Colloidal systems viscosity

An interesting problem arises when we consider solutions or colloidal sols where the diffusing component is much larger in size than the solute molecules. In dilute systems Equation (1.14) would give an adequate value of the Peclet number but not so when the system becomes concentrated, i.e. the system itself becomes a condensed phase. The interactions between the diffusing component slow the motion and, as we shall see in detail in Chapter 3, increase the viscosity. The appropriate dimensionless group should use the system viscosity and not that of the medium and now becomes... 

The major difficulty in predicting the viscosity of these systems is due to the interplay between hydrodynamics, the colloid pair interaction energy and the particle microstructure. Whilst predictions for atomic fluids exist for the contribution of the microstructural properties of the system to the rheology, they obviously will not take account of the role of the solvent medium in colloidal systems. Many of these models depend upon the notion that the applied shear field distorts the local microstructure. The mathematical consequence of this is that they rely on the rate of change of the pair distribution function with distance over longer length scales than is the case for the shear modulus. Thus... 

The emulsion polymerization process has several distinct advantages. The physical state of the emulsion (colloidal) system makes it easy to control the process. Thermal and viscosity problems are much less significant than in bulk polymerization. The product of an emulsion polymerization, referred to as a latex, can in many instances be used directly without further separations. (However, there may be the need for appropriate blending operations,... 

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. 

As noted above, dilute colloidal systems display Newtonian behavior that is, their apparent viscosity is independent of the rate of shear. Accordingly, the capability to measure 77 under conditions of variable shear is relatively superfluous in these systems. However, non-Newtonian behavior is commonplace in charged colloids and coagulated colloids (see Section 4.8). 

Equation (20), known as the Poiseuille equation, provides the basis for the most common technique for measuring the viscosity of a liquid or a dilute colloidal system, namely, the capillary viscometer. 

The viscosity of colloidal systems depends upon the volume occupied by the colloidal particles. The simple equation of Einstein,... 

See Table 1. See also Colloid Systems Fluid and Fluid Flow Stoke s Law Viscoelasticity and Viscosity. 

If particle aggregation occurs in a colloidal system, then an increase in the shear rate will tend to break down the aggregates, which will result, among other things, in a reduction of the amount of solvent immobilised by the particles, thus lowering the apparent viscosity of the system. 

The principles of colloid stability, including DLVO theory, disjoining pressure, the Marangoni effect, surface viscosity, and steric stabilization, can be usefully applied to many food systems [291,293], Walstra [291] provides some examples of DLVO calculations, steric stabilization and bridging flocculation for food colloid systems. 

Figure 7. Formation of gels from iron molybdate precipitates [5]. A very fine colloidal precipitate forms in all cases but undergoes different changes suggested by the arrows, according to concentration and Mo Fe proportions, as well as other conditions mentioned in the figure. In the upper part domain, the colloidal precipitate seems to dissolve to form a solution whose viscosity increases until formation of a gel. In the right-hand domain, the gel directly forms from the colloidal system but can become transparent if the reaction takes place in the temperature range 293-313K.

Volatile species emanate from the fluid during pyrolysis. If the concentration of bubbles at any stage is appreciable then we have a 3-phase colloidal system and the gaseous bubbles will also tend to raise the viscosity of the system. 

The viscosity of a system is a measure of its resistance to flow under an applied stress. Colloids, especially lyophilic colloids, tend to increase the viscosity of a system. The large molecular chains become entangled in one another, increasing the resistance to flow. The viscosity of a colloidal system is related to the shape, molecular weight, and concentration of the colloid. Viscosity measurement can be used to obtain an approximate molecular weight value. 

The chapter has dealt with the stability and stabilisation of colloidal systems and covered topics such as their formation and aggregation. If the particle size of a colloidal particle determines its properties (such as viscosity or fate in the body), then maintenance of that particle size throughout the lifetime of the product is important. The emphasis in the section on stability is understandable. Various forms of emulsions, microemulsions and multiple emulsions have also been discussed, while other chapters deal with other important colloidal systems, such as protein and polymer micro- and nanospheres and phospholipid and surfactant vesicles. 

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. 

In colloidal systems where the dispersion medium is solid all processes aimed at changing the degree of dispersion are retarded due to high viscosity of dispersion medium and small diffusion coefficients of components. 

In many colloid systems particles are covered with adsorbed layers (see Chapter 5). These too influence the viscosity since the effective radius, and hence the effective volume fraction, is greater than that of the core particles. In attempting to fit experimental data on dispersions of spherical particles to theoretical equations the effective volume fractions must be employed. If measurements are made on very dilute suspensions and at low shear rates, equation (8.7) (retaining only the first two terms) may be used to calculate the effective volume fraction and hence the particle size, and the thickness of the adsorbed layer if the size of the core particles is known. This is not, however, a very precise method and generally other methods of finding the adsorbed layer thickness are to be preferred. 

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