Imaging the 3D flow field of free-swimming Chlamydomonas reinhardtii with high-speed holographic microscopy *

The flow induced by microswimmers provides invaluable insights in the mode of their locomotion, as well as their interactions with each other and surrounding environment. While the two-dimensional (2D) flow fields of single microswimmers have been well-characterized in previous experiments, obtaining reliable measurements on the three-dimensional (3D) flow field around a single microswimmer remains challenging due to experimental difficulties associated with the high-speed, high-resolution velocimetry of 3D flow. Here, we present a precise measurement of the time-averaged 3D flow induced by a motile, free-swimming microalga, Chlamydomonas reinhardtii, using high-speed holographic microscopy. The flow of the alga is obtained by tracking the motion of micron-sized tracers over thousands of its flagellar beating cycle. Our experiments capture the crucial details of the 3D flow field of microorganisms in their natural swimming mode and demonstrate the power of holographic microscopy in imaging complex flow fields with unprecedented spatiotemporal resolutions.