WHEN WINE IS LEFT STANDING in a glass for a few minutes, a curious array of drops is likely to appear on the inside of the glass above the surface of the liquid, particularly if the wine has a fairly high alcoholic content. The phenomenon is known as "tears of strong wine." It was examined as early as 1855 by James Thomson, a British engineer and physicist, who concluded that it did not arise from the condensation of water on a cool surface, since the drops appeared even when the glass was at room temperature. He thought the drops formed as a result of variations in the surface tension of the wine as alcohol evaporated from a thin film of wine on the wall of the glass.

The formation of the tears has since figured in the general stud circulation patterns in liquids varying' surface tension. Although such mots have been studied for more than a century, the physical events are still not understood in detail. The motions are interesting to a physicist because they serve as a tool in the study of surface tension and the stability of circulation patterns in liquids. They are also important because they can be found in many industrial and biological processes.

Although Thomson's explanation for the tears of strong wine was basically correct, his work was apparently ignored until 1869, when the entire subject of motion resulting from surface tension was reviewed by Gustav L. van der Mensbrugghe in a Belgian journal. The priority of the research was challenged two years later in Italy by Carlo Marangoni. Thomson's early work was virtually forgotten, and Marangoni's name is now attached to liquid motions driven by surface tension.

Thomson's explanation was a simple one based on the observation that when alcohol is added to water, the surface tension of the water decreases. Hence wine has less surface tension than pure water. If the wine is exposed to air, the alcohol in it continuously evaporates, creating areas of higher tension on the surface. Such an area pulls on the adjoining liquid and starts it in motion. The more visible formation of tears on the wall of the wineglass is due to the same kind of variation in surface tension. The surface of the liquid is curved at the wall because the surface tension at the interface of the wine and the glass pulls the wine up the wall a little way.

Alcohol evaporates from this film, increasing the surface tension of the liquid there and causing more wine to be pulled up. Thomson noticed that the additional liquid tends to form a thick ring at the top of the film. As the alcohol evaporates and the surface tension in the ring increases, the liquid begins to contract into small drops. More liquid is pulled up. More alcohol evaporates. The drops get larger. Finally a drop gets heavy enough to slide back down into the wine. Soon another drop forms in its place because evaporation continues in the film and fresh liquid is constantly pulled up. The process continues until so much alcohol has been removed that the variation in surface tension gets too small to keep the cycle going.

Thomson tested the role of evaporation by corking a partly filled vial of wine. After shaking the mixture he examined the film on the interior wall. No tears of wine formed and no motion was apparent in the film except normal drainage. Then he removed the cork, so that fresh air entered the vial. Under these conditions, he reported, "a liquid film is instantly to be seen creeping up the interior of the vial with thick or viscid-looking pendent streams descending from it like a fringe from a curtain." When the cork was in place, shaking so saturated the air in the bottle with alcohol that no more evaporation was possible. Fresh air restored both evaporation and the cycle whereby liquid was driven into motion by the variation in surface tension.

I demonstrated the formation of tears with several kinds of alcoholic beverage. First I poured a small amount o 94.6-proof gin into a watch glass. (Th concentration of alcohol by volume is half the proof number.) Four tear formed almost immediately near the rim of the glass. The region between a tear and the rest of the gin was visibly wet and occasionally showed motion. I doubt that I saw gin flowing up the sides of the watch glass, since gin is transparent, it is more likely that I was seeing dust motes caught up in the flow. To improve the visibility I dusted the surface of the gin with lycopodium powder. The microscopic spores of the powder rest on top of the gin instead of sinking. Talc and other household powders that are not readily wetted by water and alcohol can serve in place of lycopodium.

With the powder in place as a tracer I could follow much of the flow of gin up the side of the glass. In general the motion

was irregular, but below a spot where a tear formed the flow was mainly upward. (The irregular flow elsewhere results because the film is evaporating and thus varying in surface tension from place to place.)

A tear would eventually slide abruptly down into the gin or would descend gradually until it touched it. In the latter case the drop jerked a little way back up the wall after touching the gin. The reason is mainly that the drop loses liquid when it touches the surface. It is then lighter and more responsive to the surface tension pulling upward than to its weight pulling downward. The jerk is also promoted by the sudden difference in surface tension at the interface of the drop and the gin.

A drop touching the gin injected a small jet of liquid into the gin. The jet is visible because it has an index of refraction different from that of the rest of the gin and therefore bends the light rays crossing through it. The difference in refractive index probably results from two factors: the jet has both more alcohol and greater density than the gin around it. The jet is denser because the liquid has been cooled by the evaporation that causes tears to form.

