Why do bubbles form in beer?

"Visible bubbles begin as invisible clusters or microbubbles of carbon dioxide molecules that grow on the rough spots (on the inside of the glass), called 'nucleation sites,' where the carbon dioxide molecules can attach themselves and coalesce.... In fact, the formation of beer bubbles is very similar to the formation of rain clouds, in which rain droplets grow on dust particles."---

Neil Shafer and Richard Zare (Stanford), Physics Today, October 1991
quoted in an article by AP writer Lee Siegel, in Phila. Inquirer, 10-20-91, p. 10A, an article on beer.

In other words:

Imperfections create the starting point.

The bubbles also rise to the surface of the beer at greatly different speeds, after suddenly detaching themselves from their "nucleation site." It appears that the larger the bubble, the faster it rises. The patterns are endlessly fascinating to the eye (especially after a beer or two). Often the bubbles seem to form interweaving lines or strings with their bubbly "traffic" moving at different speeds toward the surface of the beer. But it's hard work trying to watch one bubble until it flies upward.

All patterns and all motion need 'nucleation sites': points around which to coalesce, motes of dissonance, of texture or breakage in the smoothness to draw the eye and spur thought, downpours, brainstorms.

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