UPPER SOUND PRESSURE LIMITS ON PARTICLE CONCENTRATION IN FIELDS OF ULTRASONIC STANDING-WAVE AT MEGAHERTZ FREQUENCIES

Polystyrene microspheres (9-mu-m diameter) in air-saturated aqueous suspension were subjected to standing-wave fields over a range of pressure amplitudes at frequencies of 1.02 and 3.14 MHz. At the lower pressure levels the particles striated at half wavelength intervals and formed columns in the test chamber. The stability of the striated columns and the presence of acoustic cavitation (as evidenced by the presence of odd multiples of one-half of the driving frequency and, when present, general noise) were monitored simultaneously as the acoustic level was increased. For the 1.02 MHz experiments, the threshold pressure amplitudes for emission of odd multiples of half the driving frequency and of general noise were 525 and 1100 kPa respectively (all pressures are peak values). However the striated columns were able to retain their integrity up to pressure amplitudes of 1900 kPa. For the 3.14 MHz field, general noise in the chamber was generally not detected up to pressures of 2150 kPa but vigorous streaming in the upper quarter of the chamber disrupted the orderly columns at pressures of 1100 kPa.