  Thiovulum majus, which is one of the fastest known bacteria, swims using
hundreds of flagella. Unlike typical pusher cells, which swim in circular paths
over hard surfaces, a T. majus cell turns its flagella normal to the surface.
To probe the torques that stabilize this unusual fixed point, the trajectories
of several thousand collisions between a T. majus cell and a wall of a
quasi-two-dimensional microfluidic chamber are analyzed. Measuring the fraction
of cells escaping the wall either to the left or to the right of the point of
contact -- and how this probability varies with incident angle and time spent
in contact with the surface -- maps the scattering dynamics onto a first
passage problem. These measurements are compared to the prediction of a
Fokker-Planck equation to fit the angular velocity of a cell in contact with a
hard surface. This analysis reveals a bound state with a narrow basin of
attraction in which cells orient their flagella normal to the surface. The
escape angle predicted by matching these near field dynamics with the far field
hydrodynamics is consistent with observation. We discuss the significance of
these results for the ecology of T. majus and their self organization into
active chiral crystals.
