On the oscillation mode of gas-filled micropores.

Small pores in hydrophobic membranes trap gas when the membrane is immersed in aqueous media, and the resulting "gas bodies" may be used for studying biophysical effects of ultrasonic cavitation. In a previous study [D. L. Miller and W. L. Nyborg, J. Acoust. Soc. Am. 73, 1537-1544 (1983)], two modes of oscillation, a "pistonlike" and a "membranelike" mode, were described theoretically, and either model appears capable of qualitatively explaining observations of the linear behavior of the gas-filled micropores. In this study, low-amplitude nonlinear theory is employed with observations of second harmonic emissions to show that the membranelike model is the more appropriate model for the 3.4- and 4.0-microns-diam micropores tested at 0.81 and 1.62 MHz. In addition, this model is expanded to include the case for which the gas is asymmetrically situated in a pore, to discuss the motion of the three-phase line at the periphery of an air-water interface in a pore, and to suggest an explanation of the phenomenon of gas loss from the pores when the incident field intensity is raised. Knowledge of the behavior of the gas-filled micropores in an ultrasonic field should allow a more complete understanding of biological effects on cells exposed to this form of cavitation activity.