Seeing new depths: visualizing three-dimensional flow of free-swimming alga with holographic microscopy

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. Two-dimensional flow around different microswimmers has been well-characterized and is generally considered as a milestone in experimental fluid mechanics in the past decade, but full three-dimensional (3D) mapping of the microswimmers’ flow structures 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 algae’s flagellar beat cycles. Our measurements reveal complex near-field flow structures, defying the common wisdom on the algal flow based on three Stokelets. The flow structures reveal the significance of inertial effects of the flagella’s periodic motions at positions within 2-3 alga body lengths. We further develop a mathematical model based on the fundamental solution of oscillating point forces, which quantitatively captures the unusual algal 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.