Abstract
Escherichia coli suspension, as a paradigmatic model system of active fluids, shows different rheological responses from passive suspensions under shear. To investigate the effect of suspension volume fraction, bacterial motility, and suspension temperature on the rheological properties of bacterial suspensions, the rheological response under steady shear is systematically measured using a rheometer with a double-gap cylinder measuring system. It is found that the unusual rheological responses (shear viscosity reduction and rheopexy) of E. coli suspension at low volume fraction are not prominent while the shear viscosity reduction and rheopexy are observed in E. coli suspension at high volume fraction. Characteristic shear rates are introduced to characterize the transition of rheological behavior of E. coli suspension at different shear rates. The comparison of rheological responses between motile and non-motile bacterial suspensions highlights the effect of bacterial mobility on shear viscosity and rheopexy. The non-Newtonian properties become prominent with increase in suspension temperature due to the enhancement of the motility of bacteria. It has been proved that the coupling effect of bacterial motility and the applied shear rate on rheology properties of E. coli systems can well be described by the steady shear model. Anisotropy of the swimming stress results in a normal stress difference, which leads to the viscosity hysteresis in the active fluids. The characteristic shear rates derived from the theoretical analysis agree well with the experimental results and values of characteristic shear rates augment accordingly with the enhancement of bacterial motility.