Swimming algae microbes showcase chaotic behavior in fluid flow

Fluid flows often affect the behavior of self-propelled particles, posing unpredictable issues for systems ranging from microscopic to massive — from bacterial heart and lung infections to ship navigation and ecosystem dynamics. The motion of particles in a flow can be affected dramatically by deterministic chaos, with very complicated behavior emerging from a very simple system and sensitive dependence on initial conditions.

Nghia Le and Thomas Solomon studied how simple fluid flows affect the motion of swimming organisms, examining algae microbes swimming in a flow composed of a chain of vortices. The research presented experimental evidence, supported by numerical simulations, of the effects of chaotic motion. It also revealed dynamic trapping, when swimming microbes that manage to penetrate ordered regions become trapped in them for extended periods of time.

“This is the first study to show experimental evidence that swimming microbes in a fluid flow can undergo chaotic motion, even for very simple and well-ordered flows,” said Solomon. “It’s also the first to present evidence of dynamic trapping, which plays a considerable role in the long-range transport of swimming microbes.”

The researchers introduced a new technique that teases out more data from the experiments by averaging measured trajectories.

“From a basic, fundamental scientific perspective, the evidence that chaos can have measurable effects in this realistic, noisy experiment implies there are likely other cases where simple models of chaotic behavior will have measurable effects in real systems,” said Solomon. “And we expect that the new trajectory calculation approach will be useful for other scientists exploring chaos.”

Source: “Chaos, dynamic trapping, and transport of swimming microbes in a vortex chain flow,” by Nghia Le and Thomas H. Solomon, Chaos (2025). The article can be accessed at https://doi.org/10.1063/5.0270869 .