(358b) Effects of Elasticity and Hydrodynamic Interactions on Locomotion in a Coarse-Grained Model of Monotrichous Bacteria

The locomotion of singly-flagellated (monotrichous) bacteria in viscous fluid is a quintessential example of movement at low Reynolds number. Observations of such organisms have shown that the hook protein connecting cell body and flagellum may buckle when the flagellum generates a large enough propulsive force. This local instability is sufficient to produce complex trajectories even for relatively rigid flagella. We first model a swimmer as rigid cell body and rigid flagellum (with simple toy models and a fully detailed model) connected by a flexible hook, then discretize the flagellum as a series of rods connected by flexible springs to incorporate elasticity. In the first case, the hook always buckles above a critical flexibility, and the resulting misaligned swimmer undergoes complex trajectories. In the second case, the flagellum itself cannot maintain its equilibrium helical shape above a critical flexibility, leading to significantly less propulsion than the rigid linked swimmer. The inclusion of hydrodynamic interactions affects the stability boundaries in all cases. These considerations may explain the biology and composition of particular organisms, and provide insight into optimal designs and materials for artificial microswimmers.