A FILM of liquid enclosing a gas is called a bubble. It is not generally known that one can also have a film of gas enclosing a liquid. Such objects are called globules, boules or antibubbles, depending on their size and shape, the surrounding medium and the whim of the experimenter who is describing them. Like bubbles, these objects make fascinating playthings.

Gerard Schol, a physics teacher of Drachen in the Netherlands, has done a series of experiments with drops of water that float on water. A film of air separates the drops from the supporting surface. The drops Schol launches on a collision course frequently coalesce on impact to form floating puddles that he calls globules. Additional experiments led him to conclude that the integrity of a globule can be maintained at least in part by forces of electrical repulsion arising from the polarized nature of the water molecule [see "The Amateur Scientist, SCIENTIFIC AMERICAN, August, 1973]. Kenneth C. D. Hickman, who heads the distillation research laboratory of the Rochester Institute of Technology, has made much larger bodies of water that float in water and still larger hemispherical masses of isopropyl alcohol that float in isopropyl alcohol. He refers to these remarkable objects as boules, a term he has borrowed from the manufacturers of synthetic gems.

Hickman suggests that forces other than electrical repulsion may contribute to the support of Schol's globules, because experiments demonstrate that the integrity of a boule is maintained by a concave film of vapor flowing radially. To make boules Hickman designed an apparatus that superheats the supporting liquid, that is, raises the temperature of the liquid slightly above the point at which it would normally boil. At this temperature the surface liberates vapor at an abnormally high rate.

Hickman reduces the tendency of the superheated liquid to boil by continuously purifying it and draining flotsam from its surface by means of an associated retort. Liquid at the surface in the experimental flask drains through a tube into the boiler of the retort. Vapor that rises from boiling fluid in the retort enters the cool tube at the top, where it condenses. The condensate trickles down the cool tube and encounters a glass rod [see illustration lower left]. The now purified liquid adheres to the rod, flows down it and drips off its rounded end. The height of the rod can be adjusted experimentally to let drops fall gently on the clean surface of the superheated liquid. Typically the flasks are made of borosilicate glass with a capacity of one liter (1.057 quarts).

Drops of condensate will merge to form a boule that floats in the superheated liquid if appropriate conditions have been established. The supporting liquid must be heated at least .15 degree Celsius above the temperature at which it would ordinarily boil. The superheated liquid must be purged of absorbed gases. Its surface must be free of contaminating substances. The condensate must not be dropped to the supporting surface through a distance of more than about one centimeter (.4 inch). The boule and the supporting liquid must be kept electrically neutral, preferably interconnected by a thin wire bent into the shape of an inverted U. The upper parts of the vessel in which boules are made should be warmed above the temperature at which the vapor condenses. Finally, all parts of the apparatus that come in contact with the liquid must be scrupulously clean.

The temperature of the supporting liquid and the vapor above it can be measured by a pair of thermocouples. The probe that senses the temperature of the supporting liquid should be placed approximately one millimeter (.04 inch) below the surface and beyond the edge of the area within which boules are likely to form. The remaining probe can be put anywhere in the vapor above the liquid.

Boiling tends to purge a liquid of absorbed gas. A useful accessory for inducing a superheated liquid to boil is a glass rod that carries at its bottom an inverted test tube three or four millimeters in diameter Bubbles that form at the open end when the tube is pushed below the surface initiate boiling.

The experimental vessel in which boules are made can be superheated by air from a short metal chimney enclosing a torus of heating wire. The temperature of the torus can be adjusted by means of a variable transformer. The application of concentrated heat at the center of the experimental vessel should be avoided. Heated air will rise by convection and warm the upper part of the vessel to prevent condensation if the unit is placed in a protective housing. The housing can be fitted with an access door and observation windows of any clear plastic that can be heated without damage to 100 degrees C.

Hickman washes the assembled apparatus with household detergent. He avoids the use of abrasives and brushes that might mar the polished glass surfaces. All parts are rinsed with concentrated hydrochloric acid, lightly rinsed with ammonia and finally rinsed for five minutes in running water. The apparatus is best used right away, but it can be dried and kept in a dust-free environment. This cleaning procedure minimizes the presence of particles and substances that could function as nuclei to initiate boiling.

