Thomas J. Asaki
MS K764, Los Alamos Natl. Lab., Los Alamos, NM 87545
David B. Thiessen
Phillip L. Marston
Washington State Univ., Pullman, WA 99164-2814
The excess damping of capillary waves caused by a surfactant monolayer was demonstrated for the quadrupole shape mode of an isolated acoustically trapped bubble in water. A known amount of the water-insoluble surfactant stearic acid was deposited on the bubble surface from a microliter of ethanol solution. As the bubble dissolves, its surface area decreases thus compressing the monolayer. Compression rates ranged from 0.09 to 0.18 (Angstrom)[sup 2] per molecule per minute for different bubbles and surfactant coverage ranged from 20 to 37 (Angstrom) [sup 2] per molecule specific area. This technique was tested for accuracy and reproducibility by application to pendant bubbles and subsequent measurements of the static surface tension. Shape oscillations were induced by modulation of the ultrasonic radiation pressure and the free decay was recorded by an optical extinction technique. Local maxima in the normalized damping and frequency were observed near 26 (Angstrom) [sup 2] per molecule coverage in agreement with similar studies of capillary waves on flat interfaces. An unexpected increase in the frequency was observed at high coverage and may be associated with the solidification or collapse of the monolayer. Any effects due to the presence of ethanol were shown to be negligible. [Work supported by ONR.]