In the Classroom edited by
JCE DigiDemos: Tested Demonstrations
Ed Vitz Kutztown University Kutztown, PA 19530
Tears of Wine submitted by:
Marcos Gugliotti Instituto de Química, Universidade de São Paulo, Brazil;
[email protected] checked by:
Todd Silverstein Department of Chemistry, Willamette University, Salem, OR 97301-3922
In his article entitled “Why Do Alcoholic Beverages Have ‘Legs’?” (1), Silverstein describes an interesting phenomenon most commonly known as “tears of wine” (2–4), which is related to the formation and flow of wine drops (and also other spirits) on the internal walls of a glass. This phenomenon has been observed since the early history of mankind, and its first description appears in the Bible (5).1 Probably because of that, there are myths associated with the formation of the “tears”, but a clear explanation can be given with the knowledge of some chemical properties of the liquids. In the earlier article (1), it was stated that capillary action and mainly cohesive and adhesive forces are the most important phenomena for the formation of a thin film of wine on the walls of the glass, where the drops are formed. The explanation was based on the fact that ethanol spreads much more than water on polar solid surfaces (such as glass), because adhesive forces are stronger than cohesive forces for ethanol when compared to water, which is correct (6). However, adhesion cannot drive a liquid film up the glass surface. For example, for the same capillary tube, the stronger adhesive forces between ethanol and the glass tube are not enough to make this liquid rise to a higher level than pure water. Thus forces beyond cohesive and adhesive forces must propel a liquid film of wine up the walls of a glass. Complete Explanation: Surface Tension Gradient As pointed out by Adamson (7), “Contact angle behavior in liquid mixtures is more complicated when one of the components is volatile such as found in wine tears… Since water has a much higher surface tension than alcohol, evaporation of alcohol produces a surface tension gradient driving a thin film up along the side of a wine glass, where the liquid accumulates and forms drops or tears.” Thus, a more complete explanation for the tears of wine effect must take into account the surface tension gradient generated by evaporation of alcohol from the meniscus (2–4). A surface tension gradient is formed whenever there is a local change in the surface tension of a liquid. The liquid moves toward the region of higher surface tension (8), because this portion of the liquid tends to contract, and drains liquid from the region of lower surface tension. This effect can easily be visualized from other simple experiments (9). Since water has a much higher surface tension than ethanol (72.75 and 22.55 mN兾m at 20 ⬚C, respectively), a mixture of these liquids, such as wine, has a lower surface tension than that of pure water. When alcohol evaporates2 from the thinner region of the meniscus, a surface tension gradient is www.JCE.DivCHED.org
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generated, and the liquid climbs the glass walls spontaneously, forming a film. The gradient is maintained by continuous evaporation of alcohol from the film, and more liquid is dragged up. This phenomenon is called the “solutal Marangoni effect” (10) and can be observed not only in water and alcohol mixtures, but also in any mixture where the more volatile component has the lower surface tension. However, after continuous evaporation of alcohol, the relative quantity of water in the film increases, and because of its high surface tension, the liquid now tends to accumulate in the form of drops. When the drops become heavy enough, they roll back down the glass resembling tears—that is why this is usually called tears of wine. For a better understanding of the phenomenon, Figure 1 presents a picture of the tears (Figure 1A) and an schematic description of the sequence of events (Figure 1B). Despite the fact that surface tension is a cohesive phenomenon, in the present case the concepts of cohesion and adhesion can only be applied to explain the well-known formation of the meniscus and the tendency of the film to accumulate in the form of drops. We could also say that since ethanol has evaporated, strong cohesive forces between the water molecules remaining in the film will lead to the formation of drops. However, we should remember that, in the case of tears of wine and other related phenomena, surface tension gradients and not adhesive or cohesive forces are the driving force for the motion of the liquid. James Thomson, in 1855 (11), was the first person to arrive at this conclusion but for some unknown reason the phenomenon remains associated with the name of the Italian physicist Carlo Marangoni of Paiva and