Division of Biomedical Engineering

The word "Acoustophoresis" means migration with sound. It consists of two parts, where phoresis means "migration" and acousto where sound waves are the modus operandi of the movement. In similar concepts, electric forces move particles in electrophoresis and magnetic forces in magnetophoresis [1].

Particles in suspension exposed to an acoustic standing wave field will be affected by a radiation force [2]. The force will cause the particle to move in the sound field if the acoustic properties of the particle differ from the surrounding medium. The magnitude of the movement depends on many factors, such as the size of the particle, the acoustic pressure amplitude and the frequency of the sound wave. The direction the particle is moved depends on the density and compressibility of the particle as well as the liquid medium. If a particle suspension in present in a channel in a dense material such as steel, silicon or glass there will be pressure anti nodes at the walls. If the width of the channel is matched to half a wavelength, there will also be a pressure node in the center of the channel.

Most rigid particles and cells will be affected by the radiation force in such way that they are moved to the pressure node, in this case present in the center of the channel (blue particles), figure 1A. Liquid elements or air bubbles will move to the anti nodes at the wall (yellow particles). If the size of the flow channel is in micro domain, the flow conditions is generally laminar, and the particles passing through the standing wave field will be moved to their nodal position and keep that position even after exiting the sound field. By letting the flow channel end in a trifurcation it is possible to separate and/or concentrate the particles from the medium, see figure 1B.