h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

The landlord in my local, if he sees a less than healthy head on a pint of lager will bang the rim of the pint glass with another pint glass. The shock causes the lager to froth up.

Some of the bubbles come from the walls of the glass but the majority seem to stem from a column of bubbles at the centre of the bottom of the glass.

What causes the effervescense and why does it form a column?

When you bang the pint glass you cause a compression wave that travels through the more-dense glass and then the less-dense lager - this all concentrates itself at the middle of the bottom of the glass.

The carbon dioxide is dissolved in beer at an equilibrium - in other words if you do something to affect the equilibrium, you will change the amount of carbon dioxide that can be dissolved. Putting the beer under (temporary) pressure means it can hold less carbon dioxide in solution, so some is released as a gas.

Additionally, the carbon dioxide needs to find nucleation sites around which it can form gas bubbles. The majority of these are dust particles, which are more likely to be located at the bottom of the glass than the sides. Many beer glasses incorporate etchings or even deliberate defects into the bottom to act as nucleation sites.

B

Dust particles?

http://www.crossroad.to/articles2/007/compass-movie.htm

Real bits of dust, in this case.

B

Dust in the bottom of the glass? What kind of pubs do you go to B?

Ones not equipped with HEPA filters, presumably. Any glass in any pub will have a layer of dust on it, even if you've just polished it.

The only way you can stop there being dust is build a "clean room" with incredibly efficient air filtration, and then only enter that room wearing a respirator and an all-over one-piece dust-tight suit. Which begs the question, what sort of pub do YOU go in?

True, pub glasses aren't stored in the kind of clean room where, for instance, silicon chips, PC processors or satellites are built. As for the sort of pub I go into, it's the same as anyone else - a pub where the 'clean' glasses are cleaned often and stored upside down to avoid the kind of dust buildup in the bottom that B suggested.

these dust particles are in the air and microscopic, you don't mind breathing them in so why should it bother you that you are drinking them as well

they are the reason you get bogies!!!

http://begthequestion.info/