Common Rheometer Designs

A rheometer is a piece of experimental equipment designed to measure the viscoelastic properties of a material, and in soft systems they can be used to characterize a variety of colloidal systems, including suspensions, emulsions, and pastes. Rheological instruments are also widely used in polymer science for polymer melts, rubbers, and solutions. The often-surprising flow properties of many soft materials make rheom-etry an important industrial measurement because products must be transported, filled into containers, and dispensed. A variety of different instrumental designs for rheometers exist, and we review a few common examples here. 

FIGURE 5.19 Schematic geometries of three different rheometer designs. The fluid under investigation in each case is shown in purple (a) spinning disk rheometer (b) cone-and-plate rheometer and (c) capillary rheometer. 

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This section describes common steps designed to measure the viscosity of non-Newtonian materials using rotational rheometers. The rheometer fixture that holds the sample is referred to as a geometry. The geometries of shear are the cone and plate, parallel plate, or concentric cylinders (Figure HI. 1.1). The viscosity may be measured as a function of shear stress or shear rate depending upon the type of rheometer used. 

A number of common accessories are available to further enhance the capabilities of a capillary rheometer. The rheometer is designed to determine viscous data and minimize the effects of other properties. These other instruments work in conjunction with the rheometer to produce other types of measurements. 

Three common designs for eliminating end effects in concentric cylinder rheometers (a) conicylinder, (b) recessed bottom, and (c) double Couette. 

This chapter covers some of the methods and instruments used to determine the mechanical properties of polymers. Examples of instrument designs and typical data generated in these measurements will be introduced. In particular, automated axial tensiometers (to find elastic modulus, yield stress, and ultimate stress), dynamic mechanical analyzers (to determine storage and loss moduli), and rheometers (to measure flow viscosity) will be introduced. This chapter considers the principles behind the devices used to establish and measure the properties of viscometric flows. One of the common techniques used to determine viscous flow properties, PoisueiUe (laminar) flow in cylindrical tubes, is also important in technical applications, as polymer melts and solutions are often transported and processed in this manner. The time-temperature superposition principle is also covered as a way to predict polymer behavior over long timescales by testing materials across a range of temperatures. 

For the strain controlled LAOS experiment (LAOStrain), a pure sinusoidal strain input is needed, which is why in the past it was necessary to use a SMT-rheometer. With the new developments in the deformation control of CMT-rheometers that enable them to perform strain controlled experiments [27], the question arises whether these instruments can be used in the same way as SMT-rheometers for LAOStrain experiments. It is of great interest to know if they can deliver quantitatively identical results or if the deformation control loop of the CMT-Rheometers influences the measured nonlinearities in the stress wave. Since CMT-rheometers are much more common due to the lower price and simpler design, it would be a great advantage if they could also be used for LAOStrain experiments, therefore... 

The most commonly used experiment is start-up of steady simple extension, and the main problems that arise are the support of the sample and the marked decrease in sample cross section at Hencky strains sufficient to generate nonlinear information. The small cross-section gives rise to necking and rupture that prematurely terminate experiments. The only commercial extensional rheometers now available are devices designed for use as fixtures in standard rotational rheometers. These are easy to use and inexpensive, but the stability of the flow at large strains remains a limitation. 

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