## 5 CWS Rheology  
The definition of rheology can be presented as the study of the flow of matter, primarily in a liquid state, but also as soft solids or even solids that, in response to the application of a force, present a plastic flow rather than deforming elastically [61].  
The rheological properties of particle suspensions are of great importance in several industrial applications, including the pipeline transportation of slurries [62], and can be used to determine the relationship between the flow rate and pressure drops. Rheological parameters can also be used to determine the power required to agitate the slurry in the tank and to determine the wear rate and life of the pipeline. The rheology of a slurry depends on several parameters, such as the particles' shape and size distribution, solid concentration, and fluid properties [63; 64]. By suitably manipulating the particle size distribution, if other parameters are kept the same, it is possible to obtain a stabilized slurry suspension. This information is important to predict the requirements for pumping the slurry. The presence of solids affects the performance of the pump, and inner surfaces wear more due to the presence of solid particles [64].  
Historically, the rheology of suspensions has been investigated mostly through experimentation on equalized particulate suspensions. Correlations have been derived, on the basis of the above data, to predict the Newtonian viscosity of a suspension [65]. Furthermore, the particles of different materials will differ in various properties, such as density and shape. Thus, the actual flow pattern that exists in a slurry pipeline will differ from material to material. Moreover, the behavior of a slurry is generally non-Newtonian at the concentrations that are commercially used [66]. It is essential to obtain a good understanding of the methods of characterizing rheological properties and extrapolating these characteristics to commercial slurries [67].  
In the absence of any suitable correlations for predicting the rheological parameters of non-Newtonian slurries containing large-sized particles with a wide particle size distribution, visometric tests are unavoidable. For slurries containing large particles in a low-viscosity carrier liquid, visometric measurements are difficult because large particles tend to settle during measurements, thereby affecting the homogeneity of the suspension. The geometric interference of particles with the walls of the viscometer also places a limit on the largest size particles that can be accommodated during tests. To overcome these problems, large/heavy particles may have to be removed from the original sample before rheometric tests can be performed with the remaining fine particulate slurries. However, presently, the effect of removing large particles is not fully understood [68].  
CWSs are non-Newtonian fluids with a viscosity function that decreases with the increment of the shear rate. Pseudoplastic fluids exhibit a shear-thinning behavior without yield stress. Dilatant fluids, on the contrary, exhibit shear-thickening behavior. Expensive additives, dispersants, and stabilizers have been used to improve the rheology and the static stability for transportation, storage, and combustion [69].  
The rheology of a coal suspension depends on the existing interparticle attractive forces [70], which in turn depend on the chemical structure of the surface, which is specific to a particular coal, and the total surface area [71]. Porosity is inherent to a specific coal and affects reagents adsorption but may contribute to interparticle attraction. Interparticle forces can be modified by the presence of surface-active chemicals, inorganic electrolytes, pH, and polyelectrolytes [72].