Fig. 2. Wall trapping of 10 μm PS particles in different rectangular microchannels.

(a) Time-lapse images of particles under ultrasound. The narrower 100 μm and 200 μm width channels show excellent wall trapping, whereas the wider 500 μm and 750 μm width channels have poor wall-trapping performance due to the sine-type displacement actuation. (b) The particle tracing simulations of the microchannel cross-section from dispersed to ultrasonically manipulated condition over 15 seconds, confirming the final location of the beads in the experiments (Movie S1†). The blue and red colors indicated the minimum and maximum speed of the particles, respectively. The scale bars are 50 μm. The operating frequencies are 105 kHz for 100 μm and 200 μm width channels and 155 kHz for 500 μm and 750 μm width channels.