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  • Featured in Physics
  • Open Access

Seeing New Depths: Three-Dimensional Flow of a Free-Swimming Alga

Gregorius Pradipta1, Wanho Lee2, Van Tran1, Kyle Welch1, Santosh K. Sankar3, Yongsam Kim4, Satish Kumar1, Xin Yong5, Jiarong Hong3,6 et al.

Sookkyung Lim7 and Xiang Cheng1,6

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Phys. Rev. X 16, 021019 – Published 24 April, 2026

DOI: https://doi.org/10.1103/2rr3-lbrn

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 challenge. Here, we report direct, time-resolved measurements of the three-dimensional (3D) flow field generated by a single, free-swimming microalga, Chlamydomonas reinhardtii, a model organism for microbial locomotion and flagellar dynamics. 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 full algal flow in 3D. Moreover, we reveal unexpected low-Reynolds-number flow phenomena including micron-sized vortex rings and periodically recurring translating vortices and uncover topological changes in the underlying flow structure associated with the puller-to-pusher transition of an alga. Biologically, access to the 3D flow field enables rigorous quantification of the alga’s energy expenditure, as well as its swimming and feeding efficiency, improving the precision of these physiological metrics. Taken together, our study demonstrates rich vortex dynamics in inertialess flows and shows their influence on microbial motility. The work also introduces an experimental method for mapping the fluid environment sculpted by beating flagella.

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Physics Subject Headings (PhySH)

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3D Recordings of Swimming Algae

Published 24 April, 2026

Measurements of the 3D fluid flow around a swimming microorganism could help researchers better understand the swimming dynamics of such microbes.

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Popular Summary

As microorganisms swim, they stir the fluid around them, generating complex flow patterns that extend far beyond their tiny bodies. These flows play an important role in how microorganisms interact with their environment and with one another. Using high-speed holographic microscopy, our study provides direct measurements of the complete 3D flow field around a freely swimming microalga. Our results reveal striking features of the microscopic world. These include tiny vortex rings—micron-scale analogs of smoke rings—and topological changes in flow structure, fluid phenomena long thought to be associated only with the locomotion of much larger organisms such as birds, fish, and insects, where inertia dominates. By capturing the full 3D flow field, this work also enables more accurate measurements of key biological properties, offering insights into how these microscopic swimmers live and move.

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Phys. Rev. X 16, 021019– Published 24 April, 2026
  • Received 8 September 2025
  • Accepted 16 March 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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