Surface acoustic waves for particle fractionation – physical mechanisms relevant for device design

By integrating surface acoustic waves (SAW) into microfluidic devices, microparticle systems can be separated precisely. The driving mechanism exploited here is the acoustic radiation force, which depends on the size and acoustic properties of the suspended particles. Thus a multidimensional particle separation is possible. However, fluid motion caused by the acoustic streaming effect is superimposed and can further manipulate particle trajectories. This might have a negative influence on the separation result. A characterization of the crucial parameters that affect the pattern and scaling of the acoustically induced flow is thus essential for the design of acoustofluidic separation systems. In the presentation, the fluid flow induced by pseudo-standing acoustic wave fields with a wavelength much smaller than the width of the confined microchannel is revealed in detail, using quantitative three-dimensional measurements of all three velocity components (3D3C) accompanied by numerical simulation.