Enhanced synchronized ultrasonic and flow-field fractionation of suspensions

• Published in: Ultrasonics 1994-03, Vol.32 (2), p.113-122
• Main Authors: Mandralis, Z., Feke, D.L., Bolek, W., Burger, W., Benes, E.
• Format: Article
• Language: English
• Subjects: acoustic force; Acoustics; Exact sciences and technology; fractionation; Fundamental areas of phenomenology (including applications); Physics; standing wave; suspensions; Ultrasonics, quantum acoustics, and physical effects of sound
• ISSN: 0041-624X; 1874-9968
• DOI: 10.1016/0041-624X(94)90019-1

A fractionation method for fine-particle suspensions based on variations in the speed of response to ultrasonic standing wave fields has been developed. The method is sensitive to a combination of the particle size as well as the particle and suspending fluid density and compressibility. The fractionation is accomplished within a narrow-gap rectangular channel having a solid barrier positioned along its midplane. Ultrasonic standing wave fields of two alternating frequencies are applied across the gap to induce a partial separation at short irradiation times. A co-ordinated bidirectional laminar flow field is used to transform these partial separations into useful separations along the chamber. In comparison with similar fraction strategies that use other types of fields to accomplish the separation, the acoustic forces acting on particles can be quite strong, thereby enabling fast, continuous, and controllable fractionation of micrometre-sized particles. An analytical model based on the trajectories of particles in response to the acoustic and flow cycles was developed. Model predictions indicate how the fractionation can be controlled through the choice of cycle parameters. Experimental results using a 325 mesh polystyrene particle suspension demonstrate that sharp fractionations of nearly neutrally buoyant micrometre-sized particles can be achieved.