(408h) Hydrodynamic Trapping of Bacteria Near Walls

Recent experiments have shown bacteria like T. Majus and E.Coli can get trapped near walls with their flagella pointing away from the wall in the perpendicular direction into the fluid. The flagella continues rotating in this upright position in a counter-clockwise direction while pushing cell body towards the wall. This configuration appears to be unstable as any perturbation in the flagella orientation should make it unstable thereby causing the cell body to start swimming parallel to the wall. In this work, we focus on T. Majus cells that have a large spherical cell body and assume that it has a single helical flagellum that propels the organism. Using a combination of theory and full numerics based on boundary element method, we discover a transition from swimming to no-swimming as the cell body gets very close to the wall. In particular, we show that the lubrication forces acting on the cell body can cause it to rotate making the flagellum point perpendicular to the wall. The complex interplay between the hydrodynamic interaction of the flagellum and the cell body with the wall determines a critical flagellar length above which swimming is not possible and the bacterial cell gets trapped.