Viscosity colloidal dispersions

De Kruif, C.G. ven lersel, E.M.F. Vrij, A. Russel, W.B. Hard sphere colloidal dispersions viscosity as a function of shear rate and volume fraction. J. Chem. Phys. 1986, 83, 4717-4725. 

This work will describe a series of magnetic colloidal systems that were specifically developed for an application in ink jet printing technology. This application imposes a certain set of requirements such as particle size 100 + 50A, magnetic moment of 25 emu/g or 35%w Fe O in colloidal dispersion, viscosity of 8-10 cps, non-toxic aqueous system, shelf life of a few years, freeze-thaw stability, fast drying (2 msec) and high optical density of magnetic ink on various papers. 

The flow properties of a colloidal system are very much dependent on its microstructure, as determined by the molecular arrangement and interaction of its components. ME systems show flow typical of a Newtonian liquids, for which the shear stress is directly proportional to the shear rate. Since viscosity measurements are dynamic experiments, they will give information on dynamic properties of the ME. These will depend on the miCTOStructure, type of aggregates, or interactions within the ME, which in turn are determined by the concentration of the various components and the temperature. The dispersion of one component in another, e.g., water in oil, will generally increase the bulk viscosity in comparison to the individual components (oil and water) [58]. For at true colloidal dispersion, viscosity will increase with increasing volume fraction of dispersed phase according to the formula generated by Einstein ... 

Dekruif C.G., Vanlersel E.M.F., Vrij A., Russel W.B. Hard-sphere colloidal dispersions—Viscosity as a function of shear rate and volume fraction. J. Chem. Phys. 1985 83(9) 4717 725 Derooij R., Potanin A. A., Vandenende D., Mellema J. Steady shear viscosity of weakly aggregating polystyrene latex dispersions. J. Chem. Phys. 1993 99(11) 9213-9223 Derooij R., Vandenende D., Duits M.H.G., Mellema J. Elasticity of weakly aggregating polystyrene latex dispersions. Phys. Rev. E. 1994 49(4) 3038-3049 Derjaguin B.V., Landau L. Acta Physiocochim URSS. 1941 14 633-662... 

Colloidal dispersions often display non-Newtonian behaviour, where the proportionality in equation (02.6.2) does not hold. This is particularly important for concentrated dispersions, which tend to be used in practice. Equation (02.6.2) can be used to define an apparent viscosity, happ, at a given shear rate. If q pp decreases witli increasing shear rate, tire dispersion is called shear tliinning (pseudoplastic) if it increases, tliis is known as shear tliickening (dilatant). The latter behaviour is typical of concentrated suspensions. If a finite shear stress has to be applied before tire suspension begins to flow, tliis is known as tire yield stress. The apparent viscosity may also change as a function of time, upon application of a fixed shear rate, related to tire fonnation or breakup of particle networks. Thixotropic dispersions show a decrease in q, pp with time, whereas an increase witli time is called rheopexy. 

Almost all urethane materials are synthesized without the use of solvents or water as diluents or earners and are referred to as being 100% solids. This is true of all foams and elastomers. There are many products, however, which do utilize solvents or water, and these are known as solvent-borne and waterborne systems, respectively. In the past, many coatings, adhesives, and binders were formulated using a solvent to reduce viscosity and/or ease application. However, the use of volatile solvents has been dramatically curtailed in favor of more environmentally friendly water (see Section 4.1.3), and now there are many aqueous coatings, adhesives, and associated raw materials. Hydrophilic raw materials capable of being dispersed in water are called water reducible (or water dispersible), meaning they are sufficiently hydrophilic so as to be readily emulsified in water to form stable colloidal dispersions. 

Routh and Russel [10] proposed a dimensionless Peclet number to gauge the balance between the two dominant processes controlling the uniformity of drying of a colloidal dispersion layer evaporation of solvent from the air interface, which serves to concentrate particles at the surface, and particle diffusion which serves to equilibrate the concentration across the depth of the layer. The Peclet number, Pe is defined for a film of initial thickness H with an evaporation rate E (units of velocity) as HE/D0, where D0 = kBT/6jT ir- the Stokes-Einstein diffusion coefficient for the particles in the colloid. Here, r is the particle radius, p is the viscosity of the continuous phase, T is the absolute temperature and kB is the Boltzmann constant. When Pe 1, evaporation dominates and particles concentrate near the surface and a skin forms, Figure 2.3.5, lower left. Conversely, when Pe l, diffusion dominates and a more uniform distribution of particles is expected, Figure 2.3.5, upper left. 

The viscosity of colloidal dispersions is affected by the shape of the dispersed phases. Sphero-colloids form dispersions of relatively low viscosity, while systems containing linear particles are generally more viscous. The relationship of particle shape and viscosity reflects the degree of solvation of the particles. In... 

The high viscosity of heavy crude oils is essentially due to the high levels of asphaltene content. Asphaltene is the highest MW component of crude oil, is a friable, amorphous dark solid, which is colloidally dispersed, in the oily portion of the crude. Asphaltenes are considered to be heavily condensed aromatic molecules with aliphatic side chains and with high heteroatom content (S, N, and O) as well as high-metal content. The asphaltene fraction is physically defined as that fraction insoluble in n-alkanes, but soluble in toluene and is the most polar fraction of oil. 

Polyelectrolytes provide excellent stabilisation of colloidal dispersions when attached to particle surfaces as there is both a steric and electrostatic contribution, i.e. the particles are electrosterically stabilised. In addition the origin of the electrostatic interactions is displaced away from the particle surface and the origin of the van der Waals attraction, reinforcing the stability. Kaolinite stabilised by poly(acrylic acid) is a combination that would be typical of a paper-coating clay system. Acrylic acid or methacrylic acid is often copolymerised into the latex particles used in cement sytems giving particles which swell considerably in water. Figure 3.23 illustrates a viscosity curve for a copoly(styrene-... 

