Continuous particle separation in a microfluidic channel via standing surface acoustic waves (SSAW)

Abstract

This work introduces a method of continuous particle separation through standing surface acoustic wave (SSAW)-induced acoustophoresis in a microfluidic channel. Using this SSAW-based method, particles in a continuous laminar flow can be separated based on their volume, density and compressibility. In this work, a mixture of particles of equal density but dissimilar volumes was injected into a microchannel through two side inlets, sandwiching a deionized water sheath flow injected through a central inlet. A one-dimensional SSAW generated by two parallel interdigital transducers (IDTs) was established across the channel, with the channel spanning a single SSAW pressure node located at the channel center. Application of the SSAW induced larger axial acoustic forces on the particles of larger volume, repositioning them closer to the wave pressure node at the center of the channel. Thus particles were laterally moved to different regions of the channel cross-section based on particle volume. The particle separation method presented here is simple and versatile, capable of separating virtually all kinds of particles (regardless of charge/polarization or optical properties) with high separation efficiency and low power consumption.

Vertical Hydrodynamic Focusing and Continuous Acoustofluidic Separation of Particles via Upward Migration

“…Among the single‐layered PDMS‐based devices, similar kind of focusing is not reported before. Due to the utilization of the principal component of ARF, the present device successfully operated for particle separation at net flow rate up to 1.3 mL min −1 , which is ≈100 times higher than used in previously reported SAW devices 20, 23, 31, 32, 37, 38, 42, 43, 47, 48. The throughput of the proposed device can be further improved with ease by increasing the width of the microchannel since the microchannel is placed directly on the IDT.…”

“…Compared with the most SSAW‐based devices23, 39, 40 that usually require a pair of parallel IDTs tightly aligned with the microchannel, the present device did not require tight alignment with the microchannel similar to other TSAW‐based devices 20, 38. However, tilted angle SSAWs have been utilized to circumvent such limitations 41.…”

“…A SSAW is a combination of two constructively interfering TSAWs propagating in opposite directions. SSAWs form pressure nodes and antinodes inside the fluid and are used to push particles toward regions of low pressure inside the microchannel, achieving particle focusing and separation 22, 23, 24. On the other hand, TSAWs have been used in cross‐type acoustic particle separators to laterally migrate particles and realize separation across the microchannel width or within a sessile droplet, because particles predominantly migrate within the horizontal plane 11, 25, 26, 27.…”

“…Collins et al33 took advantage of the vertical component of the ARF and employed a focused IDT to trap and concentrate selected particles behind a microfabricated PDMS membrane. However, the particle trapping capacity of the membrane would reach a saturation value in a short period of time; therefore, a continuous flow separation device offers improved particle separation performance 20, 23…”

“…A single‐layered PDMS microchannel with a simple design was used to continuously isolate the selected particles based on their diameters by inducing vertical migration. Furthermore, particle focusing based on two sheath flows, as used in most SAW‐based particle separation devices,23, 31, 43, 44 was circumvented in the present work by focusing the particles along the vertical direction using a single sheath flow. Note that some splitt fractionation or H‐filter‐based separation devices used a single sheath flow configuration.…”