Acoustofluidic trapping of microparticles to axially centered wires in cylindrical microcapillaries.

Acoustophoresis is a powerful technique for manipulating micro- and nanoparticles in microfluidic systems, but efficient particle capture remains limited by the size-dependent nature of the primary acoustic radiation force. This limitation has motivated the use of secondary acoustic forces to capture particles to wire meshes or larger suspended seed particles; however, these approaches are constrained by fluidic drag and limited control over particle residence time in acoustic fields. Here, we present an acoustofluidic capture strategy that combines primary and secondary acoustic radiation forces with laminar flow effects near surfaces to enable efficient microparticle trapping. Our device consists of a cylindrical microcapillary containing an axially aligned stainless-steel through-wire and an externally mounted piezoelectric transducer that generates an acoustic pressure node during flow. Microparticles are driven toward the wire by primary acoustic forces, where secondary acoustic interactions and reduced near-wall flow velocity promote stable capture. Devices with no wire, a centered through-wire, and an offset through-wire were evaluated to assess the role of wire placement within the acoustic field. Precise axial alignment of the wire with the pressure node yields optimal trapping, demonstrating a 3.6-fold increase in downstream particle concentration after cessation of the acoustic field, with overall capture efficiencies of 52-91%. This work establishes a design paradigm based on precise placement of microstructures within acoustic pressure fields to maximize particle trapping and capture in laminar flow systems.