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Tears or Legs on the side of a wine glass containing wine

• You will often have observed the accumulation of a ring or film of clear liquid some distance up from the surface of a wine, contained in a wine glass (see diagram).
  
  From this ring, "tears", or liquid drops of the accumulated liquid will flow back down into the wine, forming arcs or arches and leaving streaks, "legs" or as the French say cuisses, down the side of the glass.
  
  
• Now if you try the same thing with a glass of water or alcohol only, this effect will not be observed.
  
  Similarly if you cover or seal off a glass of wine, the effect is also not observed.
  
  
• "Why is it so?"
  
  
• The "tears of wine" phenomena is often considered a indicator of alcoholic strength, glycerol concentration or quality. Sometimes the patterns are even interpreted as a fortuneteller interprets tea leave patterns in a tea cup.
  
  
• While the phenomenon is most pronounced in high alcohol wines, the affect different alcohol concentrations have on the appearance of tears and legs are not significant enough to be interpreted or provide a tool for quantitative analysis.
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  

It involves four (4) physical phenomena - surface tension, interfacial tension, evaporation and refractive index.


• Wine, is a water-alcohol mixture, and is drawn up the sides of a wine glass as a thin film due to interfacial tensions between the glass and the water alcohol mixture..
  
  
• As the film climbs up the glass, the opposing forces of gravity causes the film to thin out at the top, with a greater surface to volume ratio, higher up the glass.
  
  
• This increased surface exposure results in an increase in the evaporation of alcohol from the water-alcohol mixture with a local depletion in alcohol and an increased concentration of water.
  
  
• This local increase in water concentration results in a increase in surface tension (Surface tension for water @ 20⁰C is 72.8 mN/m and the surface tension of ethanol @ 20^oC is 22.3 mN/m)
  
  
• Chemical/physical system are always driven to their lowest possible energy state.
  
  
• This increased surface tension and higher energy state is alleviated by the transfer of more wine (mass transfer), containing a higher concentration of alcohol, to the alcohol depleted location at the top of the film.
  
  This reduces the surface tension and reduces the energy state.
  
  Note: Chemical/physical systems are always driven to their lowest energy state.
  
  This lifting of wine up the sides of the glass is a phenomenon is referred to as the "Gibbs-Marangoni affect".
  
  
• This lifting of wine continues until the the forces driving the wine up are outweighed by the forces of gravity pulling the wine down.
  
  
• At this stage the watery wine depleted in alcohol at the top minimises its surface tension by beading until it becomes to heavy and gravity wins as witnessed by the flow of "tears" from the visible ring back down to the wine forming streaks or "legs" as it does so.
  
  
• That the ring, "tears" and "legs" are clearly visible is due to the increased volume of the liquid, but more importantly the lower refractive index of the alcohol depleted wine contrasting with the wine film high in alcohol, glass and air (refractive index (RI) @ 20^oC of water = 1.33, 12-14% w/v ethanol = 1.34, ethanol = 1.36, glass = 1.5, air = 1.0003, vacuum -=1.0)
  
  
• A glass of pure water will see some water drawn up the sides of the glass because of the interfacial tensions, but only to a limited degree, forming a meniscus.
  
  
• Similarly, a meniscus will form with a glass of pure ethanol.
  
  However, the effect observed in a alcohol-water mixture (wine), described above, will not be observed.