Rising bubbles testing water contamination
by
J F Harper
Victoria University of Wellington

Bubbles of air or gas rise faster in pure than in even slightly impure water. The reason has long been known: small amounts of many impurities (surfactants) lower the surface tension by a large amount, but when a bubble moves, surfactant has to get onto it at the front and off at the rear end. Diffusion and slowness of adsorption both imply that the front has a lower concentration of surfactant than the rear, and thus a higher surface tension, which opposes the motion. Smaller bubbles are more sensitive than large ones.

It is also well known that many different experimenters used `pure' water that slowed down bubbles of a given size by the same amount, as if they all had the same amount of the same impurity. Many years ago the speaker gave an order-of-magnitude analysis for bubbles large enough to have almost free surfaces, which suggested that the impurity might have been atmospheric carbon dioxide. Since then, enough computational work has been published for the idea to be tested quantitatively. The test is actually easier for nearly rigid than nearly free surfaces, and carbon dioxide now seems insufficient to account for the experimental results. The 50-year-old question of what did affect everyone's water in the same way therefore remains open.

Date received: October 8, 2001