I next poured about a cup of 80-proof rum into a porcelain bowl and added a small amount of chocolate flavoring at one spot near the edge of the liquid. Beautiful purple patterns raced through the rum. The colored lines were quite lively, apparently driven by strong variations in the surface tension at the top of the mixture and by circulation systems within it.

I put the bowl in a warm oven (its temperature somewhat below 93 degrees Celsius, or 200 degrees Fahrenheit). Tears soon formed on the inside surface of the bowl. With a flashlight and a small magnifying glass I examined the edge of the rum. Small, undissolved particles left from the flavoring served to trace the liquid motion. The particles darted toward and away from the edge, revealing a vigorous and complex circulation of liquid there.

Food coloring turned out to be a better tracer. A small drop added to isopropyl alcohol in a porcelain cup colored one section of the perimeter. I watched as a small colored section of the alcohol reached the perimeter of the cup and began climbing through the invisibly thin film of alcohol and water lining the wall, finally entering a tear that had already formed on the wall.

I next made a simple but crucial test of Thomson's hypothesis that the drops appear only because of a difference in the surface tension in the film on the wall. Suppose the alcohol contains no water. Then the surface tension would hardly vary as the alcohol evaporated on or near the wall. Indeed, it would not vary at all except for the changes caused by the fact that the liquid cools as it evaporates.

Working with a pair of identical cups, I poured a quantity of 70 percent isopropyl alcohol into one cup. In the other I poured half as much alcohol and then added enough water to bring the surface level up to that of the first cup. I gave both cups a brief swirl to wet the sides. Many tears formed quickly above the diluted sample. Only a few small tears appeared above the concentrated sample. Thomson was right: water is necessary for a variation in surface tension.

Any factor that promotes evaporation also aids the formation of tears. A warm environment, direct sunlight and a wide, shallow container help. I made a large-scale demonstration of tear formation employing a serving platter with sides that curved upward. A tall glass with a small amount of liquid works poorly because the evaporation is slow, unless you first wet the entire inside wall. Then when the alcohol is poured into the glass, the liquid film rapidly climbs the wall, rising about as high as the remaining wetness. The appearance of tears several centimeters above the alcohol mixture seems almost magical. A watcher might say the tears are actually drops of condensation. To prove their origin you can color the liquid at the bottom of the glass. The tears will be colored too.

You might like to investigate the formation of tears with various alcoholic drinks. Beers made in the U.S. do not work, at least in my experiments, apparently because they contain too little alcohol. Can you find liquids other than alcohol and water that form tears?

As I was investigating the tears of A strong wine I was writing the article about Middle Eastern coffee that appeared in this

column last month. The drink consists of water, sugar and finely ground coffee grains, all brewed together and served in a cup along with the grains. I prepared so much of the stuff that I began to pour it into bowls instead of drinking it. When I was cleaning up one morning, I discovered a pattern in a bowl of coffee left from the previous night. Along the edge of the liquid, just below the surface, lay a neat array of dark lanes of coffee silt. They and the intervening clear lanes were each a few millimeters wide.

I was stumped by the pattern. Not one of the hundreds of unfinished cups of coffee I had left lying around had ever shown it. Is Middle Eastern coffee somehow special? I prepared another batch and left it in the same kind of porcelain bowl. I saw nothing interesting in the period of rapid evaporation from the hot surface, but in rechecking over the next several hours I began to notice the same pattern developing. Although I sat patiently with a flashlight and a magnifying glass, I could not actually see the lanes growing. Since I had tears of strong wine on my mind, I wondered if the patterns could be related. No, the liquid showed little movement up the side of the bowl.

Perhaps this pattern has been reported before, but I can find no mention of it. A silt such as the residue in Middle Eastern coffee is required, as is sugar. I prepared two bowls of the coffee, one bowl with sugar and one without. The next morning the one with sugar had a fine pattern and the other did not.

Evaporation is also needed. I put a bowl of the coffee in a warm oven and quickly got the pattern. Erasing it with a gentle swirl, I covered the bowl with plastic food wrap. After a while I shone a flashlight through the plastic. The pattern was there. I gave the bowl another swirl and waited again. The pattern would not re-form.

Apparently the pattern developed the first time because the water was able to evaporate into the air above the coffee. Thereafter the saturation of the air with water vapor prevented further evaporation. The high concentration of water vapor trapped on the inside of the plastic wrap was demonstrated when I opened the oven door for a look; the cool air from the kitchen caused condensation on the inside of the plastic.

