Upper sound pressure limits on particle concentration in fields of ultrasonic.pdfVIP

Upper sound pressure limits on particle concentration in fields of ultrasonic standing-wave at megahertz frequencies Robert K. Gould*, W. Terence Coakley and Mart in A. Grundy School of Pure and Applied Biology, University of Wales, College of Cardiff, Cardiff CF1 3TL, UK * Department of Physics, Middlebury College, Middlebury, VT 05753, USA Received 6 September 1991 Polystyrene microspheres (9#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 1 900 kPa. For the 3.1 4 M Hz field, general noise in the chamber was generally not detected up to pressures of 21 50 kPa but vigorous streaming in the upper quarter of the chamber disrupted the orderly columns at pressures of 1100 kPa. K e y w o r d s : u l t rasonic cav i t a t ion ; u l t rasonic s tanding w a v e ; acoust ic s t reami ng Particles suspended in a liquid in a standing-wave field are driven by radiation pressure forces to striate in planes separated by half an acoustic wavelength. The radiation pressure force depends on the sound frequency, on the particle size and on the density and compressibility of the particles relative to the corresponding properties of the host liquid. Several inve