lew (09/09/82)
In the December 81 Scientific American "Amateur Scientist" section, Jearl Walker discusses beer bubbles. He remarks that they expand as they rise due to the decrease in hydrostatic pressure. I find this untenable. Atmospheric pressure is equivalent to 10 meters of water, so with a 10 cm rise a bubble should expand to 1.01 times its original volume (1.003 times its radius.) This is not nearly enough to account for appearances, which show an expansion of 2 to 5 times of the radius. I first thought that relief of surface tension as the bubble expands might account for the large expansion but it turns out that the pressure due to surface tension in a bubble even .1 mm in radius is ~.01 atmosphere. Surface tension won't account for it. That means that the bubble must actually be gaining gas as it rises. Regarding the pressure as nearly constant, the bubble should gain CO2 at a rate proportional to its surface area. This means that the radius should grow linearly with time. With buoyancy proportional to volume and resistance proportional to velocity times radius (Stokes Law), we get velocity proportional to time squared. This might be right. I plan to try checking it experimentally. Some nucleation sites release bubbles at an even rate so I can get a pseudo-strobe effect with a single flash photo. Just think, the Galileo of beer! Lew Mammel, Jr. - BTL Indian Hill