Chapel Hill, North Carolina USA
Understanding Viscosity And Its Impact On Material Properties
Rheology Testing Services measures viscosity with either rotational (1-direction, see video below) or "complex viscosity (n*)" with oscillatory (back-forth, see video in "Frequency Sweep") methods with a broad range of parameters and conditions. The most requested assay is the rotational "shear rate ramp" that measures viscosity (resistance to flow) with an increasing shear rate (applied upper plate movement, Figure 1) to obtain flow curves. For reference, Figure 2 shows shear rates corresponding to common processes. Figure 3 below, compares flow curves for mayonnaise (non-Newtonian) that decreases viscosity with increasing shear rate (shear thinning); whereas, honey (Newtonian) remains relatively unchanged. This sensitive and often discriminating assay can detect important rheological properties and subtle differences that may not readily be observed with a traditional viscometer (Figures 4-5).
Depending on the questions to be answered, brief descriptions of other assays that also determine viscosity are found in the above "Assays" tab such as Yield Stress, Thixotropy, Temperature Sweep and Time Sweep. Automated and very flexible assay macros apply highly controlled continuous (non-equilibrium) or stepwise (nearer equilibrium) steps using increasing/decreasing shear rate, shear stress and temperature ramps to efficiently model various processing and handling conditions.
Depending on material properties, some materials cannot be properly assayed by rotational methods. Fortunately, various oscillatory methods that measure various viscoelasticity properties with deformation rather than flow can be used (see "Assays" tab above). It is important to note that viscosity determined with oscillatory assays is defined as "complex viscosity" (n*) that may not necessarily match values obtained with traditional rotational assays at equivalent shear.​​ Refer to "Cox-Merz Rule" under "Helpful Links" tab for more information. Although "complex viscosity" may not necessarily match the "absolute viscosity" values obtained with rotational assays, it is a very useful measurable for rheological comparison among samples. Refer to the introduction section under "Frequency Sweep" for more details.​​​​​​​​​​​​​​
Checkout the 15 second video showing plate and sample movement during an abbreviated shear rate ramp assay.
Figure 1 below illustrates the basic principles and relationships to determine viscosity with rotational methods that are based on flow. It is important to note that terms and plate movement differ for oscillatory methods such as amplitude sweep and frequency sweep that are based on deformation not illustrated below. It is also important to note that the shear rate unit "sec-1" or "1/sec" is not the same as revolutions per minute (rpm) used for rotational assays or Hz (cycles/sec) that is used for oscillatory assays.
.jpg)
Figure 2 compares the broad range of shear rates across common processes as well as the operating shear rate range of a traditional viscometer, rotational rheometer and capillary rheometer. Refer to "Rheometer vs Viscometer" for more details.
Since viscosity often changes with shear rate (or shear stress), Figure 3 highlights an important potential oversight when measuring and reporting viscosity with a single shear rate instead of measuring across a broad range of shear rates. With this in mind, when reporting viscosity it is important to know and report the shear rate or shear stress (see Shear Stress Ramp) used to measure viscosity; otherwise the viscosity value may be meaningless. Unfortunately, this is a common situation that causes confusion.
Figure 3 is a classic example illustrating using a rheometer to easily generate flow curves obtained with a continuous shear rate ramp. Mayonnaise (black curve) is more viscous than honey (gold curve) only at lower shear rates (<14/sec), both mayonnaise and honey have same viscosity at 14/sec, and then becomes less viscous than honey at >14/sec due to shear thinning. The response to increasing shear rate shows the shear thinning mayonnaise is "non-Newtonian". In contrast, honey maintaining a relatively constant viscosity is "Newtonian". This example highlights the importance to include the shear rate used to determine a specific viscosity result.
A "Thixotropy" assay is another routine and helpful assay that quantifies viscosity changes during low shear (baseline (Step1)), then high shear (thinning (Step2)), and return to the same low shear as Step 1 to measure the extent and rate of rebuilding (Step3).
Figure 4 demonstrates the ability for a rheometer to easily discern among oil-based formulations with varying amounts of surfactant that easily shear thin (non-Newtonian) at a relatively low shear rate. The viscosity difference among samples typically appreciably decreases with increasing shear rate.
Figure 4B shows that depending on sample properties, a "Newtonian plateau" at low shear rates can be helpful to determine the extrapolated zero-shear viscosity (y-intercept) to model "at rest" as well as the terminal viscosity under high shear. Zero-shear viscosity values can be useful to rank order molecular weights for macromolecules.
Figure 5 demonstrates the accuracy, precision and sensitivity achieved with a rotational rheometer to discern among water standards (black curves) and highly aqueous formulations, each assayed in triplicate having very low viscosity (1-1.5x water) within a very narrow range (1-1.5cP).
Figure 6 shows increasing, then decreasing stepwise shear rate ramps (30 second hold each step) over 0.01 to 200sec-1 (per client specifications) to quantify the different shear thinning and subsequent rebuilding properties for 2 polish products to model a manufacturing process.
A "Thixotropy" assay is another very routine and helpful assay that quantifies rheological behavior of sample during low shear (baseline (Step1)), then high shear (thinning (Step2)), and return to the same low shear as Step 1 to measure the extent and rate of rebuilding (Step3).
Figure 7 shows viscosity changes (blue curve) during temperature cycling (red curve) for a food product using a single low shear rate (0.1sec-1) to investigate the irreversible effect of temperature cycling to model a manufacturing process. More information can be found in Temperature Sweep.
