Session Y46: Invited Session: Novel Approaches to Understanding the Behavior of Self-Propelled Particles

Bacterial transport: From flagellar mechanics to unmixing

Swimming bacteria play integral roles in processes ranging from infections in the human body to bioremediation in the environment. Understanding the physical mechanisms underlying bacterial transport is key to controlling these important process. Using high-speed video microscopy and microfluidic devices, we uncover surprising mechanics at the level of individual cells that lead to complex motility patterns. We describe a novel mechanism enabling bacteria with a single flagellum to reorient, whereby their propulsive thrust induces a buckling instability in their flagellum. We also show that hydrodynamic shear produces striking spatial heterogeneity in suspensions of otherwise randomly-swimming bacteria. This shear-induced `unmixing' phenomenon directly impacts bacterial survival strategies, by suppressing chemotaxis and enhancing surface attachment.   

Swimming dynamics of flagellated bacteria in liquid crystal

A flagellated bacterium swimming in water generates a flow with a typical scale of its body length. Anisotropy of the surrounding liquid significantly affects the swimming dynamics of bacteria and modifies the flow pattern created by a single bacterium. Using the particle tracking technique and flow reconstruction method we investigated the structure of the flow generated by bacteria and pairwise interactions between bacteria in liquid crystals. We demonstrated that while the rotation rate of bacterial flagella is reduced by an order of magnitude due to increased viscosity, the bacteria swimming speed is slowed only by 25-30{\%}. Due to the strong anisotropy of viscosity in liquid crystal the bacteria-induced flow is localized along a bacterial body: the flow along a line coaxial with the bacterial body is much stronger than in perpendicular direction and decays rather slowly. We found that interaction between flagella bundles of two close-by bacteria is negligible and the observed convergence of the swimming speeds and flagella waves may occur due to viscoelastic interaction between bacterial bodies.   

Quantifying and controlling microbial swimming

Interactions between swimming cells, surfaces and fluid flow are essential to many microbiological processes, from the formation of biofilms to the fertilization of human egg cells. Yet, relatively little remains known quantitatively about the physical mechanisms that govern the response of bacteria, algae and sperm cells to flow velocity gradients and solid surfaces. A better understanding of cell-surface and cell-flow interactions promises new biological insights and may advance microfluidic techniques for controlling microbial and sperm locomotion. In this talk, I will summarize our recent efforts to quantify the surface interactions of bacteria, unicellular green algae and mammalian spermatozoa. This joint experimental and theoretical work shows that the subtle interplay of hydrodynamics and surface interactions can stabilize collective bacterial motion, that direct ciliary contact interactions dominate surface scattering of eukaryotic biflagellate algae, and that rheotaxis combined with steric surface interactions provides a robust long-range navigation mechanism for sperm cells.   

Entrapment, escape, and diffusion of swimming bodies in complex environments

We will begin by addressing the hydrodynamic entrapment of a self-propelled body near a stationary spherical obstacle. Simulations of model equations show that the swimmer can be trapped by a spherical colloid larger than a critical size, that sub-critical interactions tend to result in short residence times on the surface, and that the basin of attraction around the colloid is set by a power-law dependence on the colloid size and dipole strength. With the introduction of Brownian fluctuations, swimmers otherwise trapped in the deterministic setting can escape from the colloid at randomly distributed times. The distribution of trapping times is governed by an Ornstein-Uhlenbeck process, resulting in nearly inverse-Gaussian or exponential distributions. Analytical predictions are found to match very favorably with the numerical simulations. We also explore the billiard-like motion of such a body inside a regular polygon and in a patterned environment, and show that the dynamics can settle towards a stable periodic orbit or can be chaotic depending on the nature of the scattering dynamics. We envision applications in bioremediation, sorting techniques, and the study of motile suspensions in heterogeneous or porous environments.