Florence (1840–1925), after his work in 1871 (12). The tears of wine constitute just one example of the socalled Marangoni effects (13) in which the surface tension gradient is generated by a local change in composition. However, surface tension gradients can also be generated by a local change in temperature, and in this case the resultant motion of liquid is usually called Marangoni convection or thermocapillary flow (14).3 All these types of surface tension driven flows can influence a variety of processes and phenomena in many areas (13), such as crystal growth, motion of protoplasm, cellular mobility and transport of bacteria, soldering, printing, coating, adsorption, absorption, distillation, foam stability, and probably others. Simple variations of the experiment presented here, just like that described by Ahmad (15), can be done using a watch glass on a projector to increase the visibility to a large audience. For example, a 1 mL of a mixture of water and ethanol
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In the Classroom A
B
Figure 1. (A) A glass was filled with a mixture of tap water and commercial ethanol (60:40, v/v) using a Pasteur pipet (or a funnel) to avoid wetting the internal walls. A colorant (crystal violet) was added as a tracer. The glass was left exposed to the air without agitation and “tears” were formed within minutes a few millimeters above the level of the meniscus. Note that the walls of the glass were not wetted previously; (B) Evaporation of ethanol changes the composition of the mixture, increasing the surface tension locally at the meniscus. A surface tension gradient is created, driving the film up the walls and draining more liquid up. A mass of liquid accumulates on the glass walls, forming drops that roll back down the glass by the action of gravity. (Other alcoholic spirits, such as vodka and absinthe, show the same behavior, provided the amount of alcohol is enough for the formation of a surface tension gradient able to drain the film up.)
(50:50, v兾v) colored with blue food coloring was placed on a 7-cm watch glass making a spot of ∼3 cm in diameter. After giving the watch glass a brief swirl, it was possible to observe the continuous formation and flow of drops. Finally the example of the tears is a useful tool to visualize the motion of liquids by surface tension. It introduces the concept of the surface tension gradient, an important concept that is often ignored in introductory chemistry courses. Hazard There are no significant hazards associated with this experiment. Acknowledgments Special thanks to Guilherme A. Marson for digitizing the photograph, to Mario J. Politi and Mauricio S. Baptista for providing the materials, and to the Brazilian Financial Agency FAPESP for the support. The author is a postdoctoral student from the Institute of Chemistry, University of São Paulo, Brazil. Notes 1. The text that appears in the Bible is as follows, “Don’t look at the wine when it is red, when it sparkles in the cup, when it goes down smoothly.” 2. Evaporation of alcohol is necessary for the formation of the film and consequently of the tears. If we cover the glass, the air remaining inside the glass becomes saturated with alcohol, evaporation stops, the film does not climb the walls, and no drops are formed.
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3. Since evaporation of alcohol changes the temperature of the surface of the liquid, thermocapillary flow may also have some influence on the motion of liquid, which increases the complexity of the phenomenon.
Literature Cited 1. Silverstein, T. P. J. Chem. Educ. 1998, 75, 723–724. 2. Walker, J. Sci. Am. 1983, 248, 162–170. 3. Fournier, J. B.; Cazabat, A. M. Europhy. Lett. 1992, 20, 517– 522. 4. Vuilleumier, R.; Ego, V.; Neltner, L.; Cazabat, A. M. Langmuir 1995, 11, 4117–4121. 5. Prov. 23:31 World English Version. http://theonlinebible.com/ bibles/english-web/index.html (accessed Oct 2003). 6. Gesser, D. H. J. Chem. Educ. 2000, 77, 58–59. 7. Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th ed.; John Wiley and Sons: New York, 1997; p 371. 8. Bikerman, J. J. Surface Chemistry: Theory and Applications, 2nd ed.; Academic Press: New York, 1958; p 89. 9. Jasien, P. G.; Barnett, G.; Speckhard, D. J. Chem. Educ. 1993, 70, 251–252. 10. Fanton, X.; Cazabat, A. M. Langmuir 1998, 14, 2554– 2561. 11. Thomson, J. Philosofical Magazine 1855, 10, 330–333. 12. Marangoni, C. Ann. Phys. Chem. 1871, 143, 337–355. 13. Scriven, L. E.; Sterling, C. V. Nature 1960, 187, 186–188. 14. Levich, V. G. Physicochemical Hydrodynamics; Prentice-Hall International Series in the Physical and Chemical Engineering Sciences: Englewood Cliffs, NJ, 1962; pp 384–390. 15. Ahmad, J. J. Chem. Educ. 2000, 77, 1182–1183.
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