Concentration and fractionation of small particles in liquid by ultrasound

The efficacy of the ultrasonic standing plane wave in concentrating small particles was theoretically evaluated and compared with experimental results. Acoustic energy density was estimated by measuring the ultrasonic absorption, and particle distribution was observed by dark-field microscopy. The theory predicts that diffusion is negligible in concentrating polystyrene spheres larger than 5 µ m in diameter when they are subjected to 2 J/m³ ultrasound. The half-width of the steady-state particle distribution was of the same order of magnitude as the theoretical value for the particles of 1 µm and 2 µm diameter. We also applied this concentrating technique to fractionation of polystyrene spheres 10 µ m in diameter, and more than 90% of the particles in the laminar flow were successfully collected.