Acoustic separation of sub-micron particles in gases

Abstract

In several areas of science and technology there is a strong need for concentrating, separating and sorting small particles suspended in gaseous flows. Acoustic fields can be used to accomplish this task, an approach extensively used in liquid phase microfluidics that has great potential for aerosol treatment. This paper presents an experimental investigation of acoustophoresis for very small particles in gases, with sizes ranging from tens to hundreds of nanometers. The phenomenon is studied in a rectangular channel with variable height in which a standing acoustic field is created by a broadband electrostatic transducer operated in the 50-100 kHz range. Downstream of the separation channel, the flow is separated into enriched and depleted streams with adjustable slits for analysis. The particle number density and size distribution is measured with a Scanning Mobility Particle Sizer (SMPS) as a function of position in the standing wave pattern. From these measurements, the separation efficiency is determined as a function of the particle size and the amplitude of the acoustic field. For the very small particles used here, this yields novel information on the magnitude of acoustophoretic forces in the transition and molecular flow regimes.