Transport and dynamics of swimming microorganisms in chaotic flows: a Lagrangian perspective

Microorganisms form the basis of marine food web and play crucial roles in Earth’s biogeochemical cycles. Habitats of microorganisms, from oceans to lakes to rivers, are often characterized by dynamic fluid motion. Fluid flow exerts forces and torques on microorganisms that affect their movement and distribution, while also transporting chemicals crucial for their sensing, foraging, and mating. As a result, flow has a broad range of effects on the microbial behaviors, including locomotion, reproduction, and nutrient uptake. Despite significant efforts to understand microbiology in dynamic flow environments, it remains challenging to interpret the physical and biological behaviors of microorganisms in the presence of fluid flows, particularly unsteady and chaotic flows.

In this talk, I present a Lagrangian framework for understanding the transport and dynamics of planktonic and swimming microorganisms in chaotic flows. Unlike the classic Eulerian approach, which is based on spatial quantities such as velocity and vorticity, this framework is built upon the Lagrangian coherent structures (LCSs). First, I will discuss the interaction between swimming organisms and the hyperbolic LCSs (i.e., material surfaces maximally attracting or repelling nearby fluid trajectories). The alignment and accumulation of microorganisms near hyperbolic LCSs can reduce in large-scale transport while enhancing small-scale mixing. Second, I will show the interaction between microorganisms and elliptic LCSs, (i.e., rotation-dominated material surfaces that resemble coherent vortices). Microorganisms are found to escape and deplete within these vortex-like structures, which can lead to enhanced transport barriers for passive scalar quantities such as temperature, salinity, and contaminants. Overall, these results enable quantitative predictions of microorganism transport in chaotic flows and provide useful insights into the spread of algal blooms and the bio-mitigation of oil spills.