The trapping of particles or cells using ultrasonic radiation forces is relevant to a range of biological, chemical and medical analyses, for example, those involving cell washing, cell separation or testing of reagents. This work studies the trapping forces within a flow-through chamber designed to detect counterions based on the aggregation position of ion-exchange resin beads. Certain counterions alter the water content of the beads and therefore their acoustic properties; this alters their response to the acoustic field allowing the counterions to be detected by measuring bead position or trapping force. This paper investigates the acoustic radiation force distribution of the device and shows experimentally and computationally that flow rate and resulting fluid drag opposing the longitudinal acoustic radiation forces influence the location of trapped beads. The relationship between location and force is influenced by the uniformity of the axial field and axial displacement of the bead. It is proposed that by characterizing this relationship, the trap location of a resin bead against a constant flow can be used to measure changes in trapping forces and therefore acoustic contrast factor, and as an indicator of certain counterions.
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