Rheology of Active-Particle Suspensions

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.

Figure 1

Force dipoles for (a) a rowboat, (b) a “bacterium” with two flagella [15], and (c) a motor on a filament.

Figure 2

Disks (a) and (c) and rods (b) and (d) with active force densities attached along their symmetry axes, under shear (horizontal arrows). The parameter $W>0$ in (a) and (b) and $<0$ in (c) and (d).

Figure 3

Cytoplasmic streaming: Stress inhomogeneities induced by flow (thin arrows) induced in the ambient fluid due to active polymerization (at the right end) and depolymerization (at the left end) of an actin aggregate (cross-hatched region) should lead to mass flux from left to right.