Particle trapping using axial primary acoustic radiation force

Development in bio-particle handling microfluidic systems is subject to advancements in efficient techniques of trapping them. Contactless and gentle trapping platform offered by the ultrasonic standing wave technology makes it an attractive tool for widespread biotechnological applications. In the present study, we demonstrate an acoustofluidic trapping technique based on the standing bulk acoustic waves (S-BAW) generated inside a uniquely designed chamber – the ‘shaped trap’ that enables the particle to experience the axial primary radiation force (A-PRF) along the flow direction as the main trapping force. The study of particle dynamics reveals that the competition between A-PRF and viscous drag force governs particle trajectory. The ratio of the acoustic energy to the viscous work (β) provides a general criterion for particle trapping at a distinctive off-node site that is spatially controllable through variations in particle size, flow velocity, and acoustic energy density, creating a zone of trapping sites. Particles get trapped for β > β_cr at some distance away from the pressure nodal plane and the distance varies as β^c (c = 0.6 –1.0). Our study highlights the importance of utilizing the A-PRF as the main retention force (which originates from the more dominant potential energy gradients present away from the pressure nodal plane) against the flow unlike the conventional S-BAW trapping systems relying on lateral acoustic energy density gradient (which relies on the smaller lateral acoustic velocity field gradients effective only near the pressure nodal plane), consequently achieving an improved control over the trapping sites and higher retention force which could potentially bring in significant throughput advancements to the existing bioparticle trapping technology, thus opening up new avenues for biochemical applications.