We study the interplay of activity, order and flow through a set of
coarse-grained equations governing the hydrodynamic velocity, concentration and
stress fields in a suspension of active, energy-dissipating particles. We make
several predictions for the rheology of such systems, which can be tested on
bacterial suspensions, cell extracts with motors and filaments, or artificial
machines in a fluid. The phenomena of cytoplasmic streaming, elastotaxis and
active mechanosensing find natural explanations within our model.Comment: 3 eps figures, submitted to Phys Rev Let