Rheometry refers to the experimental technique used to determine the rheological properties of materials; rheology being defined as the study of the flow and deformation of matter which describes the interrelation between force, deformation and time. The term rheology originates from the Greek words ‘rheo’ translating as ‘flow’ and ‘logia’ meaning ‘the study of’, although as from the definition above, rheology is as much about the deformation of solid-like materials as it is about the flow of liquid-like materials and in particular deals with the behavior of complex viscoelastic materials that show properties of both solids and liquids in response to force, deformation and time.
There are a number of different rheometric tests that can be performed to determine flow properties and viscoelastic properties of a material which depend largely on the type of rheometer being used and its capabilities. These include yield stress.
Malvern Panalytical provides two main rheometry techniques:
- Rotational rheometry
- Capillary rheometry
How does it work?
Samples are loaded between two plates, or other similar geometry such as cone and plate or alternatively a cup and bob system. Applying a torque to the top plate exerts a rotational shear stress on the material and the resulting strain or strain rate (shear rate) is measured. Rotational rheometers and viscometers share the same operating principle, but the former have far greater functionality. This is most evident in the accuracy and range over which shear stress can be applied, their facility for oscillatory testing and the level of control over the normal force applied during rotational testing.
What is it useful for?
Rotational rheometers are arguably the most versatile rheological tools available and can be configured for a number of different rheological methods, to probe the structure and performance of suspensions. Test types range from the generation of simple viscosity flow curves (plots of viscosity against shear) over many decades of torque, through yield stress measurement and on to precise sequences that simulate the chewing of food. Modern, sophisticated instruments enable close matching of the test method to the specific process or in-use environment of the product. Innovative software is increasingly helpful in allowing even novice rheologists to generate and interpret relevant data.
Rotational rheometers are used for a broad range of sample types from pastes and gels to the most weakly structured liquids. Applied shear can be precisely controlled into the very low shear stress region making these instruments suitable for stability studies and the measurement of yield stress. However, rotational rheometers are optimized for operation across many decades of torque rather than for the precise differentiation of viscosity in low viscosity, weakly structured fluids. In addition, rotational rheometers face mechanical limitations in the high shear region, at shear rates, in excess of 1000 s-1.
How does it work?
A sample is forced to extrude through a barrel or die of well-defined dimensions under high pressure. The pressure drop across the barrel or die is measured to give pressure-flow rate data for the fluid, from which viscosity is calculated. Temperature and shear rate can be closely controlled to simulate the processing environment of interest.
What is it useful for?
Originating in the polymer industry, capillary rheometry is useful for measuring the viscosity profiles of suspensions and slurries containing relatively large particles, at high particle loadings. Industrial examples include polymer melts, ceramic slurries, foodstuffs, inks and coatings. Capillary rheometers can apply very high force, which enables the exploration of behavior at far higher shear rates than is possible with rotational rheometry. High shear rate performance is pertinent in many industrial processes, such as extrusion and spraying. For certain applications the sample size required for capillary rheometry, around a liter for the generation of a flow curve, is a limitation.