Apparent Non-Newtonian Behavior of Ionic Liquids

A significant viscosity
variation with the shear rate has been
observed for several ionic liquids in rheometry experiments above
a critical shear rate. Depending on the liquid and the rheological
conditions, both viscosity increase and decrease have been reported.
So far, these variations have been interpreted as a signature of a
non-Newtonian behavior. However, the measured critical shear rates
are orders of magnitude below the ones predicted by numerical simulations.
In this work, we perform new rheometry experiments with both ionic
liquids and Newtonian liquids to elucidate this discrepancy. For these
two types of liquids, both a viscosity decrease and increase have
been measured depending on the geometry of the rheometer and the zero-shear
viscosity of the liquid. We interpret the viscosity decrease as resulting
from viscous heating, since the viscosity of the investigated liquids
is also highly temperature-dependent, and the viscosity increase as
resulting from the development of instabilities at high shear rates.