. The Angle F in Figure 1 cannot be too large. The exact number depends on the thickness of your soap mixture. Angle G in Figure 1 and 2 should also not be too sharp. The sharper this angle, the slower the flow there and the more the film has time to evaporate. Experiment for best results.

Figure 2.



This figure gives some ideas for different shapes and sizes of films. We have made films like that in Fig. 1, and also the diamond shaped one in Fig. 2. There is no reason why you have to pull the film in on plane. There could also be more vertical wires. All sorts of shapes should be possible.

You can of course make things a lot fancier. In the laboratory and for day long demonstrations at the Carnegie Science Center in Pittsburgh we have used pumps to keep the water level in the reservoir bottle constant, as I suspect we will do for a permanent setup planned there early 1997. If pumps are used for this reason, use one that can pump very slowly (1-50 cc/sec), one that has no parts that can rust, since soap is notorious for making things rust. We have used pumps that cost US$1000 from scientific supply houses and medical surplus pumps that cost US$15.

In the lab we do recycle the fluid at times, but in the museum we have not yet done that since the demos were only for a day at a time. The amount of fluid needed in a day depends on the width of your bubble. Before I already said that each square meter at one micron thick takes 1cc of fluid. The film itself falls at about 2 meters per second. The fluid flow rate is then (for a typical film) 1cc/micron/square meter * 2 microns (thickness) * 1 meters / second * 2 meters width = 4 cc per second. That is just a rough estimate. During Demonstrations we used about 20-40 liters of solution per day, which certainly could have been recycled. But, since the soap concentration is so low (only 200-400 cc of dishwashing liquid per 20 liter bottle of water) it is not very expensive not to recycle. The practicality of recycling on large scale will have to be explored in the permanent museum demonstration, but I foresee little problem.

Disclaimer:
Build this at your own risk. You will need to use a knife and a drill to make this thing, and you may find yourself trying to hang it from tall overhangs within buildings. You may also get soap and water on the floor and walls of your space. I take no responsibility for any damage or injury. If you use common sense, all these can be easily avoided. Children should not attempt to make this without adult supervision.

(c) copyright, Maarten Rutgers, 1996.