Microparticle sorting with a microfluidic system and ultrasonic standing waves

In earlier work, our research group developed a technology to sort microparticles in a microfluidic system fabricated within a rigid polymethyl methacrylate (PMMA, acrylic) prism, sandwiched by lead-zirconium- titanate (PZT) wafers that operate in through-thickness mode (with inertial backing) to produce standing bulk waves. The overall configuration is compact, solid, and mechanically robust. The "tilted angle" geometry (orienting the microfluidic channel skew with respect to the standing wave node lines) enables robust separation, whereby particles of different sizes can separate by multiples of the node line spacing; the microparticles display undulating trajectories, where deviation from the straight path increases with particle diameter and excitation voltage. Our earlier work involved particles with large size differences, exemplified by separating 2 mu m particles from 15 mu m particles, or by separating red blood cells from white blood cells. In this paper we describe our recent results separating 2 mu m particles (polystyrene spheres) from 6 mu m particles. We describe challenges faced by researchers working with these experimental systems, including the following examples: internal reflections can produce standing wave nodes additional to those predicted by a simple model; particles are observed through a microscope, but the limited depth-of-field places some particles in focus and others out of focus; the conditions in the microfluidic channel vary over relatively short time intervals, including thermal variations produced by the excitation process; and, there exist tradeoffs in choosing an acoustic couplant between the PZT wafers and the PMMA prism. Nonetheless, we show effective separation of microparticles of different sizes.