Seeing new depths: visualizing the three-dimensional flow field of a free-swimming alga

Insights into the locomotion of microswimmers and their interactions with environment rely on the detailed understanding of the structure of the flow they induce while swimming. While measurements of the projected two-dimensional flow around different microswimmers have been achieved as a milestone in experimental fluid mechanics in the past decade, mapping the full three-dimensional (3D) flow structures of microswimmers remains a challenging task. Here, we utilize high-speed holographic microscopy to measure the time-averaged and phase-specific 3D flow of a free-swimming Chlamydomonas reinhardtii, a premier model for swimming microorganisms. The flow field is obtained by tracking the 3D motion of micron-sized tracers at 500 fps over thousands of the flagellar beat cycles of algae. Our measurements reveal complex near-field flow structures, defying the common wisdom on the algal flow based on three Stokelets. We further develop a modified three-sphere model of algae, which qualitatively captures the unusual time-averaged 3D flow structure. Our study sheds new light on algal swimming and demonstrates the potential of holographic microscopy in imaging complex flows induced by microorganisms in their natural habitats.