Examples of Rheology Testing Techniques
Emulsions, suspensions, solutions and gels are all examples of non-Newtonian fluids – that is, their viscosity is not a fixed value but is dependent upon the degree of shear they are exposed to. By far the most common form of non-Newtonian behaviour is shear-thinning where viscosity decreases with increasing applied shear rate. Emulsions, suspensions, polymers solutions and gels are all examples of shear-thinning materials. Shear thinning properties can provide desirable attributes to a product, such as suspension stability or drip resistance when at rest but ease of application or pouring when a stress is applied.
The image on the right shows a comparison of shear rate sweeps results of Golden Syrup and mayonnaise. The contrast is striking, with the syrup showing a near-Newtonian viscosity at around 35 Pascal seconds (35000 centipoise) while the mayonnaise shows a significantly-shear-thinning profile starting at a viscosity of around 80 Pa.s at a shear rate of 1 1/s and shearing down to 3 Pa.s at a shear rate of 100 1/s, crossing over the syrup profile at around 3 1/s shear rate.
For shear-thinning products measuring viscosity at a single shear rate obviously only provides a tiny glimpse of the "full picture" - instead a “flow curve” of viscosity across a range of shear rates is far more meaningful, from which a viscosity value at a shear rate relevant to the process or product usage conditions can be read.
In the mayonnaise example above viscosity rises with decreasing shear rate. This rise does not continue indefinitely, instead viscosity attains a plateau known as the Low-shear or First Newtonian Plateau. The viscosity at this point is the zero-shear viscosity. Zero-shear viscosity of a suspending medium is a key contributor to suspension and emulsion stability as it is inversely related to the terminal settling or creaming velocity of particles or droplets. Zero-shear viscosity is also a qualitative indicator of molecular weight of a dissolved polymer and so can be used for batch to batch comparisons of incoming raw materials such as thickeners and gelling agents. It is also a sensitive indicator of changes in a product resulting from ageing or changes in the formulation or process.
The graphic here shows the clear zero-shear plateaux forming at the low-shear end of viscosity profiles of three coating products. Note that at shear rates of 1 1/s and above the viscosities have converged and even crossed over! A single-point viscosity reading at this part of the profile - typical for a simple viscometer test - would completely miss the highly-significant differences in zero-shear viscosity between the coatings.
Zero-shear viscosity measurement necessitates the use of a highly sensitive instrument that can impart very small stresses and measure incredibly low shear rates. For this reason air-bearing rheometer such as our AR2000 is a necessity for measurement of zero-shear viscosity.
Yield Stress TestingYield stress is the stress that must be applied to initiate significant flow in a structured liquid. This flow initiation occurs when a stress is applied that is sufficient to disrupt a materials gel structure. The thickening effects of that structure are then lost, viscosity decreases markedly (sometimes by many orders of magnitude) and the material makes the transition from an apparent solid to a free-flowing liquid.
Yield stress is hugely under-appreciated; it determines many aspects of a materials processing, handling, storage and performance properties, such as:
- Application behaviour, skin feel of cosmetics and in-the-mouth texture of foods.
- Ease of process start-up, mixing, pumping and filling.
- Coating and drainage properties.
Rheology Model Fits for Pump-sizing and Process Design
A range of rheological models can be fitted to quantify flow curves and obtain parameters for inputting into process design and engineering calculations. These models are typically fitted to shear stress/shear rate flow curves or viscosity/shear stress or viscosity/shear rate profiles. Popular models include:
- Bingham and Casson
- Power Law (Ostwald) and Herschel Bulkley
- Cross and Carreau
The graphic shows a a simple Power Law model fit on a mayonnaise sample tested across the range 20 to 200s-1 shear rate. Consistency (K) and Power Law Index (n) parameters are then reported.
Viscosity Temperature Relationships
Viscosity typically, but not always, exhibits an inverse relationship with temperature. A viscosity-temperature profile can be obtained under defined imposed shear conditions relevant to the process.Viscosity temperature profiles of oils.
