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.

Theoretical investigation of the particle response to an acoustic field

“…In contrast, in some systems, agglomeration of particles due to acoustics is a beneficial phenomena and has a potential use in air pollution control. 2 Acoustic agglomeration is in this context a promising novel alternative to improve the particulate material filtration effectiveness in exhaust systems, 3,4 and, consequently, reduce the emissions of this product in diesel engines, just to mention one application.…”

“…Although several experiments 2,6 and numerical simulations 3,[7][8][9] for different scenarios of an individual particle and population of particles submitted to an acoustic field have been carried out, in our best knowledge, a theoretical approach of the problem of a particle immersed in an acoustic field considering nonlinearities and focused on the examination of the parameters role has not yet been performed, which is the main novelty of the present investigation. Partial approximations have been carried out, for example, Aboobaker et al 10 developed an approximate analytical description for the trajectory of a particle subjected to an acoustic standing wave.…”