A Question of Physics: Beer Foam and Football Games

Factual Questions
1

So I was at the game on Friday (Riders 42, B.C. 29 - yay!) and as usual, got my two per customer beers in the plastic cups. To prevent spillage during the trek to my seat through the many-elbowed masses, I put the plastic lids on the plastic cups - you know, the kind of lids with the two cross-slits for a straw.

Also as usual, by the time I got to my seat, beer foam was starting to seep up through the two slits on each beer. I took the lid off one and started drinking it, and put the second under my seat with the lid still on.

By the time I got to the second one, the beer foam was all over the lid and dripping down the sides.

Now, besides it being messy, that beer foam represented beer I’d paid for and couldn’t drink, dammit! :mad: So it got me thinking. Why does the lid seem to make the beer foam more?

If I buy a beer and don’t put the plastic lid on it, the foam stays constant. It doesn’t start building up in the cup and eventually bubbling over the side. What’s the diff with the lid?

The only thing I could think of was that the lid changes the pressure at the surface of the beer, which in turn either increases the rate at which bubbles come out of the beer, or decreases the rate at which the bubbles in the foam collapse. But these are just guesses of a definitely non-physicist type. :confused:

Any beer-physicists out there who can cast light on this one? How can I keep my beer for drinking purposes :stuck_out_tongue: , not foaming?

2

The plastic probably contains tiny unwetted discontinuities (eg little scratches) that act as nucleation points for bubble formation.

3

The lid traps the escaping gases which agitates the liquid beer causing more gas to release, etc., etc., etc.

[sup]Disclaimer: Just a wild ass guess.[/sup]

4

A covered beer loses less CO2. The released gas is trapped near the surface of the beer; the higher CO2 partial pressure under the lid opposes gas release. You can test this yourself with a trial comparing a lightly covered beer or soda with an uncovered one. You’ve probably already done this

However, this does not affect bubble formation, which is dictated by local conditions at the nucleation site and the bubble’s path through the liquid (Bubbles often pick up gas and grow as they rise. The decreasing pressure as the bubble rises through the fluid plays some role, but it’s primarily because the surface area increases with the square of the radius: the bigger a bubble gets, the faster it grows)

All other things being equal, I would assume that the rates of bubble formation wiould be equal, as a practical matter, a glass under a seat would probably be more protected from agitation, sound, solar heating, etc., and would therefore probably generate a smaller volume of bubbles.

Where it has a real advantage is in bubble survival The very factors that make the glass in your hand bubble more also tend to disrupt the bubbles in the foam: the bubbles dry, jostle, break, merge (larger bubles escape the foam more readily, and are more fragile, due to the tension/pressure relation) This is the secret of your mystery: the foam in the glass under your chair survived longer, and built up until it reached the slits and overflowed. The bubbles in the glass in your hand broke and subsided like the foam on a freshly poured soda, before it could reach the rim of the glass.

Dare I suggest that you might also have helped keep the level down by drinking some of the beer? It seems unbelieveable, I know, but there are numerous recorded instances of this behavior in humans. Is it possible that you sippeed some in the interest of frugality - to keep it from being wasted? :smiley:

I doubt that nucleations sites on the lid played any significant role. Unless the beer was (over)filled to the very limit of its capacity, the unjostledunder-the-seat beer would not reach the plastic lid, and any foam which reached is already fully “bubbled”. It won’t nucleate further.

5

two beers

three cups all 2/3 full

waste not

go stamps

6

no lid = carry and disrupt as little as possible = less foaming

lid = ability to shake it a bit more and disrupt it a bit more = more foaming
The lid just allows you to agitate more without dire spillage, but this agitation results in more foaming.

7

Also consider that the head in the second cup got more time to warm up; warm CO2 takes up more space, so those bubbles would expand slightly.

8

This dilemma would have been avoided if he had just been wearing one of these.

9

But as I said in the OP:

If I get a beer and don’t put a lid on it, carry it through the teeming hundreds, by the time I get to my seat it’s not foaming over the sides. I put a lid on it, and by the time I get to my seat it’s foaming up through the slits in the lid. If I get two and put one under the seat once I get there, the second beer under the seat has more time to foam over and so is messier, but it is the lid that make the difference, not the lack of disturbance.

10

Norther Piper Just in case you hadn’t realised, ‘the plastic’ to which I refer is the plastic of your lid.