Unexpected beads alignment in a microfluidic channel

A surprising phenomenon is observed in a microfluidic channel where suspending microbeads are spontaneously aligned into lines. Microbeads are randomly distributed in the channel at a relatively high velocity (3.27 mm/ms), but start to align into lines at a relatively low velocity (0.05 mm/ms). The alignment has been repeated with and without obstacles in the channel. The phenomenon is interpreted as an unintended acoustic wave being around the experimental environment, and the wave resulted in a standing wave which moves the microbeads towards the nodes of the standing wave. By using a frequency analyzer, it is found that a pulse-width-modulation controller in the system generated high-frequency signals, which is the most possible wave source for the alignment. The experimental results are presented, and characteristics of acoustic wave are analyzed. The phenomenon could contribute to microfluidic applications for achieving acoustic alignment without complex fabrication of interdigital transducers.