Boules have been made of many liquids, including tap water. To set up an experiment, fill the experimental vessel to the level of the drain tube. Add about 200 milliliters to the retort. Apply power to the heating torus of the experimental vessel and to the mantle of the retort. The liquid in the freshly filled experimental vessel will doubtless boil spontaneously when its temperature reaches the boiling point. (Water normally boils at a temperature of 100 degrees C. under an atmospheric pressure of 760 torr, or 14.7 pounds per square inch.)

Let the liquid in the retort boil continuously, but gradually lower the temperature of the torus until boiling stops in the experimental vessel. Then raise the temperature again. The liquid becomes increasingly pure as the retort continues to operate. Ultimately liquid in the experimental vessel can be superheated by .15 degree C. without spontaneous boiling.

At this stage induce boiling by thrusting the inverted test tube below the surface. Lower the temperature until the induced boiling stops. This procedure purges the liquid of absorbed gas. Repeat it three times. Thereafter the surface will remain undisturbed at a temperature of .15 degree C. above the boiling point.

Drops of distillate that fall from the drip rod through a distance of not more than one centimeter will float and accelerate across the superheated surface. Many of them will merge with the supporting liquid after moving a few centimeters. Others will proceed to the edge, rebound from the meniscus and continue to accelerate. The boatlike film of vapor that carries the drops is thinnest at its forward edge, where a bow wave forms, and thickest astern, where a turbulent wake resembling the exhaust of a rocket marks the escape of vapor.

Lower the drip rod toward the surface until it is no more than three-fourths of the diameter of a drop above the surface. A drop will form, its bottom resting on the superheated liquid and its top anchored by surface tension to the rod. A metal wire must electrically interconnect the drop and the supporting liquid.

The drop will slowly expand into a constantly pulsing boule. The motion is caused by the radial flow of vapor from the center of the concave film below the boule. The vapor escapes at the circular interface between the boule and the supporting liquid.

The radial flow also imparts two internal motions to the liquid of the boule: a ring vortex and a downstreaming vortex or whirlpool that is encircled by the ring vortex. The whirlpool discloses its presence by a shallow depression that forms at the center of the upper surface of the boule [see illustration at right]. The ring vortex can be observed by adding a minute crystal of potassium permanganate to the condensate at the point where it flows from the drip rod. The complex flow of the supporting liquid can be investigated with dye of a contrasting color.

When a boule has grown to a critical size, it either merges with the supporting liquid or overcomes the restraining force of the rod, whereupon it breaks away, accelerates to the wall and merges with the supporting liquid. The critical size depends partly on the purity of the liquid but mainly on the degree of superheating. Another limit is fixed by the tendency of the boule to wobble around its point of suspension. The tendency can be decreased by attaching to the rod a ring (or concentric rings) of wire gauze. Boules that grow beyond the restraining force of the rod can be anchored by lowering the rod so that a meniscus forms between the bottom edge of the gauze and the upper surface of the boule.

The ultimate size to which a boule can be grown, according to Hickman, depends significantly on the patience of the experimenter Hickman has grown a boule of water to a volume of 65 milliliters (about two fluid ounces) and one of isopropyl alcohol to 250 milliliters (about eight ounces). The rate of growth increases significantly with the temperature of the superheated liquid. The velocity at which the liquids and the vapor move increases similarly. The resting surfaces begin to ripple when the temperature of the supporting liquid is increased above .15 degree C. When it is superheated to five degrees or more, violent wave motion usually destroys the boule.

Although boules appear to have no immediate practical application, they can function as a remarkably sensitive indicator of surface contamination. Hickman and his colleague Peter Harris have demonstrated that boules grow more readily and to larger sizes in a vessel of borosilicate glass that has been well leached through use than they do in an identical vessel of new glass. When liquid at the surface of both the old and the new vessel drains into the retort through a weir, boules form in both vessels. After the vessels have been equipped for subsurface drainage, however, the relative rate of production and the lifetime of the boules decrease significantly in the new vessel. The difference can be explained only by supposing the surface of liquid in the new vessel is contaminated by flotsam dissolved from the flux in the borosilicate glass.

FULLY as interesting as Hickman's boules are objects that consist of a sphere of water enclosed by a spherical film of air submerged in water. Last summer J. E. Connett of the department of mathematics at Northern Illinois University described them as follows in a letter.

"It is possible with a small amount of care and very little equipment to produce a 'bubble' in water that consists of a drop of water surrounded by a thin spherical shell of air. I have succeeded in making such bubbles up to about a half-inch in diameter. They appear to be only slightly less dense than the surrounding water, because they rise sluggishly toward the surface and float just below it. A friend who has observed some of my experiments calls them antibubbles. Often the antibubbles break below the surface, leaving a trail of tiny air bubbles.

