In the course of time, some empirical rules have emerged that are very useful and of interest for rheological investigations.
The Cox-Merz rule states that the frequency dependence of the linear viscoelastic dynamic viscosity is equal to the shear rate dependence of the steady state shear viscosity. The advantage of this rule lies in using data taken from the linear viscoelastic dynamic experiments to complete and, to a lesser extent, substitute the steady state viscosity data. The dynamic oscillatory experiments are en general more accurate and a lot easier to perform than the steady state viscosity measurements. Although this Cox-Merz rule has often been observed to hold, at least approximately, it still has to be checked for every new material.
The mirror rule states that the time dependence of the linear viscoelastic start-up shear viscosity is equal to the shear rate dependence of the steady state viscosity. Here, also the linear start-up shear experiment is easier to perform than the steady state shear viscosity experiments.
Therefore, by only measuring the relatively simple dynamic shear viscosity, one can obtain information about the linear start-up and non-linear steady state shear viscosities.