Scattering of a fast-swimming bacterium off of a surface

The sediment bacterium Thiovulum majus is one of the fastest known
bacteria. Each bacterium is spherical and covered in approximately 200
flagella, which propel cell at a speed of 600 micron/sec. When a cell
collides with a surface, it often becomes hydrodynamically trapped. A
bound cell exerts a force normal to the surface and rotates. These
cells self-organize into active two-dimensional crystals of rotating
cells. Here we present the first experimental observations of the
scattering of a fast-swimming bacterium off of a flat surface. We show
that the probability that a cell becomes bound decays exponentially
with the angle at which the cell approaches the surface; cells
colliding head on with the surface are most likely to become
trapped. We examine the stability of bound cell and measure the
timescale over which the cell orients normal to the surface. Finally
we show that when a cell escapes, it leaves the surface at a fixed
angle. These results give insight into the the ecology of fast-swimming
sediment microbes as they navigate the water saturated pore spaces in
which they live.