Effect of an Insoluble Surfactant on Capillary Oscillations of Bubbles in Water: Observation of a Maximum in the Damping

Abstract

The excess damping of capillary waves caused by a surfactant monolayer is demonstrated to be present as well for the quadrupole shape mode of an isolated acoustically trapped bubble in water. To facilitate measurements of damping as a function of surface concentration, a method was developed for depositing a known amount of insoluble surfactant (stearic acid) on the surface of the bubble. As the bubble dissolves, the stearic acid concentration increases, and the excess damping has a pronounced local maximum near 0.26 ${\mathrm{nm}}^{2}$ per molecule specific area in agreement with capillary wave data for a flat surface. The method of depositing insoluble surfactant should be applicable to the characterization of other surface-limited processes in isolated bubbles.