Understanding and exploiting acoustic field-microswimmer interactions in acoustofluidic devices

Abstract

Acoustic microfluidic devices offer exquisite control for manipulation of objects sized from tens of nanometers to tens of microns. Our work focuses on applying various acoustofluidic solutions to understand microswimmer behavior and the mechanisms underlying the beating of propulsive cilia/flagella. Ultrasonic trapping in bulk acoustic wave (BAW) and substrate acoustic wave (SAW) fields enables observation of swimming without constraining rotational degrees of freedom and without damaging the cells. Here, we summarize recent studies using biciliate C. reinhardtii algae cells, including three-dimensional body motion, synchronization/asynchronization of cis and trans cilia waveforms, and thermal and fluid-property effects. Acoustofluidic approaches provide unprecedented flexibility as a tool to investigate cell motility. Conversely, acoustic field-microswimmer interactions can also be exploited by applying the C. reinhardtii cells as dynamic probes to assess acoustofluidic device performance. We have previously reported use of these cells to identify optimal resonant frequencies of operation and to qualitatively map the strength of the acoustic field in BAW devices. Here, we extend the method to quantify the acoustic energy density of a straight microchannel operating at the first half-wavelength resonance, achieving agreement within 1 % to a standard method based on passive particle tracking. New applications for SAW devices are also presented.