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.

principles of viscosity realting stress, rate, force, area, strain, time, displacment and height

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.

Comparing shear rates with processes with capability of viscometer and 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".

Comparing shear thinning of non-Newtonian mayannaise with Newtonian honey with increasing shear rate

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.

using shear rate ramp to compare shear thinning formulations.  Comparing log-log and linear-log plots

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.

shear rate ramp to discern among formulations having viscosity near water

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. 

Viscosity Fig 6 Temperature cycling at single shear rate.jpg