Elasticity, colloidal suspensions

In some cases the presence of slip is fairly obvious, as are its causes. For example, when an aqueous foam is sheared between smooth surfaces, the water in the foam can easily form a lubricating layer at the wall, leaving the bulk of the foam less sheared than intended (Yoshimura and Prud homme 1988 Khan et al. 1988). Gelled colloidal suspensions are elastic materials that contain solvents capable of lubricating rheometer tool surfaces, and slip is a problem (Buscall et al. 1993 Persello et al. 1994). In these and other cases, slip can be counteracted in a number of ways, for example by using roughened rheometer surfaces (Khan et al. 1988 Buscall et al. 1993),... 

Non-newtonian 1 Dispersions Elastic Yield stress Colloids Suspensions Emulsions Foams (mousses)... 

Indeed, even nonequilibrium systems do not necessarily show measurable excess noise and, thus, deviate from relation 1. An appropriate example that is relevant to the subject is a capillary channel that contains a stream of electrolyte maintained by an external pressure difference. Measurements on several aqueous polymer solutions with added electrolytes performed at up to 5000 dyn/cm2 shear stresses and zero external voltage showed that measurable excess noise can be observed only for non-Newtonian solutions exhibiting elasticity (19, 20). Similar results were obtained for colloid suspensions... 

Colloidal suspensions and polymer solutions have interesting mechanical properties. In general these materials have both viscosity and elasticity and hence are called viscoelastic. Colloidal suspensions show curious nonlinear hysteresis effects called thixotropy, rheopexy, and dUatancy. These unusual flow behaviours are the central problems of rheology. 

There are other reports on the study of pretransitional dynamics in polymeric and lyotropic nematics. Quantitative measurements of ratios of Frank elastic constants and Leslie viscosities in the pretransitional range of poly-y-benzyl-glutamate polymeric nematic are reported by Taratuta et al. [85]. McClymer and Keyes [86-88] report light scattering studies of pretransitional dynamics of potassium laurate-decanol-D20 system. An interesting study of a magnetic-field induced I N phase transition in a colloidal suspension is reported by Tang and Fraden [89]. 

Turbidimetry and nephelometry are two related techniques in which an incident source of radiation is elastically scattered by a suspension of colloidal particles. In turbidimetry, the detector is placed in line with the radiation source, and the... 

In suspension processes the fate of the continuous liquid phase and the associated control of the stabilisation and destabilisation of the system are the most important considerations. Many polymers occur in latex form, i.e. as polymer particles of diameter of the order of 1 p.m suspended in a liquid, usually aqueous, medium. Such latices are widely used to produce latex foams, elastic thread, dipped latex rubber goods, emulsion paints and paper additives. In the manufacture and use of such products it is important that premature destabilisation of the latex does not occur but that such destabilisation occurs in a controlled and appropriate manner at the relevant stage in processing. Such control of stability is based on the general precepts of colloid science. As with products from solvent processes diffusion distances for the liquid phase must be kept short furthermore, care has to be taken that the drying rates are not such that a skin of very low permeability is formed whilst there remains undesirable liquid in the mass of the polymer. For most applications it is desirable that destabilisation leads to a coherent film (or spongy mass in the case of foams) of polymers. To achieve this the of the latex compound should not be above ambient temperature so that at such temperatures intermolecular diffusion of the polymer molecules can occur. 

Chemical parameters determine the surface characteristics of the suspended colloids, the concentration of the coagulant and its effects upon the surface properties of the destabilized particles, and the influence of other constituents of the ionic medium upon the coagulant and the colloids. The extent of the chemical and physical interactions between the colloidal phase and the solution phase determines the relative stability of the suspended colloids. One speaks of stable suspensions when all collisions between the colloids induced by Brownian motion or by velocity gradients are completely elastic the colloidal particles continue their... 

Part of the process to make cheese involves the flocculation of an electrostatically stabilized colloidal O/W emulsion of oil droplets coated with milk casein. The flocculation is caused by the addition of a salt, leading to the formation of networks which eventually gel. The other part of the process involves reaction with an enzyme (such as rennet), an acid (such as lactic acid), and possibly heat, pressure and microorganisms, to help with the ripening [811]. The final aggregates (curd) trap much of the fat and some of the water and lactose. The remaining liquid is the whey, much of which readily separates out from the curd. Adding heat to the curd (-38 °C) helps to further separate out the whey and convert the curd from a suspension to an elastic solid. There are about 20 different basic kinds of cheese, with nearly 1000 types and regional names. Potter provides some classification [811]. 

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]. 

A typical result of a calculation [127] of the complex viscosity rf(co) is shown in Fig. 11. The real part of the viscosity, / (w), which describes the dissipation of energy when the fluid is sheared, is approximately frequency-independent for small cu, i.e., the fluid behaves as a Newtonian fluid. There is a characteristic frequency co where f/ (o>) drops rapidly. The imaginary part of the viscosity, rf"(o)), which describes the elastic response of the fluid to an external perturbation, increases linearly for small co and reaches a maximum at CO = CO. This behavior is not specific to microemuisions but has been observed in other complex fluids as well, such as in suspensions of spherical colloidal particles [128,129] and in dilute polymer solutions [130]. 

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