📝 SummarXiv

Abstract

A swimming microorganism stirs the surrounding fluid, creating a flow field that governs not only its locomotion and nutrient uptake, but also its interactions with other microorganisms and the environment. Despite its fundamental importance, capturing this flow field and unraveling its biological implications remains a formidable challenge. In this work, we report the first direct, time-resolved measurements of the three-dimensional (3D) flow field generated by a single, free-swimming microalga, Chlamydomonas reinhardtii, a widely studied model organism for flagellar motility. Supported by hydrodynamic modeling and simulations, our measurements resolve how established two-dimensional (2D) flow features such as in-plane vortices and the stagnation point emerge from and shape the 3D structure of the algal flow. More importantly, we reveal unexpected low-Reynolds-number flow phenomena including micron-sized vortex rings and a periodic train of traveling vortices and uncover topological changes in the underlying fluid structure associated with the puller-to-pusher transition of an alga, substantially deepening our understanding of algal motility. Biologically, access to the complete 3D flow field enables rigorous quantification of the alga's energy expenditure, as well as its swimming and feeding efficiency, significantly improving the precision of these key physiological metrics. Taken together, our study demonstrate extraordinary vortex dynamics in inertialess flows and advances fundamental knowledge of microhydrodynamics and its influence on microbial behavior. The work establishes a powerful method for comprehensively mapping the fluid environment sculpted by the beating flagella of motile cells.