Reentrant transition in the shear viscosity of dilute rigid-rod dispersions.

The intrinsic viscosity of a dilute dispersion of rigid rods is studied using a recently developed direct numerical simulation (DNS) method for particle dispersions. A reentrant transition from shear-thinning behavior to the second Newtonian regime is successfully reproduced in the present DNS results around a Peclet number Pe=150, which is in good agreement with our theoretical prediction of Pe=143, at which the dynamic crossover from Brownian to non-Brownian behavior occurs in the rotational motion of the rotating rod. For Pe>Pe(c) at which the dynamic crossover occurs, the rigid rod undergoes non-Brownian rotational motion, which means that the rotational frequency is proportional to the shear rate. This crossover occurs only when the effect of the shear rate is dominant over the effect of the thermal fluctuation for all areas. Otherwise, the rotational motion is dominated by the thermal fluctuation. The viscosity undershoot is observed in our simulations before reaching the second Newtonian regime. The physical mechanisms behind these behaviors are analyzed in detail.