Thixotropy: Viscosity and Structure Recovery After Shearing
Following a period of earlier shearing from, for example, a mixing, filling or coating process, some products will very quickly recover their viscosity whereas others will go on building viscosity slowly for hours, days or even weeks. A product that exhibits a long, slow viscosity build to a highly-structured state after shearing is termed thixotropic.
Various methods are available to quantify thixotropic recovery rates. Two excellent methods are the viscometric 3-step thixotropy test and the oscillatory 3-step thixotropy test.
The graphic below shows the Viscometric 3-step Thixotropy Test:
In this test shear rate is stepped from low (1 1/s) to high (100 1/s) and back to low (1 1/s) whilst changes in viscosity are recorded throughout. Particular attention is paid to the rate of viscosity decrease in the second step and the rate of viscosity recovery in the final step.
This graphic shows an example of the Oscillatory 3-step Thixotropy Test:
Here the sample is subjected to step changes in applied oscillation strain, from below-the-yield low values to above-the-yield high and back again. The destruction and subsequent recovery of sample network structure (i.e. colloidal gel networks) is observed throughout.
Viscoelastic Characterisation and Oscillatory Rheology Testing
Oscillatory shear testing is a general term covering a range of techniques that can be deployed to characterize and quantify the presence, rigidity and integrity of a material's internal structure resulting from, for example, flocculation and interaction of dispersed particles or droplets, or cross-linking and entanglement of dissolved polymers.
Oscillation rheology testing is almost always performed at very low applied stresses and strains, often significantly below the yield point of a sample.
Typically measured parameters include: Complex modulus (G*), Elastic (or storage) modulus (G') and viscous (or loss) modulus (G"), Phase angle (δ) and tangent of the phase angle (tan δ).
Phase angle and complex modulus together can define a viscoelastic map (see graphic) differentiating between elastic solids and viscous liquids (left to right) and high to low rigidity or viscosity (top to bottom).
Oscillation Stress Sweeps and Strain Sweeps
Oscillation stress and strain sweeps provide easy-to-interpret information about the soft-solid rigidity and yield stress (gel strength) of even delicately-structured fluids such as fruit juices and thickened drinks, "light-touch" lotions and sedimentation-resistant suspensions. These powerful tests are easy to perform and yet provide a deep insight into the rigidity and strength of the soft-solid structures that impart critical quality attributes in many manufactured products.
In the oscillation stress or strain sweep the sample is subjected to small amplitude oscillatory (i.e. clockwise then counter clockwise) shear. In the early stages of the test stress is sufficiently low to preserve structure but as the test progresses the incrementing applied stress causes the ultimate disruption of structure – the yield process – manifested as a decrease in elasticity (phase angle rises) and an accompanying decrease in rigidity (complex modulus).
Oscillatory frequency sweeps allow us to probe and identify the nature of the structuring mechanisms present in a fluid. The sample is exposed to small-deformation oscillations covering a range of frequencies to assess the structural response to deformations of longer or shorter timescales. The graphic shows how the technique can differentiate between the "relaxable" structure found in a polymer solution (where polymers disentangle to dissipate stored stresses) and the more permanent elasticity found in a flocculated suspension. This technique can prove a useful tool when attempting to match textures and flow behaviours in thickened systems.
Oscillatory Frequency Sweeps
Creep testing is a very useful technique that can be performed on a controlled-stress rheometer. The test entails applying a very small constant shear stress onto a sample and observing the resulting elastic deformation and/or viscous flow. The test has two particular uses:
- To obtain a high quality zero-shear viscosity measurement - by the sustained application of a shear stress that is significantly within the linear viscoelastic limit for the material under test.
- To simulate situations and explore product behaviour where low stresses are maintained for a period of time, for example in draining, sedimentation, sagging or leveling scenarios.
Like to learn more?
If you would like some advice on how these test methods could be applied to your products please don't hesitate to ask us a question.