Acoustic streaming in microchannels for particle sorting

Abstract

Particulate matter is associated with adverse health effects. Monitoring personal exposure is essential but the application of portable particle sorters is limited. With the goal of supporting aerosol measurement in Lab-on-a-Chip devices, this study applies a combination of acoustics and microfluidics for particle size separation. While micron particles can be manipulated by the size-dependent feature of acoustic radiation force (ARF), isolating sub-micron particles remains a challenge due to acoustic streaming effect (ASE), which causes small particles to circulate in the streaming vortices. Many studies reported a continuous stirring effect, but some confirmed that ASE can concentrate particles to vortices’ centers. To facilitate the separation of sub-micron particles using ASE, this study investigates the parameters that influence the ASE-induced particle motion. The current acoustophoretic separation technologies are comprehensively reviewed. Then, the effects of microchannel’s cross-sectional geometry on ASE-induced particle motion are evaluated. Finally, a numerical model that considers the piezoelectric effect and fluid/solid interaction is developed to reveal the actual acoustic fields in microchannels.

The acoustophoretic separation technologies are based on the use of bulk acoustic wave (BAW) and surface acoustic wave (SAW). BAW devices manipulate particles using a standing acoustic field generated by pressure waves reflecting from microchannel’s walls, causing ASE by viscous dissipation of acoustic energy at boundaries. In contrast, SAW devices create the acoustic field by SAW leaking into the fluid as longitudinal waves at the fluid/solid interface, where the wave attenuation leads to ASE. Both technologies are included in this study due to their significant contributions to particle separation.

For BAW devices, a 2D numerical model is employed to evaluate the impacts of microchannel’s cross-sectional shape. As the pressure wave and streaming flow are reflected by channel boundaries, changing the cross-sectional shapes from rectangular, to trapezoidal and triangular affects the acoustic fields and streaming velocity, thereby altering the particle’s trajectory. ASE in a non-rectangular microchannels can drive sub-micron particles away from the pressure node, enabling the separation of ARF- and ASE-dominated sub-micron particles.

For SAW devices, the ASE-induced motion is investigated experimentally. Sub-micron polystyrene particles are manipulated in microchannels with varying heights. The results show that ASE can concentrate particles as small as 0.31 µm in diameter to the vortices’ centers and increasing the channel height enhances the concentration effect. At specific channel heights, a new size category transitioning from ARF- to ASE-dominated is discovered, whose equilibrium position differs from the pressure nodes and vortices’ centers. The spatial distance between particles in different size categories enables the separation of sub-micron particle.

Lastly, a fluid/solid coupled numerical model is developed to accurately predict the particle’s motion in SAW devices. The results are validated by experimental results. It is found that the SAW amplitude changes with the thickness of the fluid, resulting in different pressure node locations and streaming flow fields. Overall, the findings indicate that ASE can be the mechanism for sorting sub-micron particles, while the numerical model would have great contributions in device development. These techniques are promising leads in aerosol measurement.