Friday, 11 March 2011
Your Wine's Got No Legs
Tonight you may to go home after a long day of work. You might stop off at the Alko (in Finland) or another store to buy wine. You might select a nice bottle of Valpollicello or something off the shelf that you select to be good. A decent bottle that is not flabby, volatile, stale or diffuse should cost you at least 10 euros here in Helsinki (in other places in the world this might differ. You get home put your feet up, pull out a clean large wine glass.
If you are a wine enthusiast or just pretentious you might swirl the glass of wine, raise it towards the light and watch with bated breath for the wine's legs to appear. These legs are also referred to by the French as tears, curtains and perhaps church windows. If they appear you will say to yourself and think back on the price, 'What a great bottle of wine I selected, great job. I have to go watch the movie Sideways again to learn more wine terminology.' So I asked myself as a scientist, a wine enthusiast (who has been to his fair share of wine tastings, vineyards and whatnot). What are these legs? And how can you tell the wine's quality by the legs they have.
Wine is a inhomogeneous mixture of alcohol and water, the alcohol has a faster evaporation rate and a lower surface tension than water, effectively forcing the alcohol to evaporate at a faster rate. This dynamic allows the water's surface tension and concentration to increase, pushing the legs up the smooth surface of the glass until the surface tension pushes the water into beads. Finally, gravity wins the battle and forces the liquid to tear down the glass in a defeated streak. The surface tension gradient can be caused by concentration gradient or by a temperature gradient (surface tension is a function of temperature) so maybe if you chill the wine you might not see as many tears. So as I mentioned earlier someone actually has studied this and they were someone who probably drank a lot of wine (why just throw your experiment down the drain when you can put it down the hatch). He called it the Marangoni effect (also called the Gibbs-Marangoni effect) is the mass transfer along an interface between two fluids due to surface tension gradient. And in the case of temperature dependence, this phenomenon may be called thermo-capillary convection (or Bénard-Marangoni convection).
So the tears are a mythical indicator of wine quality. And now if you would be pretentious if you said, 'This indicates the wines quality.' However these legs tells quite a bit about the wine's power and the wine's body (and perhaps if you have a clean glass). If the wine has a lot of alcohol or a lot of sugar it will have longer tears at the right temperature. If you are still not convinced that it's physics and not quality that drives this phenomenon? Try covering your next glass of wine and see if the legs present dramatically decrease when covered compared to when open. No evaporation, no legs. Enjoy none the less.