Rheology colloidal suspensions

Many properties of colloidal suspensions, such as their stability, rheology, and phase behaviour, are closely related to the interactions between the suspended particles. The background of the most important contributing factors to these interactions is discussed in this section. 

The behavior of colloidal suspensions is controlled by iaterparticle forces, the range of which rarely extends more than a particle diameter (see Colloids). Consequentiy suspensions tend to behave like viscous Hquids except at very high particle concentrations when the particles are forced iato close proximity. Because many coating solutions consist of complex mixtures of polymer and coUoidal material, a thorough characterization of the bulk rheology requires a number of different measurements. 

The above model assumes that both components are dynamically symmetric, that they have same viscosities and densities, and that the deformations of the phase matrix is much slower than the internal rheological time [164], However, for a large class of systems, such as polymer solutions, colloidal suspension, and so on, these assumptions are not valid. To describe the phase separation in dynamically asymmetric mixtures, the model should treat the motion of each component separately ( two-fluid models [98]). Let Vi (r, t) and v2(r, t) be the velocities of components 1 and 2, respectively. Then, the basic equations for a viscoelastic model are [164—166]... 

Barnes, H. A., Hutton, J. F., and Walters, K., An Introduction to Rheology, Elsevier, Amsterdam, Netherlands, 1989. (Undergraduate and graduate levels. A practical introduction to the rheology of suspensions and polymer solutions. Chapter 2 presents introductory concepts and measurement techniques at the undergraduate level. Effects of colloidal interactions are not described.)... 

Frisch,H.L., Simha.R. The viscosity of colloidal suspensions and macromolecular solutions, Vol. 1, Chapter 14. In Eirich,F.R. (Ed.) Rheology. New York Academic Press 1956. 

Leal, Advanced Transport Phenomena Fluid Mechanics and Convective Transport Mewis and Wagner, Colloidal Suspension Rheology... 

Frisch, H. L and R- Simha Viscosity of colloidal suspensions and macro-molecular solutions. In Rheology, Vol. I, pp. 525—-613. edited by F. Eirich, New York Academic Press 1956. 

Bergenholtz J, Brady JF, Vide M. (2002) The non-Newtonian rheology of dilute colloidal suspensions. ] Fluid Mech 456 239-275. 

In this chapter, we review the rheological properties of suspensions of solids in a liquid medium, under conditions in which the particles do not clump together— that is, do not gel. Gelled colloidal suspension are discussed in Chapter 7, while emulsions md foams—where the suspended phase is another liquid or a gas—are discussed in Chapter 9. Even within these limits, the scope of this chapter is extensive, and there is only room for major topics. Additional detail can be found in Russel et al. (1989), Hiemenz and Rajagopalan (1997), Kim and Karrila (1991), and Chapter 10 of Macosko (1994). For reviews of the most recent work, see Brady (1996) and Mellema (1997). 

Macromolecules are very important in the processing of powders into porous support layers. They are used to ensure the colloidal stability of ceramic suspensions, to control the rheology of suspensions and pastes, to avoid cracking... 

Fig. 1 A schematic comparison between polymer melt rheology and colloid rheology, (a) In a polymer melt, atypical chain (dashed curve) is constrained by many (in reality, 10 ) other chains, here represented by small circles. This gives rise to the fruitful mean-field concept of a tube in which the chain has to move, (b) In a concentrated colloidal suspension, a typical particle (hatched) is surrounded (in 3D) by 10 neighbours. This number is too small for mean-field averaging to be meaningful, (c) Large deformations in polymer melts, such as the process (i) -(ii), involves breaking covalent bonds, and so do not ordinarily occur, (d) There are no covalent constraints on order unity deformations, such as (i)—>(ii), in a colloidal suspension...

Stability in colloidal dispersions is defined as resistance to molecular or chemical disturbance, and the distance the system is removed from a reference condition may be used as a measure of stability. The stability can be analyzed from both energetic and kinetic standpoints. The kinetic approach uses the stability ratio, as a measure of the stability. W is defined as fhe ratio of the rate of flocculation in the absence of any energy barrier to that when there is an energy barrier due to adsorbed surfactant or polymer. These processes are referred to as rapid and slow flocculation with rate constants kj and kg, respectively, such that W = kjlk. The stability of colloidal suspensions can be evaluated using various techniques. In practice, two methods are mainly used sedimentation and rheology measurements. 

Coupled controlled velocity, magnetic resonance imaging (MRI)/rheology measurements of thixotropic and yielding colloidal suspensions further demonstrate the importance of paired measurements [63], Shear rate profiles obtained in laminar tube flow for both Newtonian and non-Newtonian fluids from MRI... 

L. Gunther, W. Peukert, Relationship between rheological properties of concentrated colloidal suspensions and the structure of dip-drawn coatings, Chem. Ingenieur Tech. 

In this section, we discuss the behavior of liquid crystal suspensions under the action of an external electric field. The behavior of colloidal suspensions in electric fields is of considerable technological interest with the so-called Electro-Rheological (ER) fluids [17, 18]. The main features of this behavior are now rather well understood. When an external field is applied, particles suspended in an isotropic fluid become polarized. Resultant dipole-dipole interactions between the particles lead to their chaining along the direction of the applied field. When suspended in a liquid crystal host, colloidal particles are also expected to be polarized upon the application of an electric field. However, new phenomena may take place because of the specific response of the liquid crystal. In this case, the external field is likely to alter the distortions of the liquid crystal alignment... 

Flow or deformation involves the relative motion of adjacent elements of the material. As a consequence such processes are sensitive to interatomic or intermolecular forces. In the case of liquids containing dispersed particles, interparticle forces are also important. Because the rheological properties of colloidal suspensions exhibit such a rich variety of phenomena, rheological studies not only provide information on medium-particle and particle-particle interactions but also arc of immense technological importance. 

Free colloidal particles about a micrometer or less in size settle out so slowly under gravity that they may often be considered as suspended in the flow and moving with it. As a result, the principal concern is to determine the rheological properties of the colloidal suspension. 

N, J, Wagner and R, Klein, The rheology and microstructure of charged colloidal suspensions, ColloidPolym. Sci, 269 295-319, 1991... 

If only attractive forces exist then the droplets coalesce, thereby lowering the system free energy (except in the event of extremely small particles where the entropic form can dominate). On the other hand, if repulsive forces dominate, then even very small density differences are sufficient to give rise to creaming, i.e. phase separation, for example. What is required for stability is a flocculation of droplets, which does not give rise to coalescence, to form a network throughout the system. The role of colloidal forces in the rheology of suspensions has recently been reviewed by Russel. ... 

Frisch, H. L., and Simha, R., Viscosity of colloidal suspensions and macromolecular solutions, in Rheology, Vol. 1, Eirich, E. R.,Ed., Academic Press, New York, 1956. 

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