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 Originally Posted by oskar
When you stir an espresso and tap the cup, the frequency of the tapping drops significantly while the coffee is agitated and rises back up when it settles down. This works with expresso and some foamy instant coffees, does not work with filter coffee or generally non-foamy liquids. Why is that?
WTF? Really?
Umm... Got any links to this phenomenon? It's the first I've heard of it.
It's quite counter-intuitive that the frequency would DROP when the carrier fluid has more energy. Well, the heat energy is so vastly prevalent over the rotational energy of stirring the drink, that I guess it's probably a red herring.
OK, so it's the characteristic lengths being altered by the motion of the foam that makes a difference. The bubbles in the foam might have a different average spacing when it's stirred. Or even it could be that bubbles in the foam are slightly larger or smaller when the stirring alters their base-line interactions.
Either way, I'm going to guess that it's altering the average lengths between fluid/air boundaries within the foam with a net effect of increasing either bubble size or bubble spacing or possibly both. This is because lower frequencies have longer wavelengths, and would resonate in larger spaces, than higher frequencies.
Hmmm...
Bernoullii's Principle states that flowing fluid has a lower pressure than fluid that is standing still, so the reduced pressure of the stirred fluid could cause the foam's bubbles to increase ever so slightly in size.
(This is pure speculation if you haven't gathered from the tone.)
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