When pooring a glass of soda I regularly observe the creation and destruction of froth. Based on the size of the bubbles, both their volume and my perception of the thickness of the bubbles themselves, it seems to me the froth should turn into a much smaller amount of liquid than what actually happens.
Based on my limited understanding of surface tension I find it possible that the “liquid part” of the bubbles, which are basically “all surface” are a denser medium than the liquid itself, and that this makes my froth to liquid ratio guess wrong. Surface tension is due to surface molecules having fewer neighbours binding stronger to the neighbours they do have, so I find it reasonable these bonds also become shorter than in the liquid in general.
Am I onto something, or am I just trying to justify what’s a lousy guesstimate in the first place.
A large part of what you are observing depends on what type of soda as well. Root beer, for example, has all sorts of stuff in the flavorings that promote and abet large head formation. Lemon-lime soda, not so much.
Well that’s not really relevant to my observations though. What I find is that I in general underestimate how much the liquid will rise as the froth subsides. My theory is that this in part is a result of liquids being more densely packed in bubble surfaces, throwing off my estimates, and not completely a result of my poor observational skills.
Maybe the bubbles against the glass tend to be bigger than those that you cannot directly see in the “body” of the froth, so that you are observing a higher ratio of air to liquid bubble surface than actually exists throughout the medium.
This is basically just a WAG, but I think it is fairly plausible that contact with the glass, and, in particular, with tiny irregularities in its surface, might cause the bubbles there to grow faster than those further in.
Fill a glass with foam (not from a carbonated drink, as that might still be releasing dissolved gases)
Cover the glass with cling film, stretched taut and flat
Observe the film after the foam has collapsed. If the liquid comprising the bubbles was truly pullled into a denser state by the surface tension, it should have expanded when it reverts back to normal non-bubbly liquid, and the film should be inflated.
Perhaps you are misjudging the liquid-froth interface. My guess is that some of the froth is below the liquid level (which, of course, it is initially) and the liquid level appears to rise as the foam dissipates.
If you are ambidextrous perhaps you can test this by pouring a foaming liquid and water at the same rate and same time. The water level should be a good indicator of the liquid level in the foamy solution.
Last night I poured a soft-drink into a glass that was already half-full of the same drink but did not have foam on the head. Since I knew exactly where the liquid level started, it was clear upon pouring in the additional soda that a significant portion of the foam is below the liquid. So, you are merely misjudging the foam-liquid interface.