Thixotropy in macroscopic suspensions of spheres

An experimental study of the viscosity of a macroscopic suspension, i.e., a suspension for which Brownian motion can be neglected, under steady shear is presented. The suspension is prepared with a high packing fraction and is density matched in a Newtonian carrier fluid. The viscosity of the suspension depends on the shear rate and the time of shearing. It is shown that a macroscopic suspension shows thixotropic viscosity, i.e., shear thinning with a long relaxation time as a unique function of shear. The relaxation times show a systematic decrease with increasing shear rate. These relaxation times are larger when decreasing the shear rates, compared to those observed after increasing the shear. The time scales involved are about $10000$ times larger than the viscous time scale ${\tau }_{\mathrm{visc}}{=a}^2/\nu$ and about 1000 times smaller than the thermodynamic time scale ${\tau }_{\mathrm{therm}}=\mathrm{Pe}/\dot{\gamma }.$ (a is the gap width of the viscometer, ν is the kinematic viscosity, $\mathrm{Pe}=6\mathrm{}\pi \eta \dot{\gamma }\tau {)}^3{/(k}_{B}T)$ is the Péclet number and $\dot{\gamma }$ is the shear rate.) The structure of the suspension at the outer cylinder of a viscometer is monitored with a camera, showing the formation of a hexagonal structure. The temporal decrease of the viscosity under shear coincides with the formation of this hexagonal pattern.