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Rheometer

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 viscosity and therefore require more parameters to be set and measured than is the case for a viscometer. It measures the rheology of the fluid.

There are two distinctively different types of rheometers depending on the geometry of applied stress. Rheometers that deal with shear stress are called shear rheometers, whereas rheometers that apply extensional stress are extensional rheometers.

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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 galvanometer and 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 rheology the 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 rate and the pressure drop into a value for the shear stress. Varying the pressure or flow allows a flow curve to be determined.

Rotational cylinder

The liquid is placed within the annulus of one cylinder inside another. One of the cylinders is rotated at a set speed. This determines the shear rate inside 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 r.p.m.) and therefore only gives two points on the flow curve. This is sufficient to define a Bingham plastic. It is widely used in the oil industry for determining the flow character of drilling fluids. Other types measure at more speeds and enable an equilibrium flow curve to be obtained. 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

Acoustic rheometer employes piezo-electric crystal that can easily launch a successive wave of extensions and contractions into the fluid. It applies an oscillating extensional stress. Acoustic rheometer measures sound speed and attenuation of ultrasound for a set of frequencies in Mhz range. 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 liquid attenuation yields information on the volume viscosity.

This type of rheometers works at much higher frequencies than others. It is suitable for studying effects with much shorter relaxation times than any other rheometer.

Pulled string.

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.

Capillary

Other systems involve liquid going through an orifice, expanding from a capillary, or sucked up from a surface into column by a vacuum.

References

K. Walters (1975) Rheometry (Chapman & Hall) ISBN 0412120909
A.S.Dukhin and P.J.Goetz "Ultrasound for characterizing colloids", Elsevier, (2002)