I then set about investigating the formation of the pattern by preparing many samples of Middle Eastern coffee and

pouring them into a wide beaker and an assortment of watch glasses. The vertical wall of the beaker enabled me to examine the sedimentation of the grains in the coffee with the aid of a strong flashlight. I saw nothing interesting in the beaker and expected nothing but the usual pattern in the watch glasses. Surprisingly, every batch in the watch glasses failed to form a pattern. I substituted plastic lids shaped like the watch glasses but still could not produce the patterns. I was stymied.

Back with the porcelain bowls, I began to search for circulation patterns near the edge of the liquid by adding some kind of tracer. In one trial I injected a small amount of food coloring by inserting the tip of a hypodermic syringe just below the coffee level. In another trial I dusted the top surface with lycopodium powder. I could not discern any circulation, but when I left the bowl at room temperature overnight, the pattern appeared.

I tried for similar patterns with other mixtures. Neither tea leaves nor grains of sand produced a pattern in sugar water. Apparently the pattern requires fine silt Lying on the bottom; tea leaves and sand are too big.

With a normal mixture of Middle Eastern coffee in a bowl I scraped the silt at one side of the bowl and then waited for the pattern to form. It did form but not in the area I had cleared. This result gave me a useful clue to the nature of the pattern. Apparently the silt had to be near the edge of the liquid. Whatever causes the pattern does not appreciably transport the silt up the wall of the bowl and toward the liquid's edge.

I made a mixture of Middle Eastern coffee and gin with a small amount of red food coloring as a tracer. The alcohol was poured in slowly after the coffee had cooled for a while. Soon tears formed on the upper reaches of the wall but the coffee pattern failed to appear.

From this test, however, I received my next clue to the cause of the pattern. Whenever a tear slid down the wall and into the drink, it cleared a lane through the sediment just below the spot where it entered the liquid. In this way the drops soon produced a pattern resembling the patterns I had seen only with the coffee The spacing in the pattern was about 10 times too large, and the pattern varied a lot because the points where the tears entered the liquid shifted.

Perhaps a circulation system at the shallow edge of normal Middle Eastern coffee generates the pattern in the silt. Two mechanisms can drive such a system in a naturally evaporating liquid. (Natural evaporation takes place without an additional source of heat, such as a stove, to supply the energy for vaporization.) The liquid might move because of the variation in surface tension over its surface. It might also move because the liquid in an upper level becomes denser than the liquid under it. A circulation system is created when the conditions altering surface tension and density recur as fresh liquid is brought in.

For example, circulation cells can be seen in a cup of hot coffee. Hot liquid from the bottom rises to the top, where it cools by evaporation as it flows a short distance over the surface. Then it sinks in narrow, crooked lines to the bottom. The cycle continues until the coffee gets too cool to sustain it. The cells in hot coffee are randomly shaped and constantly changing. In some circumstances and with certain liquids they have ordered shapes and can be quite beautiful.

Natural evaporation in liquids drives circulation cells because the evaporative cooling increases the density and surface tension of the liquid. Either the increased surface tension pulls in fresh liquid or the denser liquid sinks and is replaced by fresh liquid. It is usually difficult to determine which of these mechanisms is primarily responsible for a circulation system.

According to research published a number of years ago by J. C. Berg and Michel Boudart of the University of California at Berkeley and Andreas Acrivos of Stanford University, neither mechanism operates unless the liquid is deep enough. For water the required depth is one centimeter. Shallower water displays no circulation systems; deeper water has the large circulation cells that can be seen in hot coffee.

Several other liquids investigated by Berg, Boudart and Acrivos had circulation systems even when they were quite

shallow. The investigators monitored each liquid as the depth was increased. In all liquids except water circulation systems first appear when the depth of the liquid is about two millimeters. The systems are "two-dimensional, worm-like roll cells." As the depth is increased to one centimeter the cells increase in width and the lines of descending liquid become more distinct. At a depth of one centimeter the circulation cells resemble those in hot coffee. The regions where the liquid rises to the surface are then indistinct.

Why is water so different? Trace contaminants from the air, the container or even the experimenter spread a monolayer (a layer one molecule thick) over the surface, preventing any circulation generated by variations in surface tension. Then only circulation generated by variations in density is possible. Apparently a monolayer of contamination can stabilize water shallower than one centimeter.

This finding disappointed me. It implied there should be no circulation system near the shallow edge of evaporating Middle Eastern coffee. Yet the coffee silt revealed a pattern regular enough to have been made by hand. The coffee was certainly not pure water. It contained a lot of sugar and oil, and it was exposed to the air for hours. The top surface was undoubtedly coated with contamination at least one molecule thick. In some places I could even see tiny pools of oil.

Ultimately I succeeded in putting all the clues together. The pattern requires evaporation, but because of contamination it cannot arise solely from a variation in surface tension. The pattern appears in the fine silt just below the edge of the liquid. If the silt is too far below the edge, no pattern develops. It forms only on a surface of moderate slope, not on the vertical wall of a beaker and not on the shallow slope of a watch glass.

