The Marangoni effect (sometimes called “tears of wine” ) occurs when drops of a relatively strong alcohol (ethanol) solution on a wine glass slowly begin to crawl up the side of the glass due to the differential evaporation of the ethanol.
Explain this effect from a surface chemical perspective. Would this effect still occur if a less well-made glass were used such that it was significantly rougher than the original glass? What about if a smooth plastic “glass” were used?
The link below has a video of this formation:
http://rapidshare.com/files/82604143/spherical_glass_10ml_300_times.mpg
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 force 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 @ 20°C is 72.8 mJ/m2 and the surface tension of ethanol @ 20°C is 22.3 mJ/m2) 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, 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. This lifting of wine continues until 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 too heavy and gravity wins as witnessed by the flow of "tears" from the visible ring back down to the wine as it does so. 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.
If a less well-made glass were used such that it was significantly rougher than the original glass, it increases the surface tension, increases the wetting ability.