Rheometer
A rheometer is a laboratory device used to measure the way in which a viscous fluid (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.
[1] In the 19th century it was commonly used for devices to measure electric current, until the word was supplanted by galvanometer and ammeter.
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.
Liquid is forced through a tube of constant cross-section and precisely known dimensions under conditions of laminar flow.
[4][5] Capillary rheometers are especially advantageous for characterization of therapeutic protein solutions since it determines the ability to be syringed.
A dynamic shear rheometer, commonly known as DSR is used for research and development as well as for quality control in the manufacturing of a wide range of materials.
Dynamic shear rheometers have been used since 1993 when Superpave was used for characterising and understanding high temperature rheological properties of asphalt binders in both the molten and solid state and is fundamental in order to formulate the chemistry and predict the end-use performance of these materials.
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 was once widely used in the oil industry for determining the flow character of drilling fluids.
Some models allow the speed to be continuously increased and decreased in a programmed fashion, which allows the measurement of time-dependent properties.
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.
This mode of operation is used, for example, to increase the maximum achievable shear rate range or for advanced rheooptical characterization of samples.
(most polymer solutions) are best characterized with capillary breakup rheometers, opposed jet devices, or contraction flow systems.
This type of deformation can occur during processing, such as injection molding, fiber spinning, extrusion, blow-molding, and coating flows.
Because of the pre-shear induced as the fluid is transported through the upstream tube, a true extensional viscosity is difficult to obtain.
The midpoint diameter is monitored as a function of time as the fluid filament necks and breaks up under the combined forces of surface tension, gravity, and viscoelasticity.
The FiSER (filament stretching extensional rheometer) is based on the works by Sridhar et al. and Anna et al.[11] In this instrument, a set of linear motors drive a fluid filament apart at an exponentially increasing velocity while measuring force and diameter as a function of time and position.
Acoustic rheometers employ a piezo-electric crystal that can easily launch a successive wave of extensions and contractions into the fluid.
Acoustic rheometers measure the sound speed and attenuation of ultrasound for a set of frequencies in the megahertz range.
