Fluid Mechanics Seminar: David Saintillan

Active suspensions, of which a bath of swimming microorganisms is a paradigmatic example, denote large collections of individual particles or macromolecules capable of converting chemical fuel into mechanical work and microstructural stresses in a viscous liquid. Such systems, which have excited much research in the last decade, exhibit complex dynamical behaviors such as large-scale correlated motions, pattern formation, enhanced diffusivities and mixing as a result of hydrodynamic interactions. In this work, we use a combination of theory and simulations to analyze these effects. First, a kinetic model is constructed and applied to elucidate the onset of spontaneous chaotic flows in semi-dilute suspensions. In isotropic and uniform systems, a linear stability analysis reveals the existence of a long-wave hydrodynamic instability for the active particle stress arising from self-propulsion, which drives large-scale fluctuations in suspensions of rear-actuated swimmers, or pushers, when the product of the linear system size with the suspension volume fraction exceeds a given threshold. This instability is confirmed by direct numerical simulations based on a slender-body model for interacting self-propelled particles, and is shown to capture all the salient features of experiments on bacterial suspensions. Extensions of these models to describe chemotactic interactions with an external oxygen field, the effective rheology of active suspensions, as well as the effects of confinement by rigid boundaries will also be discussed.