Characterization of acoustofluidic geometric traps by means of defocus particle tracking

In this work we used the General Defocusing Particle Tracking method to measure the three-dimensional trajectories of 10-μm-diameter polystyrene beads moving inside a micro-acoustofluidic d evice. The aim i s to measure the change in acoustic energy density induced by localized modifications of the channel g eometry. The measurement were performed in a glass capillary tube with a rectangular cross-section of 2.0×0.2 mm². The wall thickness of the capillary tube is 140 μm and it was locally increased by gluing a glass slide with a thickness of 100 μm. The acoustic energies at five different frequencies around the expected resonant frequency have been estimated for the single-glass and double-glass region from the measured particle paths. The results showed a down-shift of the resonant frequency in the double-glass zone, as expected from theoretical predictions. This approach can be used to create multiple acoustofluidic traps based on simple geometrical modifications of the microchannel.