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. The flow can either be seeded with particles with a known size distribution or ambient laboratory air can simply be circulated in the channel. 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 measurement, the separation efficiency is determined as a function of the particle size, excitation frequency, bulk flow velocity, and number of nodes in the channel. Further analysis yields an estimation of the force acting on the particles, which for very small particles yields novel information on the magnitude of acoustophoretic forces in the transition and molecular flow regimes.