Rheometers: Instruments in the Rheology Laboratory
The ensemble of instruments make it a full-service laboratory for non-Newtonian liquids, or complex fluids, including suspensions, polymeric materials, pastes, and dispersions. Accordingly, a wide range of fluids have been characterized in the Laboratory, and these are listed in the website Fluids Previously Tested. The instruments enable a liquid to be characterized in both shear and extension, something which few laboratories can offer. The instruments and their features are described below.
This instrument is the workhorse of the rheometers in the Laboratory, because it has been found to be so user friendly. Made by TA Instruments, it can subject a fluid to the full range of shearing motions: steady, oscillatory, startup and cessation. Hence the instrument can measure:
Viscosity, as a function of shear rate (the flow curve)
Dynamic properties, the shear modulus G’ and the storage modulus G”
First Normal Stress Difference N1
By such measurements, the viscoelasticity of a liquid can as evaluated.
The rheometer can be operated in the controlled shear rate mode, where the shear rate is fixed and the required torque is measured, or in the controlled shear stress mode, where the shear stress is applied, and the resulting shear rate is measured. In fact, the latter mode is a feature of the instrument, enabling the yield stress to be determined for pastes such as ketchup and mine tailings. A variety of fixtures are available, enabling the rheometer to measure a wide range of viscosities, from watery liquids to polymer melts. Moreover, an oven surrounds the test sample so that, along with a Peltier plate, tests can be conducted over a wide range of temperatures.
The AR2000 specifications (from TA Instruments):
Temperature: from –20°C to 600°C
Shear rate range: from 10-4 s-1 to 104 s-1
Viscosity range: from 5 mPa.s to 104 Pa.s
This instrument is the most advanced rheometer commercially available. Also available from TA Instruments and like the AR2000, the ARES can test materials over the full range of shear modes and so the viscoelastic properties of complex liquids can be determined. A feature of the ARES is that its transducers, for torque and normal force, can be calibrated absolutely, so that data can from the instrument can be scientifically rigorous. The rheometer can be operated with at a fixed shear rate or at a constant shear stress. Like the AR2000, various fixtures are available for installation in the instrument, to provide a wide range of dynamical conditions, and an oven can enclose the test fixture, to enable tests at temperatures as high as 600°C.
The ARES specifications:
Temperatures: from –10°C to 600°C
Shear rates: from 10-4 s-1 to 104 s-1
Viscosity range: from 10-2 Pa.s to 105 Pa.s
Filament Stretching Rheometer
Measurements with this custom-made instrument yield the extensional viscosity of a viscoelastic liquid. The instrument stretches out a liquid filament at a constant rate of extension, and from measurements of filament force and diameter, the rheometer provides a proper measurement of the (transient) extensional viscosity, generally the most elusive of flow properties. The design allows the property to be measured in a shear-free flow. There are fewer than ten such rheometers in the world, all of them laboratory instruments, following the concept introduced by Professor Sridhar in Australia. With this instrument, and the shear rheometers in the Laboratory, a complex fluid can be fully characterized rheologically, that is, it can be tested in both shear and extension.
Extensional rates: from 1 s-1 to 10 s-1
Shear viscosity: 1 Pa.s and above
Strain: Hencky strain up to 6