Porous inkjet papers are in general created from colloidal dispersions. The eventual random packing of the colloid particles in the coated and dried film creates an open porous structure. It is this open structure that gives photographic-quality inkjet paper its apparently dr/ quality as it comes off the printer. Both the pore structure and pore wettability control the liquid invasion of the coated layer and therefore the final destination of dyes. Dispersion and stability of the colloidal system may require dispersant chemistries specific to the particle and solution composition. In many colloidal systems particle-particle interactions lead to flocculation which in turn leads to an increase in viscosity of the system. The viscosity directly influences the coating process, through the inverse relation between viscosity and maximum coating speed. 

Emulsions and foams are two other areas in which dynamic and equilibrium film properties play a considerable role. Emulsions are colloidal dispersions in which two immiscible liquids constitute the dispersed and continuous phases. Water is almost always one of the liquids, and amphipathic molecules are usually present as emulsifying agents, components that impart some degree of durability to the preparation. Although we have focused attention on the air-water surface in this chapter, amphipathic molecules behave similarly at oil-water interfaces as well. By their adsorption, such molecules lower the interfacial tension and increase the interfacial viscosity. Emulsifying agents may also be ionic compounds, in which case they impart a charge to the surface, which in turn establishes an ion atmosphere of counterions in the adjacent aqueous phase. These concepts affect the formation and stability of emulsions in various ways ... 

When colloidal particles are dispersed in a liquid, the flow of the liquid is disturbed and the viscosity is higher than that of the pure liquid. The problem of relating the viscosities of colloidal dispersions (especially when dilute) with the nature of the dispersed particles has been the subject of much experimental investigation and theoretical consideration. In this respect, viscosity increments are of greater significance than absolute viscosities, and the following functions of viscosity are defined ... 

Colloidal dispersions suspensions and aggregates Viscosity and transient electric birefringence study of clay colloidal aggregation. Physical Review E 65, 21407-21500... 

There are numerous ways in which viscosities are expressed in the literature. Some of the most common are defined Table 6.8. There is an entire lexicon of terms used to describe the different rheological classifications of colloidal dispersions [9-11,353,355]. 

Saturated hydraulic conductivity may also be influenced by dramatic changes in solution viscosity as well as the soil s dispersive potential. The data in Figure 10.5 represent saturated hydraulic conductivity as a function of EC. It appears that as EC increases, hydraulic conductivity decreases. The soil material in this study represents a Kentucky mine spoil. The predominant salt in the solution was an acid, MgS04. Suspension data showed that as MgS04 concentration increased, colloid dispersion increased. This could be due to an increase in solution viscosity, which also has a suppressing effect on saturated hydraulic conductivity (see Eq. 10.2). 

Attempts to describe the unlimited increase of the viscosity of dispersions and emulsions observed when their concentrations approach the maximum values (tPmax) meet great theoretical difficulties. Various approaches were developed to overcome these difficulties. Thus, for example, Russel et al. [58] suggested that account should be taken of the Brownian motion of particles in colloidal dispersions in the form of a hydrodynamic contribution. They showed that this contribution which is to be taken into account in considering a slow flow (with slow shear rates y), increases considerably with increasing dispersion concentration. For a description of the dependence of viscosity on concentration the above authors obtained an exact equation only in the integral form. At low shear rates it gives the following power series ... 

While asphalt itself consists of a complex colloidal dispersion of resins and asphaltenes in oils, introduction of liquid elemental sulfur, which on cooling congeals into finely dispersed crystalline sulfur particles and in part reacts with the asphalt, necessarily complicates the rheology of such a SA binder. Differences and changes with SA binder preparation, curing time, temperature etc. must be expected and may be demonstrated by viscosity characteristics. 

This behaviour ean be quahtatively explained by Equation (12.7) since 4>m increases as the width of the size distribution increases. A good example of the ef-feets that particles and polymers have on the rheological behavior of liquids is cloudy apple juice (Genovese and Lozano, 2000). The aqueous milieu of the juice is a solution of sugar, acids and salts (i.e., the clarified juice) that contains charged particles (0.25-5 pm in size) and pectin as a colloidal dispersion. The viscosity of cloudy apple juice has been described by the expression ... 

This section draws heavily from two good books Colloidal Dispersions by Russel, Seville, and Schowalter [31] and Colloidal Hydrodynamics by Van de Ven [32] and a review paper by Jeffiey and Acrivos [33]. Concentrated suspensions exhibit rheological behavior which are time dependent. Time dependent rheological behavior is called thixotropy. This is because a particular shear rate creates a dynamic structure that is different than the structure of a suspension at rest. If a particular shear rate is imposed for a long period of time, a steady state stress can be measured, as shown in Figure 12.10 [34]. The time constant for structure reorganization is several times the shear rate, y, in flow reversal experiments [34] and depends on the volume fraction of solids. The viscosities discussed in Sections 12.42.2 to 12.42.9 are always the steady shear viscosity and not the transient ones. 

At this voliime fraction, the viscosity diverges because the shear stress is now given by the particle-particle contact in the tightly packed structure. As a result, we obtain a fluid with visco-elastic properties similar to polymeric solids. In ceramic processing, we extrude and press these pastes into green shapes. As a result, the rheology of ceramic pastes is of importance. The rheology of very concentrated suspensions is not particularly well developed, with the exception of model systems of monodisperse spheres. This section first discusses visco-elastic fluids and second the visco-elastic properties of ceramic pastes of monodisperse spheres. The material on visco-elastic fluids draws heavily from the book Colloidal Dispersions by Russel, Saville, and Schowalter [31]. 

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