Understanding the effect of confinement on the viscosity of bacterial suspensions*

Bacterial suspensions, a premier example of active fluids, show intriguing rheology different from their counterpart colloidal suspensions. When the confinement length scale is comparable or smaller than the intrinsic length scale of bacterial suspensions, we expect to see a qualitative change of their flow behaviors. Here, by using a microfluidic channel viscometer, we investigate the viscosity of E. coli suspensions under different degrees of confinement. For low concentration suspensions, we observe strong shear thickening at low flow rates. More importantly, a strong confinement effect is found when the confinement length is comparable to the running length of single bacterium. The viscosity of bacterial suspensions decreases by a factor of 2.5, when the confinement decreases from 50 down to 25 microns. In contrast, for high concentration suspensions, we observe strong shear thinning at low shear rates. The confinement leads to an increase rather than decrease of viscosity when the confinement length is comparable to the size of collective swarming of bacterial suspensions. Our study reveals the dynamics of bacterial suspensions under confinement and provides new insights into the transport of active fluids in confined geometries.