"Here is a fairly workable method for producing antibubbles. Obtain a medicine dropper or a pipette with a fairly large nozzle. Add to a glass of tap water a few drops of liquid synthetic detergent (not soap). Liquid Lux and Swan appear to work best. Partly fill the dropper from the glass and then hold it vertically about one centimeter above the surface of the solution in the glass. Squeeze the rubber bulb to release liquid but do not squeeze too vigorously. Several trials may be necessary before the experimenter acquires the knack of forming antibubbles. Make sure there are no air bubbles in the tube of the medicine dropper.

"Usually small antibubbles break within half a minute. I have no complete explanation for these remarkable objects, although I think it is easier to see how they might persist after they have been created than how they are created in the first place. I would guess that the role of surface tension is quite different in the antibubble from what it is in an ordinary soap bubble floating in air. I suspect that the only effect of surface tension in the antibubble is to cause the enveloped water to assume a spherical shape.

"The thin film of air around the central liquid appears initially to have uniform thickness, but in time the film becomes thinnest at the bottom and thickest at the top. I suspect that if it were possible to create an antibubble in detergent-free water, the object would persist just as long as one that is made in detergent solution. I have no idea how long antibubbles might last under ideal conditions, how large they can be made or whether they can be made in other mediums. I should not be astonished to learn that they are a well-known phenomenon, perhaps a forgotten curiosity of the l9th century, although I have found no mention of them in standard references or in C. V. Boys's classic lectures on soap bubbles."

Recently Connett wrote again. "After poking around in our library," he said, "I learned that I am indeed not the first discoverer of antibubbles. A short letter by W. Hughes and A. R. Hughes, published in Nature in 1932, describes how they formed antibubbles with soap in a trough of rainwater. Their antibubbles measured only one millimeter to four millimeters (.04 to .16 inch) in diameter. They concluded that the drops were enclosed by a film of soap.

"During recent months I have succeeded in making antibubbles in cold water, salt solution, sugar solution at room temperature and flat beer. A few drops of liquid detergent must be added to each of these liquids. By means of a plastic squeeze bottle with a nozzle about three millimeters (.12 inch) in diameter I have made antibubbles with a diameter of more than an inch and a half. They are less stable than the smaller antibubbles but are far more striking in appearance.

"It is possible to keep an antibubble submerged by directing a jet of solution downward at it with the nozzle of the squeeze bottle. It is even possible with sufficient practice to inject a smaller antibubble into a previously formed large one. The experimenter learns by trial and error just how abruptly and forcefully to squeeze the bottle.

"As I have mentioned, antibubbles usually break soon after they rise to the surface. To prevent them from rising I added salt to the detergent solution from which the antibubbles were formed and injected the resulting antibubbles of higher density into a detergent solution of plain tap water. Much to my dismay the antibubbles broke when they sank to the bottom.

"After more experimentation I managed to surmount this problem by coating the bottom of the container with a layer of honey roughly one centimeter thick in which density increased with depth from about 1 to 1.4. To form the layer I poured a few ounces of honey slowly into the container of plain detergent solution. The honey in the slender entering stream spread across the bottom of the container and diffused upward within a few minutes to form the desired layer. The depth of the vessel was approximately 18 centimeters (seven inches).

"When antibubbles were made in this container of liquid, I observed an unexpected effect. The antibubbles fell rapidly to the layer of honey, bounced upward and (after several oscillations) came to rest just above the honey solution. Some broke spontaneously. Others that appeared to have about the same density as the surrounding solution hung for a time and then rose slowly to the top. Usually they made the trip to the surface in two or three minutes, but I observed one that required 7-1/2 minutes to ascend.

"A rising antibubble accelerates as it approaches the surface. Doubtless the cause is in part the diminishing hydrostatic head of pressure that allows the film of air to expand proportionately. I have repeated the experiment many times with a variety of solutions. The results are invariably the same: some antibubbles break spontaneously on the bottom and others become buoyant. The behavior does not seem to depend on the relative temperature of the antibubble and the solution.

"For a time I suspected that the change in density of the enclosed solution might arise from the diffusion of water molecules through the thin barrier of air-a sort of osmotic effect across a membrane of air. Molecules of water that evaporate from the solution might condense on the enveloped sphere until both solutions approach the same concentration. I discarded this hypothesis, however, when I observed that the gaseous film surrounding an antibubble' clearly absorbs air at a higher rate where the experiment is made with a solution of fresh tap water. The mysterious added buoyancy is contributed not by the dilution of the enveloped solution but by air released from the tap water that joins the thickening film of the antibubble.

