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. Viscosity is a measure of the resistance of a Fluid which is being deformed by either Shear stress or Extensional stress. A viscometer (also called viscosimeter) is an instrument used to measure the Viscosity of a Fluid. It measures the rheology of the fluid. Rheology is the study of the flow of matter mainly liquids but also soft solids or solids under conditions in which they flow rather than deform elastically

There are two distinctively different types of rheometers depending on the geometry of applied stress. Geometry ( Greek γεωμετρία; geo = earth metria = measure is a part of Mathematics concerned with questions of size shape and relative position Rheometers that deal with shear stress are called shear rheometers, whereas rheometers that apply extensional stress are extensional rheometers. A shear stress, denoted \tau\ ( Tau) is defined as a stress which is applied Parallel or tangential to a face of a material 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 Stress is a measure of the average amount of Force exerted per unit Area. 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

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. A galvanometer is a type of Ammeter; an instrument for detecting and measuring Electric current. An ammeter is a Measuring instrument used to measure the Electric current in a circuit. 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. Rheology is the study of the flow of matter mainly liquids but also soft solids or solids under conditions in which they flow rather than deform elastically (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. The Oxford English Dictionary ( OED) published by the Oxford University Press (OUP is a comprehensive Dictionary of the English Laminar flow, sometimes known as streamline flow occurs when a fluid flows in parallel layers with no disruption between the layers 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. Shear rate is the rate at which a Shear is applied Simple Shear Shear rate is defined using the following equation \dot \gamma = \frac {\nu} {h} A shear stress, denoted \tau\ ( Tau) is defined as a stress which is applied Parallel or tangential to a face of a material Varying the pressure or flow allows a flow curve to be determined.

Rotational cylinder

Rheometer with cylinder measuring system (left) and cone/plate measuring system (right)

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. Shear rate is the rate at which a Shear is applied Simple Shear Shear rate is defined using the following equation \dot \gamma = \frac {\nu} {h} 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. A torque (τ in Physics, also called a moment (of force is a pseudo- vector that measures the tendency of a force to rotate an object about A shear stress, denoted \tau\ ( Tau) is defined as a stress which is applied Parallel or tangential to a face of a material 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 plastic model which used to be widely used in the oil industry for determining the flow character of drilling fluids. A Bingham plastic is a Viscoplastic material that behaves as a rigid body at low stresses but flows as a viscous Fluid at high stress The petroleum industry includes the global processes of exploration, extraction, refining, transporting (often by Oil tankers and pipelines Drilling a very deep hole is a very costly business The hole size is kept very small as the drilled depth increases because it is to cased and cemented after wards In recent years rheometers that spin at 600, 300, 200, 100, 6 & 3 RPM have been used. This allows for more advanced 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. A torsion spring is a spring that works by torsion or twisting that is a flexible elastic object that stores Mechanical energy when it is twisted 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. A torsion spring is a spring that works by torsion or twisting that is a flexible elastic object that stores Mechanical energy when it is twisted A shear stress, denoted \tau\ ( Tau) is defined as a stress which is applied Parallel or tangential to a face of a material Shear rate is the rate at which a Shear is applied Simple Shear Shear rate is defined using the following equation \dot \gamma = \frac {\nu} {h} 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. Acoustic rheometer employes Piezo-electric Crystal that can easily launch a successive Wave of extensions and contractions into the Fluid Piezoelectricity is the ability of some materials (notably Crystals and certain Ceramics including bone to generate an Electric potential in response to In Materials science, a crystal is a Solid in which the constituent Atoms Molecules or Ions are packed in a regularly ordered repeating It applies an oscillating extensional stress. Oscillation is the repetitive variation typically in Time, of some measure about a central value (often a point of Equilibrium) or between two or more different states Stress is a measure of the average amount of Force exerted per unit Area. Acoustic rheometer measures sound speed and attenuation of ultrasound for a set of frequencies in the megahertz range. Sound is a vibration that travels through an elastic medium as a Wave. In Physics, attenuation (in some context also called extinction) is the gradual loss in intensity of any kind of Flux through a medium Not to be confused with Supersonic. Ultrasound is cyclic Sound pressure with a Frequency greater than the upper Frequency is a measure of the number of occurrences of a repeating event per unit Time. The hertz (symbol Hz) is a measure of Frequency, informally defined as the number of events occurring per Second. Sound speed is a measure of system elasticity. It can be converted into fluid compressibility. In Thermodynamics and Fluid mechanics, compressibility is a measure of the relative volume change of a Fluid or Solid as a response Attenuation is a measure of viscous properties. It can be converted into viscous longitudinal modulus. In Linear elasticity, the P-wave modulus M also known as the longitudinal modulus, is one of the elastic moduli available to describe isotropic homogeneous In the case of Newtonian liquid attenuation yields information on the volume viscosity. A Newtonian fluid (named for Isaac Newton) is a Fluid whose stress versus rate of strain curve is linear and passes through the origin Volume viscosity (also called bulk viscosity or second viscosity) appears in the Navier-Stokes equation if it is written for Compressible fluid

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. Relaxation time is a general concept in Physics for the Characteristic time in which a system changes to an Equilibrium condition from a non-equilibrium

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)

See also

• Rheology

External links

• [1] - Physica Rheometer from Anton Paar
• [2] - Thermo Fisher Scientific
• [3] - MonTech Rubber Testing Instruments, Germany
• [4] - Rheometer/Viscometer from RheoTec Messtechnik GmbH
• [5] - A R Series Rheometer from T A instruments Inc
• [6] - Rotational and Capillary Rheometers from Malvern Instruments
• [7] - DMA+ series from 01db-metravib