What Do "Legs" in a Wine Mean?

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  April 3, 2012

  George Vierra

  Legs in a
  Glass of Wine

   

   

  “Legs” in a glass of wine or
  spirits have been noted since millennia. As noted in Proverbs 23:31, “Look not
  thou upon the wine when it is red, when it giveth his colour in the cup, when
  it moveth aright.” Descriptions since Biblical times have changed. We now may
  state, “Wow, just look at those legs!” 
  The language has changed, but the chemistry and physics have remained
  the same.

   

  Wine and spirits are basically
  mixtures of water and ethanol (ethyl alcohol). As individual compounds, they
  have two different physical distinctions. Alcohol has a lower boiling point
  than water and hence, evaporates faster than water. Water has a higher surface
  tension than ethanol.

   

  The attractive forces between
  molecules in a liquid are called surface tension. These forces hold the liquid
  together. The same type a force acts between molecules of a liquid and those of
  a solid surface. This force is called ‘interfacial tension’. If the interfacial
  tension between a wine and a glass is a bit greater than the wine surface
  tension, then this causes wine (or spirits) to climb the inside walls of a
  glass. A point is reached at which the weight of the wine clinging to the glass
  just balances the force trying to lift more. A pure liquid would arrive at a
  steady state and a specific film height on the glass would be maintained.  Wine is not a pure liquid. It’s a water
  alcohol solution. On the wine film, alcohol evaporates faster than the water.
  Once the inside of the glass is covered with a thin film, the wine film loses
  some of its ethanol by evaporation. With the concentration of water increasing,
  the film surface tension increases, as does the index of refraction. In the
  areas where alcohol evaporates, the watery-wine left behind assumes a drop-like
  form. The drops become heavier and the force of gravity becomes controlling and
  the drops slides down the glass wall to the wine in the bowl. These legs can be
  seen because the change in the refractive index makes the boundary between the
  watery legs and the more alcoholic film visible. The channels of falling wine
  appear as “legs”. Since this “surface tension engine” is driven by the ethanol
  evaporation in the film, the higher the alcohol, then the greater the legs.
  Glasses of pure water or alcohol show no legs. Different evaporation rates are
  necessary. Place two glasses of the same wine side by side. Notice the legs in
  both. Put a lid on one glass. In the lidded glass, evaporation ceases and legs
  stop forming.

   

  The elder brother of Lord Kelvin,
  James Thompson, published a paper in 1855 in Philosophical Magazine,
  titled “On certain curious Motions observable at the Surface of Wine and other
  Alcoholic Liquors”. Thompson, therein, described the effect caused by
  capillarity, or surface tension. Thompson described them as “tears of a strong
  wine”.

   

  “The phenomenon stems from the
  dipole-dipole intermolecular forces in aqueous solutions. The combination of
  the cohesive forces within the liquid and the adhesive forces between the
  liquid and solid surface can explain, among other things, surface tension and
  capillary action. In solution, the ethanol and water have cohesive forces
  weaker than that of the molecules of pure water. The adhesive forces toward
  glass surfaces are about the same as those of water. These adhesive forces are
  stronger than these cohesive ones. This causes wine to adhere to and climb the
  wall of the glass. As the ethanol evaporates, the cohesive force increases
  until the wine falls in a thin stream. Upon reentering the surface of the wine,
  the ethanol concentration is restored, the cohesive forces weaken and again the
  wine climbs the walls of the glass.”

   

  Many wine writers have attributed “legs”
  to the glycerin in wine. There is no glycerin in wine. There is glycerol, an
  alcohol. It is in minuscule percentages in wine and may add a touch of
  sweetness. It has a boiling point of 554º F and does not evaporate like ethanol to
  create this struggle of forces.

   

   

  George Vierra                                                                                                                                                        3
  April 2005

  Viticulture & Winery Technology

  Napa Valley College

  Napa, CA USA