Frequency-programmable particle routing and separation via steerable SAWs from azimuthally tapered interdigital transducers.

This study presents a frequency-programmable, acoustofluidic particle routing and separation platform that enables frequency-dependent adjustment of the trajectories and destinations of micro- and submicrometer particles in continuous flows, thereby achieving outlet-switchable particle separation and on-demand switching of particle routing destinations. These capabilities arise from our platform's ability to steer traveling/standing surface acoustic waves (SAWs) via frequency tuning. To achieve these particle routing and separation and SAW control abilities, particularly, our platform employs two azimuthally tapered, frequency-steerable interdigital transducers (AFIDTs), each with azimuthally continuously tapered electrode widths and spacings. Arranged with rotational symmetry, paired AFIDTs can generate steerable standing SAWs whose node-line orientation rotates with excitation frequency. This frequency-steering function was validated by laser vibrometry and particle patterning. We then successfully demonstrated on-demand, frequency-programmable adjustment of micro- and submicrometer particle routes and switching of target outlets, when continuously flowing micro- and submicrometer particles (e.g., polystyrene beads, silica particles, and Jurkat cells) in a microchannel with three exits. Moreover, mixed 5 and 10 μm particles were effectively separated by exploiting size-dependent acoustic radiation forces. Numerical simulations of the SAW and streaming fields corroborated experimental observations. Our frequency-steerable AFIDT platform provides a compact, label-free approach for on-demand, programmable particle routing/separation, with strong potential for integration into microfluidic separation and diagnostic systems.