Characterizing three-dimensional flow fields of free-swimming microorganisms through high-speed holographic microscopy*

In understanding the locomotion of microswimmers and their interactions with the surrounding environment, one could obtain invaluable insight through characterizing the flow they induce. Previous studies have well-characterized the two-dimensional (2D) flows around numerous single microswimmers, though obtaining such measurements in three-dimensional (3D) space remains a challenging endeavor due to difficulties in performing velocimetry for 3D flows with high spatiotemporal resolution. Such flow descriptions might be crucial in understanding swimming behaviors in unconfined systems, where 3D motion becomes more apparent. In this work, we present precise measurements of the time-averaged 3D flow induced by a free-swimming Chlamydomonas reinhardtii through high-speed holographic microscopy. The flow is obtained by tracking the 3D motion of 1μm tracer particles at 500fps over thousands of the flagellar beating cycle. Our measurements capture crucial 3D features of the microorganisms' flow in their natural swimming behavior, demonstrating how potent holographic microscopy can be in imaging complex flow fields at high spatiotemporal resolutions and providing the means to elucidate the mechanism of more complex swimming behaviors.