Fluids rheological measurements

Additional complications can occur if the mode of deformation of the material in the process differs from that of the measurement method. Most fluid rheology measurements are made under shear. If the material is extended, broken into droplets, or drawn into filaments, the extensional viscosity may be a more appropriate quantity for correlation with performance. This is the case in the parting nip of a roUer in which filamenting paint can cause roUer spatter if the extensional viscosity exceeds certain limits (109). In a number of cases shear stress is the key factor rather than shear rate, and controlled stress measurements are necessary. 

Rheologic measurements conflrmed that a soluble delayed-release acid could be used to convert a borate-crosslinked fluid into a linear gel [1353]. 

Many materials are conveyed within a process facility by means of pumping and flow in a circular pipe. From a conceptual standpoint, such a flow offers an excellent opportunity for rheological measurement. In pipe flow, the velocity profile for a fluid that shows shear thinning behavior deviates dramatically from that found for a Newtonian fluid, which is characterized by a single shear viscosity. This is easily illustrated for a power-law fluid, which is a simple model for shear thinning [1]. The relationship between the shear stress, a, and the shear rate, y, of such a fluid is characterized by two parameters, a power-law exponent, n, and a constant, m, through... 

Prud homme, R.K. Ellis, S. Constien, V.G. "Reproducible Rheological Measurements on Crosslinked Fracturing Fluids," SPE paper 18210, 1988 SPE Annual Technical Conference and Exhibition, Houston, October 2-5. 

The science that deals with the deformation and flow of matter is called rheology. An important rheological concept is the shear force, sometimes called the shear stress, or the force that causes a layer of a fluid material to flow over a layer of stationary material. The rate at which a layer of a fluid material flows over a layer of stationary material is called the shear rate. A fluid flowing through a tube, for example, would be the fluid material, while the tube wall would be the stationary material. An important rheological measurement that is closely related to the resistance to flow is called viscosity. The technical definition of viscosity is the ratio of shear stress to shear rate ... 

See Fluidization Fluid mechanics Rheological measurements Sedimntation. 

There are a number of types of rheological measurement, some are appropriate for Newtonian fluids only, while others may be used for Newtonian or non-Newtonian fluids. Some of the principal types are listed in Table 6.4. Some very useful descriptions of experimental techniques have been given by Whorlow [355] and others [215,352,353,356,357]. The principal methods are discussed in the next several sections. 

Index Entries Com stover rheological measurement shear stress shear rate non-Newtonian fluids Power Law parameters. 

To avoid the apparent complications with absolute rheologic measurement techniques, a number of investigators (4,5). have used relative measurement systems to make rheologic measurements. The major difference between the relative and absolute measurement techniques is that the fluid mechanics in the relative systems are complex. The constitutive equations needed to find the fundamental rheologic variables cannot be readily solved. Relative measurement systems require the use of Newtonian and non-Newtonian calibrations fluids with known properties to relate torque and rotational speed to the shear rate and shear stress (6). 

Sometimes, this expectation is not met. At high flow rates, there can be hydrodynamic instabilities that lead to secondary flows which ruin the rheological measurement. Such instabilities occur in Newtonian fluids, due, for example, to inertial effects, such as those in Poiseuille flow at Reynolds numbers exceeding 2000 (Drazin and Reid 1981). For some complex fluids, even at low Reynolds number there are instabilities that are driven by elastic effects (Larson 1992). 

In addition to these impediments to rheological measurements, some complex fluids exhibit wall slip, yield, or a material instability, so that the actual fluid deformation fails to comply with the intended one. A material instability is distinguished from a hydrodynamic instability in that the former can in principle be predicted from the constitutive relationship for the material alone, while prediction of a flow instability requires a mathematical analysis that involves not only the constitutive equation, but also the equations of motion (i.e., momentum and mass conservation). 

A useful way to think about slip, and its effect on rheological measurements, is to define an extrapolation length b. The extrapolation length is the distance from the fluid-solid... 

While rheological measurements are wonderfully quantitative, they are usually poor qualitative probes of fluid structure. This is because in rheological experiments, the structural changes responsible for the measured relaxation behavior remain hidden. Thus, rheometry is often most useful when supplemented by other experimental methods that characterize fluid structure and flow-induced structural changes. Some of the most useful methods are microscopy, light, x-ray, and neutron scattering, and polarimetry. 

Vekas, L., Rasa, M., and Bica, D., Physical properties of magnetic fluids and nanoparticles from magnetic and magneto-rheological measurements, J. Colloid Interface Sci., 231, 247, 2000. 

The cone-plate geometry is widely used in rheological measurements of viscoelastic fluids. The fluid is placed between a plate of radius and a cone of the same radius. The angle, a, between the cone and the plate is usually smaller than 3° (see Fig. 13.19). 

---