THE GLISTENING BOUQUET OF colors in a soap film arises from optical interference. The light waves being reflected from the front surface of the film interfere with the ones being reflected from the rear surface. This phenomenon provides scope for a number of demonstrations.

Suppose the film is illuminated by light of a single wavelength, which is to say a single color. If the interference is constructive (the two sets of waves from a particular part of the film are in step), that part of the film looks brightly colored. If the interference is destructive (the waves are out of step), that part of the film is dark.

A variety of splotches and bands of color are created when the film is bathed with a beam of white light, which consists of many wavelengths and therefore of many colors. Wherever you see a certain color, light waves with the corresponding wavelength interfere constructively. Waves at other wavelengths interfere destructively, so that the associated colors are pale or absent. The thickness of a region of the film determines what wavelengths interfere constructively there. Since the thickness usually varies, the film has an array of colors.

If the film is vertical, the liquid in it moves slowly downward, mainly because of gravity. The result is a smooth variation in thickness from top to bottom. At any given height the thickness is approximately constant horizontally. Therefore you see horizontal bands of color, each band related to a certain thickness of the film. If the film is disturbed so that the thickness changes, the colors fluctuate, perhaps wildly.

To demonstrate such fluctuations Andreas Kay of Cologne in West Germany has recently devised a way to project the colors of a soap film by making the film function as a mirror. To prepare a mount for the film, Kay cut away the interior of a lid from a can of Sir Winston tea. (Although any lid might serve, the shape of the one from Sir Winston tea is advantageous.) When the film is mounted vertically, the fluid that slowly flows to the bottom collects in a gutter around the border of the lid and then drains off the mount. If the mount has no gutter, the draining fluid collects at the bottom of the film and distorts it.

In Kay's design the film serves as a concave mirror reflecting light onto a screen where the interference colors can be examined. Kay dips the lid into a soap solution, mounts it on the can and then positions the can in a beam of light with the film vertical. Wetting a straw in the soap solution, he pierces the film with the straw and sucks some of the air out of the can. Because of the reduced air pressure in the can, the external air pressure pushes the film inward, making it concave. The curvature determines the film's focal length as a concave mirror. Removing more air makes the film curve more and shortens the focal point.

The film reflects only a small part of the light illuminating it-approximately 3 percent of what reaches the front and back surface. The rest of the light enters the can. Unless it is prevented from reflecting back out, it will mask the faint interference colors reaching the screen. To reduce the glare Kay paints the interior of the can flat black. He says the glare will be virtually eliminated if the back of the can is also fitted with an inclined black wall that serves as a light trap. When light is reflected by the wall, it travels farther into the can and is reflected several more times, being partially absorbed at each point of reflection.

Kay illuminates the film with a brilliant slide projector adjusted for a long focus and aligned so that the light reflected from the film passes through a lens system. He positions the equipment and the screen to maximize the clarity of the image on the screen. In addition he fine-tunes the curvature of the film by means of a syringe that fits snugly through a hole in the can. As the absorption of light by the paint warms the air inside the can, the air pressure increases, thereby decreasing the film's curvature. Kay removes some of the air by pulling the plunger outward, restoring the curvature.

Kay finds that to produce a clear image he must pass the reflected light through the lens system, which he took from another slide projector. The system, which has a focal length of 150 millimeters, is placed at the focal point of the film so that nearly all the light reflected by the film passes through 19 the lenses. Normally the plastic barrel that holds the lenses extends well beyond them so that it can slide into the projector. Unless the extension is removed it will block some of the light from the film. Kay cuts off the extension near the lenses.

To animate the colors on the screen Kay plays loud music on a bass speaker placed near the film. The fluctuations in air pressure distort the film, varying the direction in which light is reflected. The fluctuations also change the thickness of the film and thus shift the colors that are created by constructive interference of the reflected waves. The colors dance over the screen somewhat in time with the music. Kay has found that the strong beat of rock music provides a better show than the fuller and less percussive sound of other music.

In some frequency ranges the film begins to resonate with the sound. Parts of the film then vibrate vigorously. The movement creates beautiful patterns of vortexes and symmetrical streams in the film and in the image on the screen. When the film is exposed to loud ultrasound, the patterns on the screen become crazed. Small sprays leap from the film. Some of the activity in the film is created by streams of air forced across the film by the pulsating speaker.

Kay's soap solution consists of 1.4 grams of triethanolamine, 100 grams of glycerin (which is supplied at 85 percent dilution) and two grams of oleic acid. After mixing the chemicals he stores the solution in an airtight bottle kept in the dark so that air does not oxidize the oleic acid. The solution should sit for 24 hours before films are prepared. By then it should be clear. If it is not, it probably contains too much oleic acid. Such a solution causes small drops to form on the film. Adding more triethanolamine to the solution fixes the problem. Kay's films become thin within minutes and last for about an hour.

Kay has ways other than rock music to arouse a dance of colors on the screen. A whiff of ammonia drives the colors into a frenzy. Ammonia increases the surface tension of the film wherever it is absorbed. As the affected region pulls inward on the surrounding film, its weight increases until it is finally so bloated that it falls along the film. On the screen the fall seems to drag along entrained colors.

