Colloidally stable suspension

Reversal of coagulation or flocculation, i.e., colloidally stable suspension or emulsion. [2]... 

Oc is the geometric constant for the shape of the ceramic particles and [1 + (Lg/a f] is the volume fraction correction for the adsorbed layer on the ceramic particles. Again, this equation is only good for colloidally stable suspensions. Fleer et al. [19] verified this equation for cubic particles with polytvinyl alcohol) adsorbed at the surface. For polymer solution concentrations (i.e., p) that give essentially monolayer coverage of the particle surface, the value of [1 + (LJaf] is nearly constant for a wide range of ceramic powder concentrations (Le., d>c)-... 

For example, an alumina coating with a median pore size of typically 100 nm can be prepared from a suspension (in water) of commercially available submicron alumina powder with a mass based median diameter of 500 nm. In such a suspension colloidal interactions determine to a large extent the properties of the suspension. The particle packing properties are disturbed by the presence of a fraction of aggregates which always exist in such commercial powders. This fraction can be removed from a colloidally stable suspension by means of sedimentation fractionation (see Ref. [5] for an example). 

With increasing shear, Pe — the relative viscosity of suspensions — usually decreases (see Fig. 6.19). This shear thinning effect is quite moderate in colloidally stable suspensions, which actually can behave as nearly Newtonian up to... 

Figure 7 illustrates the total interparticle potential, E, for colloidally stable systems and flocculated systems, where d is the particle diameter and r is the distance between the centers of two approaching particles. A colloidally stable suspension is characterized by a repulsive interaction (positive potential) when two particles approach each other (Figure 7a). Such a repulsion varies with distance, and hence it is termed soft repulsion. In the extreme, owing to the short range of the repulsive...

Most colloidal stable suspensions show more or less reversible response to compression and decompression. However, in the case of flocculated suspensions, the compressive properties are irreversible. In concentrated flocculated suspensions, a continuous particle network forms. The particle network can support some stress up to a critical value. Once this critical stress, also called the compressive yield stress Fy, is exceeded, the network consolidates to a higher volume fraction with a higher critical stress. 

In many ceramic systems it is not possible to create a stable suspension simply by controlling pH. Large additions of acid or base can result in dissolution of the particles, or provide a too high ionic strength. Hence, addition of suitable polymeric dispersants is commonly used to create colloidally stable suspensions. These polymeric additives can induce an interparticle repulsion that prevents coagulation. Upon the close approach of two particles covered with adsorbed polymer layers, the interpenetration of the polymer layers give rise to a repulsive force, the so-called steric stabilization (10). There are some simple requirements for steric stabilization of colloidal suspensions, as follows ... 

Normally, in sediment volume measurements, one compares the initial volume Vo (or height Hq) with the ultimately reached value V (or H). A colloidally stable suspension gives a close-packed structure with relatively small sediment volume (dilatant sediment referred to as clay). A weakly flocculated or structured suspension gives a more open sediment and hence a higher sediment volume. Thus by comparing the relative sediment volume V/Vo or height H/Hq, one can distinguish between a clayed and flocculated suspension. 

The relaxation time may be used as a guide for the state of the suspension. For a colloidally stable suspension (at a given particle size distribution), t increases with increase of the volume fraction of the solid phase, rj). In other words, the cross-over point shifts to lower frequency vdth increase in rj). For a given suspension, t increases with increasing flocculation, providing the particle size distribution remains the same (i.e. no Ostwald ripening). 

States (a)-(c) in Fig. 1.4 represent the case for colloidally stable suspensions. In other words the net interaction in the suspension is repulsive. Only state (a) with very small particles is physically stable. In this case the Brownian diffusion can overcome the gravity force and no sedimentation occurs. This is the case with nanosuspensions... 

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