Concentrated dispersions

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. 

Aqueous Dispersions. The dispersion is made by the polymerization process used to produce fine powders of different average particle sizes (58). The most common dispersion has an average particle size of about 0.2 p.m, probably the optimum particle size for most appHcations. The raw dispersion is stabilized with a nonionic or anionic surfactant and concentrated to 60—65 wt % soHds by electrodecantation, evaporation, or thermal concentration (59). The concentrated dispersion can be modified further with chemical additives. The fabrication characteristics of these dispersions depend on polymerization conditions and additives. 

The cost of the individual colorants plus the method of addition (concentrate, dispersion or raw colorant) may be a significant portion of a colored part s costs. These costs often can be rapidly changing because of raw material avaUabUities. An accurate up to date cost profile of colorant raw materials should be kept for every formulator. 

Contaminant concentrations Dispersal of airborne contaminants such as odors, fumes, smoke, VOCs, etc. transported by these airflows and transformed by a variety of processes including chemical and radiochemical transformation, adsorption, desorption to building materials, filtration, and deposition to surfaces evolution of contaminant concentrations in the individual zones air quality checks in terms of CO2 levels cross-contamination evaluation of zones air quality evaluations in relation to perception as well as health. Methods ate also applicable to smoke control design. 

Kandyrin, L. B. and Kuleznev, V. N. The Dependence of Viscosity on the Composition of Concentrated Dispersions and the Free Volume Concept of Disperse Systems. Vol. 103, pp. 103-148. 

Uriev NB (1980) Highly Concentrated dispersed systems Khimiya, Moscow... 

Third, a complicated question on the role of the dispersion of particles dimensions of particles dimensions is of independent value it is known that the viscosity of equi-concentrated dispersions of even spherical particles depends on the fact if spheres of one dimension or mixtures of different fractions were used in the experiments and here in all the cases the transition from monodisperse particles to wide distributions leads to a considerable decrease in viscosity [21] (which, certainly, is of theoretical and enormous practical interest as well). 

Many sea trials of dispersant chemicals to demonstrate the effectiveness of specific products or to elucidate the processes of oil dispersion into the water column have been described. Most tests have proved inconclusive, leading many to believe that dispersant chemicals are only marginally effective. Tests in a wave basin have been conducted to measure dispersant effectiveness under closely controlled conditions [261]. These tests show that dispersed oil plumes may be irregular and concentrated over small volumes, so extensive plume sampling was required to obtain accurate dispersant effectiveness measurements. In large-scale sea trials, dispersants have been shown effective, but only when sufficient sampling of the water column was done to detect small concentrated dispersed oil plumes and when it was known that the dispersant was applied primarily to the thick floating oil. 

Investigations of the rheological properties of disperse systems are very important both from the fundamental and applied points of view (1-5). For example, the non-Newtonian and viscoelastic behaviour of concentrated dispersions may be related to the interaction forces between the dispersed particles (6-9). On the other hand, such studies are of vital practical importance, as, for example, in the assessment and prediction of the longterm physical stability of suspensions (5). 

Any fundamental study of the rheology of concentrated suspensions necessitates the use of simple systems of well-defined geometry and where the surface characteristics of the particles are well established. For that purpose well-characterized polymer particles of narrow size distribution are used in aqueous or non-aqueous systems. For interpretation of the rheological results, the inter-particle pair-potential must be well-defined and theories must be available for its calculation. The simplest system to consider is that where the pair potential may be represented by a hard sphere model. This, for example, is the case for polystyrene latex dispersions in organic solvents such as benzyl alcohol or cresol, whereby electrostatic interactions are well screened (1). Concentrated dispersions in non-polar media in which the particles are stabilized by a "built-in" stabilizer layer, may also be used, since the pair-potential can be represented by a hard-sphere interaction, where the hard sphere radius is given by the particles radius plus the adsorbed layer thickness. Systems of this type have been recently studied by Croucher and coworkers. (10,11) and Strivens (12). 

