EFFECT OF AN INSOLUBLE SURFACTANT ON CAPILLARY OSCILLATIONS OF BUBBLES IN WATER - OBSERVATION OF A MAXIMUM IN THE DAMPING

The excess damping of capillary waves caused by a surfactant monolayer is demonstrated to be present as well for the quadrupole shape mode o f an isolated acoustically trapped bubble in water. To facilitate meas urements of damping as a function of surface concentration, a method w as developed for depositing a known amount of insoluble surfactant (st earic acid) on the surface of the bubble. As the bubble dissolves, the stearic acid concentration increases, and the excess damping has a pr onounced local maximum near 0.26 nm(2) per molecule specific area in a greement with capillary wave data for a flat surface. The method of de positing insoluble surfactant should be applicable to the characteriza tion of other surface-limited processes in isolated bubbles.