  Self-propelled microorganisms, such as unicellular algae or bacteria, swim
along their director relative to the fluid velocity. Under a steady shear flow
the director rotates in close orbit, a periodic structure that is preserved
under an oscillatory shear flow. If the shear flow is subjected to small
fluctuations produced by small irregularities in the microchannel or by other
swimmers nearby, the director dynamics becomes stochastic. Numerical
integration of the swimmer motion shows that there is stochastic resonance: The
displacement in the vorticity direction is maximized for a finite noise
intensity. This transverse displacement resonance is observed when the
displacement is coarse grained over several periods, although the director is
preferentially oriented along the flow. The resonant noise intensity is
proportional to the oscillation frequency and independent of the shear rate.
The enhanced displacement can have effects on the transverse diffusion of
swimmers and the rheology of the suspension.
