Tiny Bubbles Revisited

This is the one thousandth column produced under various names--Science Forum, Diggings, and others--since Neil Davis sat down in March 1976 and cranked out the first one in this series. Although the column itself is no subject for a science article, I couldn't let the anniversary pass without note, and a comment or two.

Each of us who produced these informal discussions into science--Davis, Larry Gedney, Sue Ann Bowling, and I--have discovered that some items produce more comment than others. People will call in, or write, or we'll encounter someone at the post office who has something to say about a column. Sometimes the articles that catch people's interest prove to be surprising to their authors. That's the case of the all-time interest-getter, the one that brought more comments on the order of, "You know, I always wondered about that..." than any other. Thus this column returns to the subject of Neil Davis's #486, dated February 1981: Bubbles in Beer .

Davis had been challenged by reader Glenn Estabrook of Fairbanks. His question: "In a carbonated beverage, why do the rising bubbles always seem to stream up from individual spots, as though emanating from distinct localities on the surface of the container?"

Davis had to hunt for a couple of months before he found the answer in a book called Butter Side Up, by Magnus Pyke. Thanks to Geophysical Institute librarian Judie Triplehorn, I found a more elaborated answer in a 1989 issue of Discover magazine. Both sources agree on the principles in action.

A properly sealed bottle of carbonated beverage shows no bubbles. The bottle's contents are under pressure, so the carbon dioxide cannot expand to form bubbles. Pop the top, release the pressure, and bingo! Bubbles.

But those newly visible bubbles don't show up just anywhere. They start as invisible protobubbles, tiny clusters of carbon dioxide molecules. Those clusters have formed at what a beverage physicist would call nucleation sites--microscopic pits and cracks on the smooth-looking interior of the bottle (or glass, if the beverage is poured out). Carbon dioxide accumulates at these points of roughness.

The gas also accumulates on floating particles, which offer places for the molecules to land and cling just as the bottle wall's cracks and pits do. Whether floating or fixed, a mediocre nucleation site may give rise to a single bubble, but one offering just the right conditions for harboring carbon dioxide can give rise to a steady string of rising bubbles.

The bubbles at the starting point of such strings may be invisibly small, so that an observer can't detect where the nucleation site actually is. As the bubbles ascend, they accumulate more carbon dioxide. Thus they grow bigger and more buoyant as they rise, so that bubbles near the top of a string are more widely spaced and are moving faster than those lower down.

Bored beer drinkers have sometimes amused themselves by shaking a sprinkling of salt into their brews. The result is a quick flare of bubbles at the surface and bubble strings tracing the downward path of the salt grains. It may look like a chemical reaction, but it's still physics in operation. Salt grains aren't the perfect little crystals they seem--their surfaces are covered with the microscopic flaws that make perfect gathering places for carbon dioxide molecules. The drinkers have added nucleation sites as well as a salty taste to their beverages.

There you have the explanation of one of life's little mysteries. Look on it as an appropriate way to toast the thousandth appearance of these miscellaneous sharings of science--with a bubbly subject.