Today, a rheometer is a laboratory device used to measure the way in which a liquid, suspension or slurry flows in response to applied forces. It is used for those fluids which cannot be defined by a single value of
viscosityand therefore require more parameters to be set and measured than is the case for a viscometer. It measures the rheologyof the fluid.
There are two distinctively different types of "rheometers" depending on the
geometryof applied stress. Rheometers that deal with shear stressare called shear rheometers, whereas rheometers that apply extensional stressare extensional rheometers.
Meanings and origin
The word rheometer comes from the Greek, and means a device for measuring flow. In the 19th century it was commonly used for devices to measure electric current, until the word was supplanted by
galvanometerand ammeter. It was also used for the measurement of flow of liquids, in medical practice (flow of blood) and in civil engineering (flow of water). This latter use persisted to the second half of the 20th century in some areas. Following the coining of the term rheologythe word came to be applied to instruments for measuring the character rather than quantity of flow, and the other meanings are obsolete. (Principal Source: Oxford English Dictionary)
Types of shear rheometer
Pipe or Capillary
Liquid is forced through a tube of constant cross-section and precisely known dimensions under conditions of
laminar flow. Either the flow-rate or the pressure drop are fixed and the other measured. Knowing the dimensions, the flow-rate can be converted into a value for the shear rateand the pressure drop into a value for the shear stress. Varying the pressure or flow allows a flow curve to be determined.
The liquid is placed within the
annulusof one cylinder inside another. One of the cylinders is rotated at a set speed. This determines the shear rateinside the annulus. The liquid tends to drag the other cylinder round, and the force it exerts on that cylinder ( torque) is measured, which can be converted to a shear stress.One version of this is the Fann V-G Viscometer, which runs at two speeds, (300 and 600 rpm) and therefore only gives two points on the flow curve. This is sufficient to define a Bingham plasticmodel which used to be widely used in the oil industryfor determining the flow character of drilling fluids. In recent years rheometers that spin at 600, 300, 200, 100, 6 & 3 RPM have been used. This allows for more complex fluids models such as Herschel-Bulkley to be used. Some models allow the speed to be continuously increased and decreased in a programmed fashion, which allows the measurement of time-dependent properties.
Cone and plate
The liquid is placed on horizontal plate and a shallow cone placed into it. The angle between the surface of the cone and the plate is of the order of 1 degree—i.e. it is a very shallow cone. Typically the plate is rotated and the force on the cone measured. A well-known version of this instrument is the Weissenberg Rheogoniometer, in which the movement of the cone is resisted by a thin piece of metal which twists—known as a torsion bar. The known response of the torsion bar and the degree of twist give the
shear stress, while the rotational speed and cone dimensions give the shear rate. In principle the Weissenberg Rheogoniometer is an absolute method of measurement providing it is accurately set up. Other instruments operating on this principle may be easier to use but require calibration with a known fluid. Cone and plate rheometers can also be operated in an oscillating mode to measure elastic properties, or in combined rotational and oscillating modes.
Types of extensional rheometer
Acoustic rheometeremployes piezo-electric crystalthat can easily launch a successive wave of extensions and contractions into the fluid. It applies an oscillating extensional stress. Acoustic rheometer measures sound speedand attenuationof ultrasoundfor a set of frequenciesin the megahertzrange. Sound speed is a measure of system elasticity. It can be converted into fluid compressibility. Attenuation is a measure of viscous properties. It can be converted into viscous longitudinal modulus. In the case of Newtonian liquidattenuation yields information on the volume viscosity.:"This type of rheometer works at much higher frequencies than others. It is suitable for studying effects with much shorter relaxation times than any other rheometer".
The liquid is placed between two solid surfaces which are pulled apart, drawing out a string of the liquid. Typically one is driven at a fixed speed and the force on the other measured. This is particularly used for polymer melts.
Other systems involve liquid going through an orifice, expanding from a capillary, or sucked up from a surface into column by a vacuum.
*K. Walters (1975) "Rheometry" (Chapman & Hall) ISBN 0412120909
*A.S.Dukhin and P.J.Goetz "Ultrasound for characterizing colloids", Elsevier, (2002)
* [http://www.malvern.com/rheology] - Rotational and Capillary Rheometers from Malvern Instruments
* [http://www.anton-paar.com/rheometers/physica-rheometers_CXSN-5PPFLE.en.0.jsp] - Physica Rheometer from Anton Paar
* [http://www.thermo.com/mc] - Thermo Fisher Scientific
* [http://www.rubber-testing.com] - MonTech Rubber Testing Instruments, Germany
* [http://www.rheotec.de/en/?doc=products-index] - Rheometer/Viscometer from RheoTec Messtechnik GmbH
* [http://www.tainstruments.com] - A R Series Rheometer from T A instruments Inc
* [http://www.01db-metravib.com/dma-instruments.7/dma-instruments.150/?L=1] - DMA+ series from 01db-metravib
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