  The accumulation of swimming bacteria near surfaces may lead to biological
processes such as biofilm formation and wound infection. Previous experimental
observations of Vibrio alginolyticus showed an interesting correlation between
the bacterial entrapment near surfaces and the concentration of NaCl in the
swimming medium. At higher concentrations of the ions, V.alginolyticus in the
puller mode (with flagella in front of the body) tends to stay close to the
surface whereas in the pusher mode (with flagella behind the body) it is more
likely to escape from the surface. Motivated by these observations, we
numerically investigate the locomotion of a uniflagellated model bacterium in
unbounded fluid and near a planar surface. In our elastohydrodynamic model, the
boundary integral technique and Kirchhoff rod model are employed respectively
to calculate the hydrodynamic forces on the swimmer and model the elastic
deformations of the flagellum consisting of a short, flexible hook and a long,
relatively stiff filament. Our numerical results demonstrate that hydrodynamic
interactions between the model bacterium and the solid wall cause the puller
type to be attracted to the surface, whereas the pusher type is either repelled
from or attracted to the surface depending on the flagellum and hook stiffness,
the ion concentration (which determines the motor torque), and the cell body
aspect ratio. We also show that large deformations of a very flexible hook can
lead to an abrupt reorientation of the cell when the bacterium encounters a
solid surface. These research findings can be used not only in understanding
uniflagellated bacterial behavior but also in designing bacteria-mimicking
micro-robots with biomedical and environmental monitoring applications.