Contamination over the surface of the coffee surely interferes with the natural evaporation of the water from the bowl but may not reduce the evaporation at the edge as much. When some of the water evaporates from an area of liquid at the edge, the remaining liquid is made denser because the concentration of sugar and oil is then higher. This section sinks. As it descends it sweeps out a lane in the silt. Other liquid flows in to replace it. Since the circulation depends on evaporation, it is normally quite slow. Part of the flow can be toward the edge along a lane of silt. If the surface is not too well stabilized by contamination, part of the flow can be along the surface.

As evaporation continues more of the clear lanes are swept clean of silt. Some of the intervening lanes of silt may be extended toward the edge by the gentle circulation system, but the diffuse flow does not carry the silt far. The pattern is enhanced by the sugar, which makes the section of liquid left by the evaporation of water near the edge denser. The pattern appears only on moderate slopes. If the slope is too steep, the circulation has no chance to sweep out lanes of silt If the slope is too gentle, the liquid layer near the edge is too shallow for circulation. I suppose the layer of contamination is then too close to the layer of silt to allow any stabilized flow. The descent of the dense liquid from the edge may also be too gradual for the formation of a pattern.

I did one final experiment, mixing a teaspoon of milk into my usual Middle Eastern coffee. The oil in the drink and in the milk made a film over the surface. As a pattern developed in the silt along the edge, this film formed into a similar pattern. Above the silt lanes was the film, extending to the liquid edge. Above the clear lanes the surface had no film. This arrangement seems to fit exactly the circulation system I had visualized at the edge of the evaporating coffee.

Another observation may support this analysis. I had made instant coffee with milk (the drink is often called white coffee) and had not finished it. The next morning a pattern of white lines streaked the liquid surface. Another pattern of distinct radial lines appeared in the liquid left in the spoon with which I had stirred the coffee. The depth of the liquid did not seem to matter. It was about a centimeter in the cup but only a few millimeters in the spoon.

To investigate these patterns I made another preparation of white coffee and poured the mixture into several watch glasses and one metal spoon. I dusted one watch glass with lycopodium powder. Off and on for about 10 hours I checked the samples, particularly near the edge, where I employed a strong flashlight and a magnifying glass. Large patterns of white lines appeared after several hours of evaporation, but I could detect no consistent motion of the lycopodium powder. In one watch glass the pattern resembled the veins in a leaf. In another it consisted of parallel lines across the entire surface. One pattern had what looked like an upswelling because the white lines extended outward from a single point. If I gently disturbed one of these patterns with the tip of a spoon, it usually reestablished itself within an hour or so.

Near the edge of the liquid in a watch glass fine-scale patterns appeared. The hair-thin lines of dried milk were roughly perpendicular to the edge. In the spoon the shallow coffee developed the same kind of fine-scale patterns along the edge of the liquid until evaporation had reduced the depth to less than a millimeter. Sometimes the radial lines in the spoon became pronounced and more widely spaced. The coffee itself was apparently not necessary, but it did facilitate my observations. Thin layers of milk evaporating from a watch glass displayed the same kind of fine-scale patterns along the liquid edge.

I believe the two types of patterns reveal the circulation systems in the evaporating liquid. The larger patterns in coffee cups resemble the worm-like cells. As the liquid flows across the surface and then descends, milk particles collect over the lines of descent. Hardening there, they mark the paths of descent of the coffee. In addition they may help to stabilize the location of the circulation cells, otherwise the cells would be no more stable than those in hot coffee. The cells also appear to be coupled to the gentle circulation of air over the liquid surface. When a bowl of coffee is exposed to the cool air moving downward from a cold window, the same milk patterns develop.

The fine-scale patterns along the edge resemble the patterns in coffee silt and may be due to the same circulation system. Once these milk lines are established, they might stabilize the circulation along the edge. As the water continues to evaporate from the liquid, the white lines harden on the container. When white coffee has fully evaporated from a watch glass, I can hold the glass in sunlight and see a beautiful radial pattern of lines.

Bibliography

THE MARANGONI EFFECTS. L. E. Scriven and C. V. Sternling in Nature, Vol. 187, No. 4733, pages 186-188; July 16, 1960.

INTERFACIAL PHENOMENA. J. T. Davies and E. K. Rideal. Academic Press, 1963.

NATURAL CONVECTION IN POOLS OF EVAPORATING LIQUIDS. J. C. Berg, M. Boudart and Andreas Acrivos in Journal of Fluid Mechanics, Vol. 14, pages 721-735; 1966.

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