Title: Experiments and Simulations of Bacterial Motility Near Surfaces 

Abstract: Motile bacteria commonly experience hydrodynamic effects of a nearby surface in their fluid environment. We use biological experiments, numerical simulations, and dynamically similar macroscopic experiments to quantify the forces and torques on microorganisms that swim near a boundary using a helical flagellum. Using the computational method of images for regularized Stokeslets with parameter values measured from biological experiments, we compute the forces and torques exerted on bacteria of different body sizes and helical geometries moving near a boundary. These results are compared with dynamically similar macroscopic experiments and theoretical predictions. Our numerical simulations are verified for rotating cylinders and rotating helices of different wavelengths. We calculate the propulsive efficiency of bacterial motion and find it increases for all helical wavelengths near a boundary and that the maximum increase in efficiency occurs for biologically relevant helical wavelengths. These results suggest that hydrodynamic boundary effects are an important factor in bacterial evolution.