Technical question about nucleation of beer?

[steve0606]

Technical question about nucleation of beer!?

I work as a barman and am also a physics student. But I just can't seem to get my head around something: I understand that beer (referring to lager really) consists of water with CO2 dissolved in it with other, irrelevant, stuff also. You cannot see the gas because it is dissolved; however if the glass it is in contains a nucleation point (a widget) the gas is released as bubbles which float to the top of the beer and stay there. However, sometimes bubbles do not form i.e. you pour the pint and there are no bubbles being formed afterwards. My managers tell me this is because either the glass is warm, wet or dirty. But they cannot tell me WHY.
Surely, if the glass is warm, the gas should be released more readily;
I can't understand why it being wet would have much effect;
If it is dirty, nucleation should still occur.
Also, if you tap the glass the glass a flat pint is in such that it vibrates, bubbles form for a very short moment (presumably whilst the glass is stil vibrating) some bubbles form.
Can somebody please tell me why pouring lager into a widget glass that is warm, dirty or wet somehow 'deactivates' the widget unless you tap it. Thanks!

 

[Andy Resnick]

<snip>I understand that beer (referring to lager really) consists of water with CO2 dissolved in it with other, irrelevant, stuff also. <snip>

Heretic! :)

AFAIK, beer is flat when the CO2/dissolved gas has already come out of solution. If you see an errant bubble or two, most likely you simple was able to coax a little bit of remaining gas out of solution.

 

[steve0606]

Sorry when I say "flat" I actually mean there is no apparent bubble formation. There is gas still in the beer because if I pour a "flat" pint, put into another glass, there is sometimes bubble formation like normal (even if I try this with more than 5 glasses!) Also, if the gas escaped, there is so much gas in beer that you would definately see Effervescence due to lager being a supersaturated liquid and due to Marangoni effect.
I think I've found the answer though:
http://en.wikipedia.org/wiki/Beer_head
This explains the situations when the glass is dirty or wet and I think I may know why when the glass is hot: The glass is usually hot straight from a glasswasher. Given that a glass is used when the glass hasn't cooled down might mean that it is still wet. So it may appear nucleation is prevented from the heat of glass when really it is because the glass hasn't dried properly! Thanks anyway!

 

[Andy Resnick]

Interesting- I've never noticed that. Time to do some experiments- cheers!