THE NONLINEAR RESPONSE OF GAS-FILLED MICROPORES TO ULTRASOUND AT MHz FREQUENCIES

In this work air was trapped in the pores of specially ordered Nuclepore ® filters by immersing them in water, thereby producing stable arrays of cylindrical bubbles with diameters of about 3 and 4 microns. The arrays were irradiated by beams of c.w. and pulsed ultrasound at frequencies near the volume resonance of the trapped bubbles. By examining the spectrum of frequencies scattered by the arrays it was found that the bubbles oscillate nonlinearly while still remaining trapped in the filters. Using short pulses similar to those used in ultrasonic medical diagnostic equipment, the bubbles remain stable up to excitation pressures of several bars.