Microswimmers

"For me this was among all the marvels that I have discovered in nature the most marvelous of all, and I must say that, for my part, no more pleasant sight has yet met my eye than this of so many thousands of living creatures in one small drop of water, all huddling and moving, but each creature having its own motion"

                                   -- Antonie van Leeuwenhoek, Letter to the Royal Society of London (1676)

Ever since Antonie van Leeuwenhoek first pointed his simple, single-lens microscope at a drop of water and described what he observed as “the most marvelous of all,” the motion of microscopic life has captivated both scientists and the public. In this hidden world, microorganisms exhibit remarkable agility, navigating their environments in ways that are both visually striking and scientifically rich. At these small scales, where inertia is negligible, microswimmers adopt propulsion strategies that differ fundamentally from the swimming behaviors familiar at macroscopic scales. Our research employs advanced imaging techniques to resolve the dynamics and motion of microswimmers, as well as the flow fields they generate. In parallel, we develop macroscopic model systems to investigate the hydrodynamic principles underlying the beating of flagella and cilia.

Representative topics of our research include:

(1) Swimming in complex fluids

(2) Flow fields around microswimmers

(3) Dynamics of beating flagella and cilia

The natural habitats of microorganisms in the human microbiome, ocean and soil ecosystems are full of colloids and macromolecules. Such environments exhibit non-Newtonian flow properties, drastically affecting the locomotion of microorganisms. Although the low-Reynolds-number hydrodynamics of swimming flagellated bacteria in simple Newtonian fluids has been well developed, our understanding of bacterial motility in complex non-Newtonian fluids is less mature. Even after six decades of research, fundamental questions about the nature and origin of bacterial motility enhancement in polymer solutions are still under debate. Here we show that flagellated bacteria in dilute colloidal suspensions display quantitatively similar motile behaviors to those in dilute polymer solutions, in particular a universal particle-size-dependent motility enhancement up to 80% accompanied by a strong suppression of bacterial wobbling. By virtue of the hard-sphere nature of colloids, whose size and volume fraction we vary across experiments, our results shed light on the long-standing controversy over bacterial motility enhancement in complex fluids and suggest that polymer dynamics may not be essential for capturing the phenomenon. A physical model that incorporates the colloidal nature of complex fluids quantitatively explains bacterial wobbling dynamics and mobility enhancement in both colloidal and polymeric fluids. Our findings contribute to the understanding of motile behaviors of bacteria in complex fluids, which are relevant for a wide range of microbiological processes and for engineering bacterial swimming in complex environments. 

• S. Kamdar, S. Shin, P. Leishangthem, L. F. Francis, X. Xu, and X. Cheng, “The colloidal nature of complex fluids enhances bacterial motility”, Nature 603, 819-823 (2022). 

• S. Kamdar and X. Cheng, “Swimming faster despite obstacles: a universal mechanism behind bacterial speed enhancement in complex fluids”, Microb. Cell 9, 139-140 (2022).

To swim, a microorganism must displace the surrounding fluid, generating a flow field with complex spatiotemporal structure that extends far beyond its own size. Far from being a mere byproduct of locomotion, this flow field plays a central role in microbiological processes, including nutrient uptake, sensing and communication with other cells, adaptation to changing environments, the rheology of active suspensions, and the emergence of collective, multicellular behaviors. As such, the flow field of a swimming microorganism is regarded as one of its fundamental characteristics. More broadly, resolving these flow fields provides insight into the essential functions of flagella and cilia in fluid transport—processes that are ubiquitous across all three domains of life. 

By combining conventional microscopy and holographic imaging with hydrodynamic modeling and simulations, our group has conducted one of the first studies to fully resolve the three-dimensional flow fields generated by unicellular microorganisms. Our results reveal striking and previously unrecognized features of the microscopic world. These include micron-scale vortex rings—analogues of smoke rings—and topological transitions in flow structure, phenomena long thought to arise only in the locomotion of much larger organisms, such as birds, fish, and insects, where inertia dominates. By capturing the complete three-dimensional flow field, this work also enables more accurate quantification of key biological properties, offering new insights into how microscopic swimmers move, interact, and survive.

• G. Pradipta, W. Lee, V. Tran, K. Welch, S. K. Sankar, Y. Kim, S. Kumar, X. Yong, J. Hong, S. Lim, and X. Cheng, “Seeing New Depths: Three-dimensional Flow of a Free-swimming Alga”, Phys. Rev. X 16, 021019 (2026) (Featured in Physics).

Flagella—motile, hair-like appendages that extend from the surface of cells—are ubiquitous across all three domains of life. They perform a wide range of essential biological functions, including motility, sensory perception, and fluid transport, primarily through their ability to drive flows in the surrounding fluid. Understanding the interaction between flagella and their fluid environment, particularly their capacity for fluid transport, is therefore central to addressing many fundamental questions in biology.

Our research group develops novel physical model systems alongside advanced imaging techniques to elucidate fluid-mediated flagellar dynamics in key biological processes. In particular, we investigate the mechanisms of flagellar bundling in bacteria, both in vivo and in macroscopic analog systems, to uncover the underlying biological and physical principles.

• S. Kamdar, D. Ghosh, W. Lee, M. Tătulea-Codrean, Y. Kim, S. Ghosh, Y. Kim, T. Cheepuru, E. Lauga, S. Lim, and X. Cheng, “Multiflagellarity leads to the size-independent swimming speed of peritrichous bacteria”, Proc. Natl. Acad. Sci. USA 120, e2310952120 (2023).   

• C. Zang, L. Omodt, M. Dasgupta, and X. Cheng, “Dynamics of rotating helices in a viscous fluid”, J. Fluid. Mech. 1013, R1 (2025).