VISCOSITY
IMPORTANT, BUT MUCH MORE TO IT!
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 with discrete steps that allow the sample to stabilize at each increment. These assays can be performed across temperatures ranging from 0 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 illustrates the basic principles and relationships of a viscosity determination.
Figure 2 compares the broad range of shear rates across common processes as well as the relative shear rate range of a traditional viscometer, rotational rheometer and capillary rheometer.
Figure 3 highlights an important potential oversight when using a viscometer to determine viscosity at a single shear rate instead of a rheometer measuring across many shear rates. This classic example using a rheometer shows mayonnaise (black curve) being more viscous than honey (gold curve) at lower shear rates (<14/sec), both have same viscosity at 14/sec, and then become less viscous than honey at >14/sec due to shear thinning. The response to increasing shear rate shows the shear thinning mayonnaise to be "non-Newtonian". In contrast, honey maintaining a constant viscosity is "Newtonian". The extent and rate of a shear thinned sample to rheologically rebuild can be quantified with a "Thixotropy".
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. Depending on product requirements, it can also be helpful to determine the extrapolated zero (low)-shear viscosity to model at rest as well as the terminal viscosity under high shear.
Figure 5 demonstrates the accuracy, precision and sensitivity of a rotational rheometer to discern among water standards and highly aqueous (Newtonian) formulations having very low (1-1.5x water) and narrow viscosity range.
Figure 6 demonstrates an approximately 50% viscosity increase of a food product using a single low shear rate to investigate the irreversible effect of temperature cycling to model a manufacturing process.
