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Name: Priya C.
Status: student
Age: 16
Location: N/A
Country: N/A
Date: Thursday, April 25, 2002


Question:
I have been asked for my physics assignment to explain the phenomenon behind why when alcohol is added to an air bubble on water, the bubble ends up pulsating up and down after a certain amount of alcohol is added. I have interpreted this to be a surface tension question however am unsure as to whether I am on the right track! Does the fact that alcohol is added to the bubble have any effect?


Replies:
The effect you describe is called the Marangoni effect or Gibbs-Marangoni effect.

The effect is graphically illustrated by a high alcohol content wine or brandy. In either case what happens is the alcohol/water solution "wets" the side of the glass and begins to "crawl" up the side against the flow of gravity. This is a result of an increase in the surface tension of the solution as the concentration of alcohol decreases due to evaporation. Water has a surface tension of about 72 ergs/cm^2 and ethanol has a surface tension of about 23 ergs/cm^2. As the surface tension increases there is a tendency to "pull" the solution up. However, at the same time the evaporation of the ethanol cools the solution increasing its density and decreasing the rate of evaporation. Then gravity "wins" and the liquid flows back down into the bulk solution. The process then repeats, so there is a convection flow set up. The quantitative analysis of the physics is rather messy, but if you search the term(s) using say: www.google.com you can surely find it discussed at varying levels of sophistication. The effect also occurs on the windshield of a car when the alcohol/water cleaner is squirted on the glass, but it is less noticeable.

Vince Calder



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