Size-dependent microparticles separation through standing surface acoustic waves

This study presents a method that uses a standing surface acoustic wave (SSAW) to continuously separate particles in a size-gradient manner in a microchannel flow. The proposed method was applied to a colloidal suspension containing poly dispersed particles with three different sizes (1, 5, and 10 mu m) but the same density and compressibility. Particle suspension was focused hydrodynamically at an entrance region, and particles were forced actively toward the side wall where SSAW-pressure nodes were generated by two interdigital transducers (IDTs) across the channel. The particles placed in the middle stream, in which the shear rate was minimized, were separated successfully in a size-gradient manner by acoustic force. In addition, this study further developed an analytical model to predict the displacement of particles in microchannel flow by considering viscous, acoustic, and diffusive forces. The predicted values of particle displacement showed excellent agreement with the experimental results, and diffusion was found to be important and not negligible. The advantage of this method is to minimize the shear rate on particles, which would be useful for potential applications of shear-dependent cells such as platelets.