Suspension models flocculated systems

In fact we have lost something over Chapter 1. The Finger tensor B is able to give us normal stresses and extensional thickening. We will have to wait until Chapter 4 to get these factors back into our models. But in the next chapter we will see how to bring in the phenomenon of time dependence, which is so important for polymeric systems. We should note that for concentrated suspensions, especially flocculated systems, there is little elastic recovery, and time dependence is often either very short or extremely long. Thus the viscous models of this chapter are often quite adequate (recall Figures 2.5.3 and 2.5.4 and look ahead to Chapter 10). 

As mentioned above, kaolin suspensions are also often used as model dispersed systems for studying flocculation processes to solve important problems in water treatment such as the removal of dispersed impurities. Among the synthetic flocculants, polyacrylamide ones are the most widely used. These flocculants are rather easily available, cheap, and economic. 

There are three chapters in this volume, two of which address the microscale. Ploehn and Russel address the Interactions Between Colloidal Particles and Soluble Polymers, which is motivated by advances in statistical mechanics and scaling theories, as well as by the importance of numerous polymeric flocculants, dispersants, surfactants, and thickeners. How do polymers thicken ketchup Adler, Nadim, and Brenner address Rheological Models of Suspensions, a closely related subject through fluid mechanics, statistical physics, and continuum theory. Their work is also inspired by industrial processes such as paint, pulp and paper, and concrete and by natural systems such as blood flow and the transportation of sediment in oceans and rivers. Why did doctors in the Middle Ages induce bleeding in their patients in order to thin their blood ... 

The second model introduced by Hunter and cowor)ters (20,21) is the elastic floe model. In this case, the structural units (which persist at high shear rates) are assumed to be small floes of particles (called floccules) which are characterised by the ability of the particle structure to trap some of the dispersion medium. In this energy dissipation is considered to arise from two processes, namely the viscous flow of the suspension medium around the floes (which are the basic flow units) and the energy involved in stretching the floes to brealc the floe doublets apart so that the amount of structure in the system is preserved inspite of the floc-floc collision. This model gives the following expression for the yield value. 

The filtration equations describing contacts between particles and media and between particles in suspension and previously retained particles are not developed as well as the equations used to describe interparticle contacts in flocculation. This lack of knowledge about some aspects of the hydrodynamics and chemistry of the system is the principal reason for limiting filter models to monodisperse suspensions and also means that some empirical coeflBcients are necessary. In this research, values for these coeflBcients were taken directly from those reported by O Melia and Ali (25). 

Garrido R, Concha F. and Burger R., 2003. Settling velocities of particulate systems 14. Unified model of sedimentation, centrifugation and filtration of flocculated suspensions, Int. J. Min. Proc., 72, 57-74. 

Systematic investigations with model suspensions and with real systems lead to clarification of flocculation mechanisms. Knowledge of the applicable flocculation mechanisms is a fundamental requirement for understanding and optimizing flocculation processes. The fiber-optic flocculation sensor is, furthermore, useful as a screening method which is close to practice for developing and for assessing the effectiveness of flocculants. 

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