Boundary Effects on Low Reynolds Number Bacterial Robots

The dynamics of prokaryotic motility at low Reynolds number is important in a wide range of fields. Our experiment models the locomotion of a bacterium near a boundary using a robotic swimmer. Our robot uses a computer controlled DC motor that drives a helical flagellum made of wire. We maintain a low Reynolds number by placing the robot in highly viscous silicone oil with viscosity 10⁵ that of water and measure the forces and torques on the flagellum. Previous research measured helical propulsion far from a boundary (Rodenborn et al., PNAS 2013), but proximity to a boundary strongly affects the results. We have designed a system to precisely control the distance from the flagellum to the tank wall, and have made some of the first macroscopic measurements of boundary effects on helical propulsion. We find an approximately exponential dependence of the propulsive force and torque near the boundary, but we also find and unexpected asymmetry between CW and CCW rotation caused by the flow induced by the flagellum.