Generally speaking, fluids with larger, more complex, molecules will have higher viscosities. In such liquids the time delay needed for the viscosity to change suggests that a certain amount of time is needed to re-arrange or align the structural components that results in decrease in the viscosity of the test material. Put in another way, the material shows thinning behaviour with time when it is sheared at a constant rate. Another type of viscous behaviour exhibited by fluid foods and polymer systems is thixtropy which is again shear-thinning of the material with increasing rates of shear, but is also dependent on the duration of shear. This is referred to as time independent (steady state) flow and materials showing this type of behaviour are called pseudoplastic. The viscosity of some fluids is dependent on the rate used to shear the material, a high rate of shear making the fluid thinner compared with the fluid that was sheared more slowly. If this is not observed then the liquid is non-Newtonian.
An easy way to demonstrate Newtonian behaviour is to double the shear stress during a viscosity test and this should result in doubling of the shear rate. Newtonian behaviour is displayed by simple liquids consisting of small molecules that do not interact or form any connected structure. However, it must be pointed out that long chain polymers at low concentration can also show Newtonian behaviour.
