subject: The Use Of Rheology, Rheometry And Rheometers [print this page] Imagine for a moment the difference in movement between two common liquids, water and honey. Although one is much thicker and flows more slowly, both are Newtonian liquids and behave in a regular expected fashion despite external stress or pressure being applied to them.
The opposite is true of non-Newtonian liquids like 'oobleck', a suspension of corn flour in water, which is often used in laboratories where studies related to rheology or fluid mechanics are being performed.
Non-Newtonian liquids can flow like water but apply pressure and they behave very differently. Punch a bucket of water and your hand powers straight through. Punch oobleck and it momentarily becomes a solid and stops your hand from penetrating beyond the surface.
Oobleck and other non-Newtonian fluids like blood, mayonnaise, multigrade engine oils and slurries hold a great deal of fascination to physicists and chemists and their study is called rheology from the Greek word 'rei' to flow.
The other name for rheology is Non-Newtonian Fluid Mechanics although the ancient Greeks were studying fluid mechanics long before Newton originated the concept of viscosity. Some of the earliest tracts on the matter were written by Archimedes more than 200 years BC.
Rheometry describes the techniques used to study the flow properties, viscosity and characteristics of fluid under pressure, while rheometers are the instruments used to do so.
There are two different types of rheometer. The first called a rotational rheometer measures shear stress - the force exerted when one layer of liquid flows past another. Those that measure extensional stress or the liquid's characteristics when stretched are called extensional rheometers.
Rotational Rheometers - pipe or Capillary: a pipe of exact dimensions used to covert flow rate shear rate and pressure drop into shear stress.
Rotational Cylinder - a cylinder measuring system that measures the shear stress and rate of various fluids.
Cone and Plate - when set to oscillate this apparatus measures the elastic properties of a liquid or in a rotational state will measure shear stress and shear rate.
Extensional Rheometers - capillary Breakup Rheometers: best used to identify the characteristics of liquids that have a low viscosity range.
Filament Stretching Rheometers - used to test mid-range viscosity liquids.
Constant Length Devices - best used to test liquids with a high viscosity.
An example of a filament stretching rheometer is the Falling Plate. Liquid to be tested is held between two plates. The top one remains in position while the second falls away drawing with it a string of the liquid.
Rheometry is applied in industry both in research and development and in quality control where for example a material's resistance to pressure or temperature is tested as well as its characteristics of flow, mixability, misting or many other parameters.
It is also a mainstay of clinical and diagnostic labs where the characteristics of blood and bodily fluids can pinpoint or discount various disorders like cardiovascular disease, sickle cell disease or leukaemia and the food and pharmaceuticals industries where among other things, rheometry is used for testing shelf-life, processability and cohesiveness of products.
