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Re: beer cans



On Mon, 18 Mar 2002, John Barrer wrote:

I haven't tried this lately, but I remember from my
youth that excessive foaming of a can of beer that had
been shaken a bit could be prevented by giving the top
of the can a couple of sharp raps with a metal object
before opening. I may be the victim of selective
memory, but I swear this worked for me.

You can have lots of fun with this once you know the mechanism.
"Explosions" of effervescence are caused by clouds of microscopic bubbles.
Outgassing can only occur at fluid/gas interfaces, and since the smallest
bubbles tend to dissolve, bubbles only arise spontaneously when the system
is immensely far from equilibrium. Usually the "nucleation" isn't
nucleation at all, it's caused by existing bubbles which happen to be
invisibly small.

For shaken-up canned drinks, you just have to wait for a while for the
bubble cloud to rise. But that doesn't remove the bubbles that stick to
the walls. Whacking the top of the can helps detach them.

So if you WANT to make somebody's beer spray out entirely, shake it with a
violent rotating motion (for immense shear and tiny bubbles), then tilt it
so lots of microscopic bubbles cling to one side.

Another trick: a little dab of water-thinned whipped cream in the bottom
of a cup will contribute a huge load of micro-bubbles to any carbonated
liquid poured in. (Or just use sub-zero ice cubes having a dry frosty
surface.)


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