BASIC, BUT IMPORTANT!
Viscosity can be determined by several approaches across a wide range of conditions. These may include assaying across an increasing followed by decreasing shear rate (or shear stress) with either a continuous non-equilibrium ramp or step-wise that allows the sample to more stabilize at each increment. These assays can be performed with temperatures ranging from 5 to 180C. This sensitive and often discriminating assay can detect important rheological properties and subtle differences that may not be observed with a traditional viscometer.
Figure 1 compares the effect of stepwise increasing shear rate on liquids and solids showing a viscosity decrease due to shear thinning.
Figure 2 highlights an important oversight when using a viscometer instead of a rheometer to compare viscosity vs shear rate. In this classic example using a rheometer, mayonnaise (black curve) is more viscous than honey (gold curve) at lower shear rates (<14/sec), both have same viscosity at 14/sec, and then becomes less viscous than honey with increasing shear rate due to shear thinning.
Figure 3 demonstrates the sensitivity of this assay showing viscosity of a polysaccharide in PBS at increasing concentrations within a relatively low and narrow viscosity range. Visually they appeared to have same viscosity.
Figure 4 demonstrates the ability to easily discern among oil samples with increasing amounts of surfactant that easily shear thin at a relatively low shear rate.