  As a paradigmatic model of active fluids, bacterial suspensions show
intriguing rheological responses drastically different from their counterpart
colloidal suspensions. Although the flow of bulk bacterial suspensions has been
extensively studied, the rheology of bacterial suspensions under confinement
has not been experimentally explored. Here, using a microfluidic viscometer, we
systematically measure the rheology of dilute E. coli suspensions under
different degrees of confinement. Our study reveals a strong confinement
effect: the viscosity of bacterial suspensions decreases substantially when the
confinement scale is comparable or smaller than the run length of bacteria.
Moreover, we also investigate the microscopic dynamics of bacterial suspensions
including velocity profiles, bacterial density distributions and single
bacterial dynamics in shear flows. These measurements allow us to construct a
simple heuristic model based on the boundary layer of upstream swimming
bacteria near confining walls, which qualitatively explains our experimental
observations. Our study sheds light on the influence of the boundary layer of
collective bacterial motions on the flow of confined bacterial suspensions. Our
results provide a benchmark for testing different rheological models of active
fluids and are useful for understanding the transport of microorganisms in
confined geometries.
