The physical form of any personal care product is one of the first attributes that potential customers evaluate when considering purchasing a product, along with color and fragrance. If the form doesn’t allow for easy access from the package or smooth application, customers may not purchase or repurchase. Even the way a product flows while in the package can impact this perception. Although this is commonly referred to as viscosity, the accurate scientific term is rheology and there is more to it than simple viscosity.
Rheology is the study of the deformation and flow of matter and can provide a foundational understanding of material behavior. The relationship between stress and deformation is a property of the material, so rheology helps link the properties of a sample to its overall performance. A rheometer applies stress or strain to a sample and the resulting deformation or stress is measured.
Rheology investigates materials in terms of deformation and flow. Deformation is how much a material moves when a force is applied.
Flow is the measurement of a fluid’s movement from one point to another. The opposition to flow in a material is called viscosity, with highly viscous materials having lower degrees of flow.
Viscosity is the property of resistance to flow in any material with fluid properties
viscosity = shear stress (the force per unit area required to move one layer of fluid in relation to another)/ shear rate (the measure of the change in speed at which intermediate layers move with respect to one another)
Since a material’s molecular structure affects its rheological properties, rheology is often used for quantitative and qualitative molecular analysis. This rheological data can provide material insights and inform structural design changes.
Rheology is frequently used to evaluate processing conditions and product performance since it can be used to link final properties back to the material’s underlying structure.
Newtonian and Non-Newtonian Fluids
Within viscous fluids, there is an important distinction based on their viscosity. A Newtonian fluid’s viscosity is independent of shear rate or shear stress. Viscosity measurements as a function of shear rate or shear stress show a flat line like in the figure below, meaning their viscosity is constant. Examples of Newtonian fluids include water, ethanol, and oil.
Non-Newtonian fluids exhibit a viscosity change with shear rate and shear stress. If viscosity increases, the fluid is shear thickening, like the materials measured on the plot below. For example, a highly concentrated cornstarch slurry is shear thickening and becomes more viscous as you stir it.
If viscosity decreases with shear rate and shear stress, the fluid is shear thinning, shown in the plots below. Hand soap is shear thinning – it becomes less viscous as you apply stress by rubbing it between your hands.
Non-Newtonian fluids often have viscosity properties that are time-dependent. In these cases, the shear rate or stress is kept constant and time is the variable instead. If the viscosity increases over time, the material is known as rheopectic, like the green line below. If it decreases with time, the material is known as thixotropic, like the blue line below. Printer ink is rheopectic, and shaving cream is thixotropic.
Overall, rheology provides a way to characterize material deformation and flow based on its response to applied stress. Rheometers apply and measure wide ranges of stress, strain, and strain rate, while simultaneously controlling the material’s environment. Rheology thus offers important information for successful material processing and optimal end-use performance.
There are various manufacturers of rheometers, including: Anton Paar, Thermo Fisher Scientific, TA instrument
