Bacterial and Complex Fluids

In the first half of the talk, I will focus on the hydrodynamics of a single bacterium swimming in an infinite medium and near a surface. Peritrichously-flagellated bacteria, such as Escherichia coli, self-propel in fluids by using specialised motors to rotate multiple helical filaments. The rotation of each motor is transmitted to a short flexible segment called the hook which in turn transmits it to the flagellar filament, enabling swimming of the whole cell. Using a combination of computations and theory, I will show that the swimming of peritrichous bacteria is enabled by an elastohydrodynamic bending instability occurring for hooks more flexible than a critical threshold. It is also well-known that bacteria swim in circles near walls. However, certain species of bacteria with large cell bodies have been shown to become dynamically bound to surfaces with their flagellum pointing perpendicular to the surface. Using numerical simulations and a simplified theoretical analysis, I will show the existence of a fluid-structure interaction instability that causes cells with relatively short flagella to become surface bound. 

In the second half of the talk, I will talk about an electrohydrodynamic instability called Quincke rotation that involves the application of a uniform electric field giving rise to spontaneous rotation of dielectric solid particles. Placed near surfaces, this phenomena has been exploited to create ideal self-propelled particles to study collective motion. However, this system like most artificial active matter systems require a surface, consequently confining the collective motion to two dimensions. I will show that it is possible to convert this spontaneous Quincke rotation into spontaneous translation in a plane perpendicular to the electric field in the absence of surfaces by relying on geometrical asymmetry instead. Suspensions of randomly shaped particles under Quincke rotation interacting electrohydrodynamically are thus expected to perform collective motion by exploring the full three-dimensional space, thereby opening doors to a potentially new type of active matter.