Wine lovers have all seen the thin film of wine drops around the side of the glass. These “tears” or “legs” of wine are actually falling down the side of the glass and reforming and re-falling again. What the heck explains this?
The Marangoni effect explains this odd phenomenon. Liquids are always “pulling” on each other. A liquid with a high surface tension pulls more strongly than a liquid with lower surface tension. Water has a higher surface tension than alcohol. After swirling some wine around the side of a glass, the wine at the top of the wine “film” actually has higher surface tension than the wine sitting in the glass. This is because the alcohol has evaporated at the top and there is less alcohol at the very top film of wine. This difference in surface tension will pull wine up the side of the glass. Eventually, enough wine is pulled up the side of the glass that gravity pulls down the heavy drops of wine.