Advanced Res. Lab., Hitachi Ltd., 2520 Akanuma, Hatoyama-chou, Hiki-Gun, Saitama 350-03, Japan
Central Research Lab., Hitachi Ltd., Tokyo 185, Japan
The efficacy of using an ultrasonic standing plane wave to concentrate small particles in a liquid was theoretically estimated and compared with experimental results. Acoustic energy density was measured by ultrasonic absorption, and particle distribution was observed by darkfield microscopy. The theory predicts that the effect of diffusion is negligible in concentrating polystyrene spheres larger than 5 (mu)m in diameter when they are subjected to 4 J/m[sup 3] ultrasound at 500 kHz. The half-width of the steady-state particle distribution in the experiment was of the same order of magnitude as that of the theory. This concentrating technique was applied to the fractionation of polystyrene spheres 10 (mu)m in diameter by using a capillary tube. More than 90% of the particles in a laminar flow were successfully collected. A separation technique based on the competition between the acoustic radiation force and the electrostatic force was also tested. A mixture of polystyrene spheres with two different radii, which lined up on a pressure node of a stationary standing wave, was successfully separated according to their radii by applying a uniform electrostatic force parallel to the acoustic radiation force.