Thixotropic suspensions

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. 

The coefficient Tj is termed the modulus of rigidity. The viscosities of thixotropic fluids fall with time when subjected to a constant rate of strain, but recover upon standing. This behavior is associated with the reversible breakdown of stmctures within the fluid which are gradually reestabflshed upon cessation of shear. The smooth sprea ding of paint following the intense shear of a bmsh or spray is an example of thixotropic behavior. When viscosity rises with time at constant rate of strain, the fluid is termed rheopectic. This behavior is much less common but is found in some clay suspensions, gypsum suspensions, and certain sols. 

Time-dependent fluids are those for which structural rearrangements occur during deformation at a rate too slow to maintain equilibrium configurations. As a result, shear stress changes with duration of shear. Thixotropic fluids, such as mayonnaise, clay suspensions used as drilling muds, and some paints and inks, show decreasing shear stress with time at constant shear rate. A detailed description of thixotropic behavior and a list of thixotropic systems is found in Bauer and Colhns (ibid.). 

A study was made of the comparative stabilities at various exposures of an upset stomach remedy suspension. This product consisted of a dispersion of bismuth subsalicylate and phenylsalicylate in an aqueous system. Methyl-cellulose and magnesium aluminum silicate were selected as the suspending agents, because the presence of polyvalent metallic ions precluded the use of hydrocolloids affected by these ions. In addition, it was found that methyl-cellulose contributed a demulcent effect. The viscosity, as well as the suspension characteristics of the combination of protective colloids used, was of a synergistic nature. These colloids formed a thixotropic system. The thixotropy undoubtedly aids in stabilizing this system. In order to make this product palatable and impart elegance, color and flavor were added. Sample 1 (with protective colloids) showed no separation, while sample 2 (without... 

Thixotropic suspensions are generally required for static coating due to the long column preparation time. 

Many food preparations and some paints are deliberately designed to be thixotropic so that the solid matter remains in suspension when the product is standing but, on being shaken, the apparent viscosity falls and the product can be poured. 

Flow and self-leveling characteristics of these products are governed by the rheological behavior of the slurrylike materials. At the low water-cement ratios required to ensure proper suspension of the solids, most selfleveling compositions are characterized by a yield stress and thixotropic behavior [75]. To obtain self-leveling properties, the yield stress has to be reduced and this is achieved by the selection and combination of suitable mix ingredients at... 

Thixotropic gels are also useful as propellants for rocket applications. These are commonly called thixotropes or gels. Normal fuels and oxidizers are converted into thioxotropes by the addition of a small quantity of a gelling agent in order to ensure uniform suspension of solid additives (usually metal additives) during storage. It is reported that if very finely pulverized particles of beryllium or aluminum are suspended in the liquid fuel, it is possible to increase the specific impulse substantially. 

There are more complicated structures intermediate between pyrophyllite and talc with variable substitution of A1J and Mg2. Electroneulrality is maintained by hydrated cations between layers. Thus the montmorillonites arc unusual days forming thixotropic aqueous suspensions that arc used as well-drilling muds and in nondrip puints. They are derived from the formulation AU(OH)jSi40 ,-x-H2o with variable amounts of water, Mg3+ (in place of some Al5 ), and compensaUng cations. M"+ (M = Ca in fuller s earth, which is converted to bentonite, M = Na). Vermiculite likewise has variable amounts of water and cations, (t dehydrates to a talc-like structure with much expansion when heated (see page 750). 

Rheopectic. If certain thixotropic suspensions are rhythmically shaken or tapped, they will set or build up very rapidly, a phenomenon termed rheopexy. Apparent viscosity of a rheopectic substance increases with time (duration of agitation) at any constant shear rate. 

Rheopexy is time-dependent shear-thickening, and is sometimes observed as an acceleration of thixotropic recovery - for example, bentonite clay suspensions often set only slowly on standing but quite rapidly when gently disturbed. 

The life of an Avicel suspension can be extended by coprecipitating the rodlike structures with a protective colloid after trituration. Avicel-RC19 is limit cellulose that has been physically modified by coprecipitation with CMC to facilite dispersibility. Avicel-RC water suspensions simulate the properties of a hydrosol. At low aqueous concentrations, the apparendy hydrated crystallites assemble into a thixotropic, heat- and acid-stable structure whose viscosity depends direcdy on pH to about pH 10, whereupon it declines precipitously. The suspension coalesces at low pH. The addition of salt after mixing increases viscosity above what it would be if the salt were added at the time of mixing or shearing. 

The thixotropic characteristics provided by fumed sihca are due to its ability to develop a loosely woven, latticelike network by hydrogen bonding between particles. This network raises the apparent viscosity of the system, increases the cohesive forces, and contributes to the suspension of the solid. Because the hydrogen bonds themselves are relatively weak, they are easily disrupted through the action of an apphed stress or shearing force and quickly reform when the stress or shearing force is removed. 

The continuous decrease in A1203 incorporation in copper is also explained by a change in particle surface composition.43 Chloride present as an impurity forms CuCl, which adsorbs on the particles and thereby inhibits the adsorption of copper ions on the particles. This is an effect characteristic for a copper sulfate bath and was not found in nickel or cobalt baths. A different type of aging was reported for the codeposition of aggregated polystyrene particles with zinc.54,76 Polystyrene incorporation increased continuously in successive experiments, where the rotation speed of a cylinder electrode was randomly varied. Thixotropic viscous behavior of the aggregated suspension causes changes in aggregate size and suspension viscosity with rotation speed of the same time-scale as the experiments. 

Many suspensions, and also some polymer solutions, change in time (Figure C4-15). This is usually because stmctures are broken or formed by shearing. The result can be that the viscosity decreases in time (a thixotropic liquid) or increases (a rheopectic liquid). Yoghurt is a good example of a thixotropic liquid rheopectic fluids are rare. The changes in viscosity are often, but not always, reversible. Note that these time effects are not the same that we saw in viscoelastic liquids. 

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