To (obtained from extrapolation of the ascending part of the flow curve) as a function of C. The shear modulus, GQ, measured using the pulse shearometer, is also shown as a function of C in the same figure. A measurable x and GQ is obtained above a critical value of C, which in both cases is -0.22 mol dm 3. As we will see later, this electrolyte concentration should be taken as the critical flocculation concentration (CFC) for the concentrated dispersion. Above the CFC, xg increases rapidly with increasing C whereas G initially increases gradually with increasing C until C = 0.3 mol dm 3, above which there is a more rapid increase of Gq. 

The CFC obtained with the dilute dispersion ( 10 %) using the Nanosizer was -0.28 mol dm, i.e. significantly larger than that for the concentrated dispersion. 

Figure 5 shows the variation of Xg with temperature at two C values (0.20 and 0.25 mol dm 3). In both cases Xg is essentially zero until a critical temperature is reached, above which xg increases rapidly with increasing temperature reaching a maximum above which there is a tendency forXg to fall again with further increase in temperature. The critical temperature corresponding to the abrupt increase inTg is 20 and 25°C for C equal to 0.25 and 0.20 mol dm, respectively This temperature may be identified with the critical flocculation temperature (CFT) of the concentrated dispersion. 

Gmin.in the free energy-particle separation curves. Since AS g is reduced in concentrated dispersions,the flocculation of the dispersion occurs at relatively lower Gm- n than that observed with dilute dispersions. Thus, this effect would result in a reduction of the CFC for concentrated dispersions. However, the net result of reduction of CFC may be due to a combination of this effect and depletion of electrolyte from the dense region of the adsorbed layers. 

Note that err = y (crr)a3/k Tand recall that in a concentrated dispersion the Peclet number is Pe = 67ry (crr)a3/k T. The use of the suspension viscosity implies that the particle diffusion can be estimated from an effective medium approach. Both Krieger and Cross gave the power law indices (n and m) as 1 for monodisperse spherical particles. In this formulation, the subscript c indicates the characteristic value of the reduced stress or Peclet number at the mid-point of the viscosity curve. The expected value of Pec is 1, as this is the point at which diffusional and convective timescales are equal. This will give a value of ac 5 x 10 2. Figure 3.15 shows a plot of Equation (3.57a) with this value and n = 1... 

For a concentrated dispersion the particles feel the effects of all their neighbours and the sedimentation velocity can be represented semi-empirically as8... 

Owing to the simphcity and versatility of surface-initiated ATRP, the above-mentioned AuNP work may be extended to other particles for their two- or three-dimensionally ordered assemblies with a wide controllabiUty of lattice parameters. In fact, a dispersion of monodisperse SiPs coated with high-density PMMA brushes showed an iridescent color, in organic solvents (e.g., toluene), suggesting the formation of a colloidal crystal [108]. To clarify this phenomenon, the direct observation of the concentrated dispersion of a rhodamine-labeled SiP coated with a high-density polymer brush was carried out by confocal laser scanning microscopy. As shown in Fig. 23, the experiment revealed that the hybrid particles formed a wide range of three-dimensional array with a periodic structure. This will open up a new route to the fabrication of colloidal crystals. 

The magnetic interaction energy increases if the interaction works between multiparticles. The multi-interaction becomes important in a concentrated dispersion (3). 

Silica gel is synthetic amorphous silica consisting of a compact network of spherical colloidal silica particles. Its surface area is typically between 300 and 850 m2/g. The predominant pore diameters are in the range 22-150 A. Silica gel is produced via the following procedure a sodium silicate solution reacts with a mineral acid, such as sulfuric acid, producing a concentrated dispersion of finely divided particles of hydrated Si02,... 

Even the traditional methods discussed in this chapter can be used for concentrated dispersions through contrast matching. For example, silica particles coated with silane coupling agents in a refractive index-matched mixture of ethanol and toluene can be used in combination with visible probe particles to study the dynamics of particles in dense systems. In the case of microemulsions (Chapter 8), selective deuteration of a component (oil, water, or surfactant) can be used in neutron scattering experiments even to measure the curvature of the oil-water interface. 

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