Date of Thesis

Fall 2023

Description

We present experiments studying the motion and active mixing of swimming mi- crobes in laminar, vortex-dominated fluid flows. We are testing a theory that predicts the existence of swimming invariant manifolds (SwIMs) - invisible, one-way barriers blocking the paths of self-propelled tracers in the flow in one direction. We also pro- pose that the SwIMs together can form chute structures in three-dimensional phase space that facilitate cross-vortex transport of the microbes. We also observe evidence of how these structures promote long-range transport at different non-dimensional velocities (microbe’s velocity relative to flow velocity). Long-range transport is quan- tified by measuring the growth of the variance of an ensemble of particles over time. Our experiments show that at v0 = 0.4, the system is diffusive-like, as opposed to the sub-diffusion observed at lower velocities, while at v0 = 0.5, the transport looks super- diffusive. Sub-diffusive behaviors can be explained by considering the co-existence of ordered and chaotic regions in the flow for small swimming speed, where microbes can get caught around islands of ordered trajectories, mimicking trapped-like behav- iors. Evidence of the coexistence of ordered and chaotic trajectories will be presented through Poincar ́e sections of the flow, and chaotic mixing is observed through expo- nential separation between trajectories.

Keywords

Biophysics, Active matter, Fluid Mechanics, Chaos, chaotic mixing, active matter, Swimming microbes

Access Type

Honors Thesis

Degree Type

Bachelor of Science

Major

Biophysics

Minor, Emphasis, or Concentration

Mathematics

First Advisor

Tom Solomon

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