Homogeneous flow rheometer

The capillary viscometer. The most common and simplest device for measuring viscosity is the capillary viscometer. Its main component is a straight tube or capillary, and it was first used to measure the viscosity of water by Hagen [28] and Poiseuille [60], A capillary rheometer has a pressure driven flow for which the velocity gradient or strain rate and also the shear rate will be maximum at the wall and zero at the center of the flow, making it a non-homogeneous flow. 

Since pressure driven viscometers employ non-homogeneous flows, they can only measure steady shear functions such as viscosity, 77(7). However, they are widely used because they are relatively inexpensive to build and simple to operate. Despite their simplicity, long capillary viscometers give the most accurate viscosity data available. Another major advantage is that the capillary rheometer has no free surfaces in the test region, unlike other types of rheometers such as the cone and plate rheometers, which we will discuss in the next section. When the strain rate dependent viscosity of polymer melts is measured, capillary rheometers may provide the only satisfactory method of obtaining such data at shear rates... 

Birefringence setups can be designed to characterize molten materials undergoing isothermal homogeneous flow. The ranges of strains and strain rates also often coincide with those of rheometers, and consequently may be limited relative to those used in fabrication. Similarly, time-temperature superposition approaches may be used to expand the rate window. State-of-the-art setups suitable for rapid screening of new materials with research-scale quantities (5-20 g) are available for shear flow [72] and startup of uniaxial extensional flow [73,74]. 

Equation 2.3.22 is independent of the constitutive equation because of the small angle. The result is a homogeneous shear field, like simple shear between sliding parallel plates or between closely fitting cylinders. Because stress and deformation rates can be determined independent of a constitutive equation, these flows are very useful as rheometers and are discussed further in Chapter S. 

Extensional flow geometries. As indicated in Figure II. 1, only the first three geometries can be used as homogeneous rheometers. The cooi inate systems indicated are chosen to give C I > 22 > 33. Numbers in parentheses indicate section in which each geometry is discussed. 

To generate equibiaxial extension, sheet inflation, lubricated squeeze flow, and the rotary clamp technique have all been used [9, p. 261]. The most rehable of these is the one based on the rotary clamp technique, and the latest version of this instrument has been described by Hachman and Meissner [160]. This impressive instrument is presently the only rheometer capable of generating homogeneous biaxial extension, but it is somewhat complex and difficult to use. Lubricated squeeze flow is much simpler, but there are important limitations on its capabilities due the difficulty of maintaining lubrication [220].Isakief a/. [161] used lubricated squeeze flow to carry out stress relaxation experiments for determination of the damping... 

The cone-and-plate rheometer (Fignre 8.12) is a simple variant of the parallel plate device considered in Section 8.5. In a cone-and-plate rheometer, polymer samples are sandwiched between a blnnt cone and a flat plate. Shear is generated by rotating either the cone or the plate at an angnlar speed Q, while the other fixture is maintained stationary. Typically, the cone angle a is maintained small a < 6° to minimize measurement artifacts caused by secondary flow [18]. The main advantage of the cone-and-plate rheometer over the parallel plate device is the homogeneity of the shear field it creates... 

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