"In recent experiments I have enjoyed the cooperation of Ewing Lusk, an enthusiastic investigator and gifted amateur photographer, who succeeded in photographing many antibubbles as well as the minute air bubbles that remain when an antibubble breaks. We concluded that an antibubble could survive indefinitely if a technique were devise for counteracting the effect of gravity. I wondered what would happen to a antibubble trapped at the center of a spinning vessel of solution. Lusk and I decided to try the experiment.

"We made an antibubble in a conventional container. Then we manipulated a solution-filled test tube over it, let the antibubble rise inside and corked the tube. Next the test tube was inserted into a snugly fitted Styrofoam holder, which was attached to the shaft of an electric food mixer. We turned the mixer on. The antibubble moved to the center of the test tube and stayed there.

"On our first attempt the antibubble lasted until the food mixer began to overheat. We had to shut the mixer off after about 15 minutes. The antibubble then drifted to the wall and prompt broke. An antibubble of the same size in an ordinary container would not last more than two or three minutes. We have repeated the experiment a number of times with the same result.

"The most frantic step in this experiment is confining the antibubble in the test tube. We made the antibubble in deep dishpan that contained a fresh detergent solution of tap water. The test tube was immersed and filled completely. One of us made the antibubbles. Th other manipulated the inverted tube vertically to a position where an antibubble would rise into it. The tube was promptly corked under water and inverted to prevent the antibubble from rising to the stopper and breaking.

"Next, we began madly twirling the tube with our fingers. With continue axial twirling and a vertical reversal o two we made the liquid spin at a rat sufficient to hold the bubble near the axis. We then quickly pushed the test tube into the Styrofoam holder and switched on the mixer. Our test tube was 15 centimeters (six inches) long and 2 centimeters (one inch) wide. The mixer turned at 400 revolutions per minute, but we believe a speed of 100 r.p.m. would have been enough to keep the antibubble from striking the wall. We think an antibubble that survives until the mixer is turned on will last as long as the machine runs.

"I determine the approximate thickness of the air film surrounding an antibubble with nothing more than a beaker of clear glass. The antibubble is made to rise into the inverted, water-filled beaker and to break. I estimate the diameter of the rising bubble; after it breaks I estimate the diameter of the resulting air bubble. Next I calculate the surface area of the antibubble, which is equal to 12.57 times the square of its radius, and the volume of the air bubble, which is equal to 4.19 times the cube of its radius. Finally I divide the volume of the air bubble by the surface area of the antibubble to find the approximate thickness of the film of air.

"The estimate may err on the high side because most antibubbles have a thickish bulb of air at the top. Even so the estimates seem reasonable. For example, the area of an antibubble one centimeter in diameter would be 12.57 x .5 x .5 = 3.14 centimeters. The approximate volume of an accompanying air bubble .152 centimeter in diameter would be 4.19 x .076 x .076 x.076 = .0018 centimeter. The thickness of the air film would be approximately .0018/ 3.14 = .0005 centimeter, which is equivalent to .0002 inch.

"Hickman mentioned in one of his articles a baffling phenomenon that has also puzzled Lusk and me, namely that it is sometimes inexplicably difficult to make floating globules and antibubbles. As Hickman put it: 'They are very capricious: they might refuse to form on a cold dry day or at the approach or retreat of a tall person or a short person.' One evening Lusk and I were trying without success to make antibubbles form in a jar in his living room. When we carried the jar into the kitchen, they formed easily. We repeated the experiment several times with the same result. Then we opened the window in the living room to get some fresh air. Thereafter antibubbles formed readily in the living room.

"Hickman reports that he has twice had samples of water and of carbon tetrachloride that for 10 days refused to form boules. Then, for reasons still unknown to him, they lost this property and never regained it. Experiences such as these convince me that anyone who takes up this hobby will encounter enough puzzles to keep him delightfully busy."

Bibliography

FLOATING DROPS AND LIQUID BOULES-A FURTHER LOOK. Kenneth Hickman, Jer Ru Maa, Andrew Davidhazy and Olivia Mady in Industrial & Engineering Chemistry, Vol. 59, No. 10, pages 18-41; October, 1967.

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