Force field particle filter, combining ultrasound standing waves and laminar flow

A continuous flow microparticle filter that combines megahertz frequency ultrasonic standing waves and laminar flow is described. The filter has a 0.25 mm, single half wavelength, acoustic pathlength at right angles to the flow. Standing wave radiation pressure on suspended particles drives them towards the centre of the acoustic pathlength. Clarified suspending phase from the region closest to the filter wall is drawn away through a downstream outlet. Experimental tests achieved >1000-fold clearance of 5 μm yeast cells, at a sample flow rate of 6 ml min−1, from which the clarified aliquot is 1 ml min−1. At this flow rate the average residence time in the sound field was <1 s. Latex particles of 25, 9, 5.7, 2.8 and 1.5 μm diameter were also tested. The temperature increase during an operating period >1 h was less than 1 K. The design criteria considered in the fabrication of this high performance device are discussed. A theoretical model of the filter’s efficiency, which considers the action of primary radiation force and the particle distribution across a laminar flow profile is presented here. The model predicts that totally clarified filtrate (i.e. zero suspended particles) may be drawn from the downstream outlet. The system described offers a generic approach to automated filtration in some applications. It is continuous flow thereby solving many of the problems of automation presented by batch filter methods and centrifuges. It could be developed for both larger scale and microfluidic applications.