  The viscosity of supercooled water has been a subject of intense study, in
particular with respect to its temperature dependence. Much less is known,
however, about the influence of dynamical effects on the viscosity in its
supercooled state. Here we address this issue for the first time, using
molecular dynamics simulations to investigate the shear-rate dependence of the
viscosity of supercooled water as described by the TIP4P/Ice model. We show the
existence of a distinct cross-over from Newtonian to non-Newtonian behavior
characterized by a power-law shear-thinning regime. The viscosity reduction is
due to the decrease in the connectivity of the hydrogen-bond network. Moreover,
the shear thinning intensifies as the degree of supercooling increases, whereas
the cross-over flow rate is approximately inversely proportional to the
Newtonian viscosity. These results stimulate further investigation into
possible fundamental relations between these nonequilibrium effects and the
quasi-static Newtonian viscosity behavior of supercooled water.
