Soap Films Made Easy

by
Dr. Maarten A. Rutgers


In this document I hope to answer the following questions:


What is a soap bubble?

A soap bubble is simply a very thin sheet of water sandwiched between two layers of soap molecules (also called surfactant molecules. These molecules are called amphiphilic. This means that part of this molecule likes water (hydrophilic) and another part hates water (hydrophobic). These molecules are often drawn as little tadpoles. The head likes the water, whereas the tail does not. When such a molecule is put in water, as many as possible will crowd to the surface, so that the heads can stay in the water, while the tails stick out into the air. This is why soap-like molecules are called surfactants, since they mostly affect the surface of water. It is these molecules that make soap bubbles stable. Without such molecules on the surface, the bubble would spontaneously break apart into tiny water droplets. (The detailed reasons for this stability is a bit too advanced for this document.)

In the picture to the left I have drawn a little cartoon which shows a detailed picture of what the surface of a soap bubble looks like. The hydrophilic head groups of the surfactant molecules are drawn as little red balls, while the hydrophobic tails are black. In reality the molecules are much smaller than drawn, about 1000 times smaller than the thickness of the bubble wall. Bubble walls are typically a few micrometers thick, while the molecules are a few nanometers long. In reality there are also molecules inside the water of the bubble wall, some of them by themselves, and others in little balls known as micelles. In such micelles many molecules ball up with their tails bunched up inside the ball, and their heads on the surface. This way the tails do not touch water and the heads get to touch the water, without the molecules being on the surface.

In terms of bubbles this is a handy way of storing soap inside the bubble wall. For instance: if the bubble is stretched, the surface is enlarged, which causes the molecules on the surface to be spread apart. If there are additional molecules stored in micelles, they replenish the surface so it remains nice and stable.



What is the difference between a film and a bubble?

Well, a bubble is a film, but a film is not a bubble. Bubbles usually start as a film, for instance in a bubble wand. Once you have blown on this film, it will separate to form a free floating entity called a bubble. Soap films must be bounded on all sides, or their surface tension will pull them into tiny droplets. In the bubble wand the edges of the wand hold onto the film. A bubble is a very clever case which has no edges; it is like a sheet folded to meet itself along all its sides. What keeps a bubble from being pulled into a small droplet is the pressure of the air inside.

Soap films always try to minimize their surface area as this minimizes the total energy of the film. The minimum area for a bubble is a sphere. Any other shape cost more energy and will only last for a short while before the bubble becomes spherical again. In the experiments we do in our laboratory we generally only deal with soap films, not bubbles. The films are most often surrounded by nylon wires, which all lie in the same plane. The minimum surface in this case is a flat sheet. You can also make films that are not flat. Imagine taking a loop of metal wire and twisting it a little so that it cannot lie flat on the table. Now dip this into soapy water. A soap film will form what is called the minimal surface, the surface with the smallest possible area, which in this case will not be flat but warped.


How do we make flat soap films?

The easiest way is to just dip a metal frame into some soapy water. Unfortunately this type of film does not last too long. If you keep the film level, it will eventually pop due to evaporation. If you set it upright, all the fluid will drain to the bottom due to gravity. The top will get thinner and thinner and pop even quicker due to evaporation. You can of course just put the film in a sealed bottle, with a little extra water in it. This way you will rule out evaporation and a film can last for a very long time. The physicist James Dewar kept some films for over three years in a sealed bottle! That is no good for our research group though since we want to study the flow of soap films around rigid obstacles. In that case it is very difficult to have things sealed in a bottle.

Our purpose has been to study fluid dynamics in two dimensions using soap films. The above figure outlines the brief history of such experiments, including references in scientific journals. During the mid to late 80's Yves Couder performed a series of experiments in Paris, France, where objects were dragged through a stationary film. This allowed a one shot type of observation of 2D like flow patterns. These experiments required that the film be remade after every try. In the U.S. Morteza Gharib developed a device which is more like a 2D version of a wind tunnel, where the fluid moves past a stationary object. In their device two rails connect a bucket of soapy water to a thin sheet of water forced from a thin slot in a metal pipe. A soap film is drawn across the rails from the bucket to the "waterfall" which then pulls the film forward. The film can now run for many minutes. Obstacles were then inserted into the film and the flow downstream from them was studied.

A third device was developed by Hamid Kellay, Xiao-lun Wu, and Walter Goldburg in our lab at Pittsburgh. In this case the film is vertical and is driven by gravity. Two wires connect upper and lower soap reservoirs. Soapy water leaks between the two wires through small holes in the upper reservoir. Using a plastic ruler we would draw a film between the top bucket and the lower water surface. Such a film could last for hours, but the thickness was not very uniform and the speed was not easily controlled.

Our current setup was designed by myself and is a slight modification with very significant consequences. The wires at the top and the bottom of the setup now come together. The soap is dribbled onto the wires and the flow rate is controlled by a valve. At the bottom the wires are tied to a weight. At first both wires are left to hang together. Then some soapy water is left to dribble onto the wires. At this point you pull the wires apart and a film forms between them. In the picture I have drawn some extra wires (red lines) we use to pull the main ones apart. This device works very well. By controlling the valve you can control the film speed and thickness. The thickness is very uniform and has allowed us to make beautiful photographs and very accurate measurements you can find elsewhere on this web site. The film will also last basically forever. We have let it run for up to a day without breaking. Evaporation is no longer a problem since the film is always being recreated at the top and destroyed at the bottom. The film moves downward at about 3 meters per second, so in our laboratory setup of about 3 meters tall, the film is never in contact with air for more than a second.



What are the dynamics of a falling film?

Think of the film emerging as a skydiver jumping out of a plane. As she jumps she quickly accelerates, and so does the film. As the film spreads out, air drag across it's ever expanding surface becomes important. The film quits accelerating, and may even slow down, as is the case with the parachutist as she pulls open the chute. Once the film reaches the parallel section of the channel, its speed will be constant, as it is for the skydiver, who now glides down to earth.



How big can we make our films?

The method works so well that we first tried it with very long wires in the five story stairwell of the University of Pittsburgh Physics Dept. Then we contacted the Carnegie Science Center (Pittsburgh Science Museum) where I built the giant film outlined in the figure above. The museum has a large atrium with a spiral walkway around it. We attached our film to these walkways. The method is the same as that used in the lab. The size of the films we made was basically limited by the size of the building. A film of 14 feet wide was once made, but only when there are no other people in the museum, since such films are quite sensitive to any air drafts. Films of few meters in width would last for many minutes, until some child would blow at it hard enough. A few scanned photos of these truly magnificent films conclude this document. It is truly amazing that something as fragile as a soap film can also be so robust. The films in the photos are 10 million times wider and longer than they are thick and contain up to 30 or 40 cubic centimeters of fluid.