By changing the liquid solution to 15 grams of triethanolamine, 60 grams of glycerin and 25 grams of oleic acid Kay demonstrates a thinning behavior called critical fall. Several minutes after the film is mounted it begins to thin dramatically at the top. Soon it is so thin that the light reflected from the front surface is always out of step with the light reflected from the rear surface. The film is then black. The black region marches to the bottom of the film, preceded by a kinetic display of convection in the regions that are still colored.

Jurjen K. van Deen of The Hague also studies soap films. In particular he has wondered how one ever manages to blow a soap bubble from a ring-something children commonly do. As the bubble escapes from the ring, how does it avoid breaking because of the hole left in its side by the ring? The hole must close just as the bubble escapes, but how?

To answer the riddle van Deen tried to photograph a bubble blown from a ring by an assistant. He found that unless he used a high-speed camera and was careful to steady the ring, the photographs were always blurry and ill-timed. He then worked out an arrangement that is quite steady. A soap solution siphoned from an elevated reservoir flows through a tube mounted on a laboratory stand. The rate of flow is regulated by a clamp. Attached to the tube is a second tube through which air flows. The air tube extends downward past the end of the solution tube. When the soap solution leaves the solution tube, it drains to the opening of the air tube. In that position it forms a film.

The air is supplied by a 25-liter plastic bottle kept under pressure by the discharge from a vacuum cleaner. Another tube clamp regulates the airflow. A water-filled U-tube monitors the air pressure. All connections with the bottle are made through a specially fitted piece van Deen attached to the top of the bottle. The solution is synthetic dishwashing soap, diluted with water to a ratio of 1:5 or 1:10, with a dash of glycerin and a teaspoon of sugar added. Photographs are made in strobe light to keep the time of illumination brief (from 30 to 50 milliseconds). Longer periods of illumination yield blurry photographs.

After the soap solution forms a film over the end of the air tube, the airstream inflates the film into a bubble. As the bubble steadily increases in size its weight eventually overpowers the surface tension that makes it adhere to the tube. In due course it breaks free. The airstream then begins to inflate another bubble. Bubbles form in a steady procession. As a result the camera and the flash can be triggered accurately. Van Deen controls the growth rate of the bubbles by adjusting the flow of air; he is able to establish the size of each bubble by adjusting the flow of the soap solution.

With this arrangement van Deen discovered how a bubble survives its launching. As the bubble grows it remains attached to the tube by an umbilical section that is initially cylindrical but narrows as the weight of the bubble increases. Just as the neck collapses onto itself, the bubble breaks free; thus the hole in the side of the bubble closes. Surface tension jerks the rest of the umbilical section back onto the tube to form a new film for the next bubble. Similar events no doubt take place when a child blows a bubble from a ring.

If the flow rate of the air in van Deen's experiment is high, the bubble may be blown away before it has had time to form properly. He therefore rests it on a ring. In this arrangement his photographs reveal that the umbilical section sometimes develops several necks along its length before fully collapsing. He suggests that a study of the dynamics of the umbilical section might be rewarding.

Great fun can be had with a new toy called Bubble Thing, manufactured by David Stein, Inc., of New York. The mechanism produces enchanting bubbles, several meters long, that look like amoeba. The unofficial length record is about 15 meters.

The device consists of a plastic tube with a loop of narrow cloth ribbon at one end. One side of the loop is attached to the end of the tube and the other side is attached to a runner that slides along the tube, the function of the runner is to alter the width of the loop. A weight hangs from the bottom of the loop.

Following the instructions, I mix one unit of Joy dishwashing soap with 10 units of water and a quarter unit of glycerin (which I obtained from a pharmacy). Closing the loop by moving the runner to the far end of the tube, I dip the loop into the solution, thoroughly soaking it. Lifting the device from the solution, I hold the tube horizontally while slowly moving the runner to open the loop and expose the soap film that stretches over it. On a breezeless day one walks backward to inflate the film. Otherwise the breeze will do it. The bubble grows, stretching away from me as more fluid is drawn out of the ribbon by surface tension. If the breeze is too strong or I walk too fast, the film pops, leaving gossamer threads floating in the air or gently gliding to the ground.

When I succeed, a tubular bubble stretches for many meters, wriggling like some wild animal. To free it I close the loop, allowing an umbilical section to form, narrow and finally collapse on itself. If I wait too long, the bubble breaks away without closing the hole at the loop. In this case it disintegrates slowly. When I release the bubble properly, the hole closes.

As the bubble floats, surface tension acts to wrestle the tube into a sphere so that the surface area is minimized. This action fights against the force of gravity and the nonuniform distribution of liquid in the bubble. In the ensuing dance of shapes the bubble may burst. If it survives the battle of forces and the buffeting by the breeze and if it avoids certain disintegration as a result of touching something solid, it floats down the street like a zeppelin. Through all these movements it glistens with delicate colors that change constantly.

Bibliography

DYNAMIC PROCESSES IN SOAP FILMS. Karol J Mysels in Journal of General Physiology, Vol. 52, No. 1, Part 2, Pages 113s-124s; July 1968.

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