A rheometer is a device used to measure 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.
Unlike a viscometer, which can only measure the viscosity of a fluid under a limited range of conditions, a rheometer is capable of measuring viscosity and elasticity of non-Newtonian materials under a wide range of conditions. Some of the most important properties that can be measured using a rheometer include viscoelasticity, yield stress, thixotropy, extensional viscosity, creep compliance and stress relaxation behavior, as well as process relevant parameters such as die swell, melt fracture.
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.
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