Below are some examples of the typical rheological profiling methods available from our lab. By employing high performance research rheometers we are able to provide viscosity and rheology data covering a range of shear conditions far in excess of those achievable on regular laboratory viscometers.
Viscosity Profiling
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. Most commonly, shear-thinning is observed where viscosity decreases with increasing shear rate. For such products a single viscosity value can be meaningless - instead a “flow curve” of viscosity against shear rate is required, from which the value at a shear rate relevant to the process or product usage conditions can be read.
“At-rest” Viscosity
Zero-shear viscosity, the viscosity of a product when effectively at rest, is a contributor to suspension and emulsion stability and a sensitive indicator of changes in a product resulting from ageing or changes in the formulation or process. A highly sensitive air-bearing rheometer such as our AR2000 is a necessity for measurement of zero-shear viscosity.
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. Models include: Bingham and Casson Power Law and Herschel Bulkley Sisko Cross and Carreau Power law model fit on mayonnaise viscosity profile Power Law model fit for Power Law Index and Consistency parameters.
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. Shown here is a 3-step thixotropy test for comparing recovery after application of two cosmetic skin products. The sample is subjected to an initial low-shear conditioning step, then a high-shear structure-breakdown step and finally a recovery step at the original low shear rate. The proportion of viscosity recovered at the end of the final step can
then be easily quantified. The second sample exhibits a poor recovery compared to the sample in the first graph.
Less thixotropic dispersion This sample exhibits a near-full recovery in step 3 following the high-shear of step 2.Highly thixotropic dispersion This sample exhibits only a minimal recovery after the high shear of step 2
Viscoelastic Characterisation and Oscillatory
Testing Oscillatory shear testing is employed to characterise and quantify the presence, rigidity and integrity of internal structure resulting from, for example, flocculation of dispersed particles or droplets, or cross-linking and entanglement of dissolved polymers.
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).
Oscillation Stress 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.
Oscillatory Frequency Sweeps
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.
