A rheometer is a kind of viscometer that measures visco-elastic properties of materials beyond just viscosity. Rheology is the flow of fluids and deformation of solids under various kinds of stress and strain. This tool, therefore, measures material behavior such as yield stress, kinetic properties, complex viscosity, modulus, creep, and recovery.
Most rheometer models belong to three specific categories: rotational, capillary, or extensional. The most commonly used of these is the rotational rheometer, which is also called a stress/strain rheometer, followed by the capillary type.
The rheometer has become important in the building and maintenance of roads. Measuring the rheology of asphalt binders helps predict pavement performance over time as it is affected by changing climate and traffic conditions. In 1993, the US government introduced the measure of rheological properties to the paving industry when it funded the Strategic Highway Research Program. This program has led to a broader industry understanding of the use of rheological properties toward improving asphalt binder performance qualities.
This tool is also used to measure the rheology of semi-solids, suspensions, emulsions, and polymers in industries such as pharmaceuticals, foods, cosmetics, and consumer products. These measurements can help predict shelf life of products under various stress conditions. In addition, it can be used in the medical field. In 1999, the elasticity and fracture strain of blood clots was measured at the Institute of Hydrodynamics, Academy of Science in the Czech Republic, using a rotational rheometer with a controlled stress system. The results indicated a decrease of elasticity and an increase of fracture strain in blood clots as hematocrit increased.
A surprising application of the tool can be found in a 1999 experiment that took place at the University of Nottingham in the United Kingdom. In this experiment, a rheometer and a twin-screw extruder were used to determine the force it would take to destroy a heat resistant vegetative bacterial species, microbacterium lacitum. This experiment has many promising applications within the medical field.