Tears in alcohol
Pull up a chair and pour a small glass of port. Any other fortified wine or spirits will do. Wait a minute! Don’t drink the stuff - yet. This is a real chemistry experiment.
Now that you have the correct
glassware and reagents, pause to investigate. Take a look at the sides of the glass, just above the beverage. You should be able to see a common and important phenomenon. If you can’t see anything, try swirling the drink about in the glass, then holding it still in your cupped hands. At least at this stage you should be able to keep the glass steady and see clearly. A thin film of the drink resides on the glass above the liquid, and within a few seconds “tears” of liquid start to swell and slowly drip from the top of the liquid film.
This is the Gibbs-Marangoni effect. Ethanol evaporates from the beverage where it is wetting the sides of the glass. In the absence of ethanol, the surface tension of the liquor on the sides increases, so it contracts to minimise the
surface area. This in turn brings more ethanol-laden liquor to the top and starts to form ‘tears’. The tears form at the highest point, since there is less ethanol there. As the process continues, the tears become large enough that they roll down under their own weight.
Late last century Gibbs and Marangoni described this surface effect, which is the underlying force stabilising bubbles by giving them elasticity. It is critical whether you want to shampoo your hair or use froth flotation to separate sulfide minerals. So, the next time you see a chemist sitting at a bar and gazing into a glass, don’t be so quick to criticise. He might be paying homage to a couple of great scientists.
Anyway, back to that experiment…
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