Fig. 1.

Bacterial suspensions under planar oscillatory shear. (A) Bacterial swarming at a concentration $n=80n_0$. (Scale bar, 20 $\mathrm{\mu }$m.) The fluorescently tagged $E.\hspace{0.17em}coli$ serve as tracer particles for particle imaging velocimetry (PIV). (B) Schematic showing our custom shear cell. A Cartesian coordinate system is defined, where $x$, $y$, and $z$ are the flow, shear gradient, and vorticity directions, respectively. (C) Temporal variation of mean suspension velocities $\dot{x}\left(t\right)$ at different heights, $y$, above the bottom plate. Red curves are for shear-rate amplitude ${\dot{\gamma }}_0=0.52$ s⁻¹. Black curves are for ${\dot{\gamma }}_0=0.21$ s⁻¹. Velocities are normalized by the imposed velocity amplitudes, $V_0$. Time $t$ is normalized by shear period $T=1/f$. $y$ is normalized by gap thickness $H$. $\dot{x}\left(t\right)$ at different $y$ are shifted vertically for clarity. Dashed lines are sinusoidal fits.