The Tears of Wine are the most famous manifestation of the Marangoni Effect. This is a physical phenomenon that was first discovered in the 1800s and is still being investigated by space agencies. Why? I'm guessing because there is wine involved.
Images by John W.M. Bush/MIT, via RedOrbit.
Chances are that you've had wine at some point in your life. And chances are that you've swirled the wine around in your glass either because you're pretending to be, or because you actually are, a wine snob. (If you're pretending to be — good for you! You showed them! If you actually are, I doubt I'm the first to break it to you, but all the rest of us hate you.) After you completely the swirl, you probably noted, through your wine haze, that there was a little wine residue left on the glass. It starts off as a neat layer all around the glass, but soon it seems to thicken at the top and condense into droplets that eventually trickle down the glass. Those are the Tears of Wine.
Liquids have a natural tendency for their molecules to cling together. This clinging, called cohesion, is a big part of why they're liquids in the first place, since there's not solid bonds that are forcing them together. At the surface, this cohesion is called surface tension and tends to form a kind of skin. The insects that are seen scooting across the surface of ponds are doing so because the force they exert downwards isn't enough to break the way the water molecules cling to each other. Those insects wouldn't do as well if placed on ethanol. It has a lower surface tension. In wines and liquors, ethanol is mixed in with water and other liquids, and the whole jumble tends to have its own surface tension strength. Ethanol evaporates more readily than anything else in wine. It will evaporate away, and the resulting liquid will have a higher surface tension than any liquid in its vicinity that's still loaded with all those weak ethanol molecules. This doesn't usually do a great deal, unless its in certain conditions.
The film of wine clinging to the glass has a lot of exposed surface, and not a lot of depth. Surface tension plays a big part in its dynamics. As ethanol evaporates away from the film, it gets a higher surface tension. The Marangoni Effect shows that, when given a choice between high surface tension and low surface tension, liquid will flow towards high surface tension. It's a bit like being attached to two rubber bands, one of them strong and one of them weak. Objects would get in the direction of the strongest rubber band, and liquid gets pulled in the direction of the highest surface tension. The tension is so high, compared to that of the ethanol-laden wine, that it will pull liquid up the glass, and form those tears.
That's all well and good, but why are space agencies studying this stuff? The only thing that brings the tears trickling back down the glass is gravity. Eventually they get big enough to be unable to cling to the glass, and drop. In space, there's no gravity to do this. The Marangoni Effect has been shown to be a good way to get heat and mass moving through a liquid. It's also been shown to be a bad way to encourage crystal growth and the growth of biotechnology materials. Space agencies want to know how removing gravity from the equation changes everything around. Or they want to get boozed up in space. Either is good, as long as we get video of it.
Top Image: Mick Stephenson