Flocculation dilatant

Although the production of highly deflocculated suspensions is a primary objective for formulation of suspension concentrates, these systems tend to settle under gravity forming dilatant sediments (clays). The latter must be prevented either by controlled flocculation or by the addition of a second disperse phase to the continuous medium (1). One method which may be applied to sterlcally stabilised dispersions, is to add a free (ie. non-adsorbing) polymer to the continuous medium. 

Normally, in sediment volume measurements, the initial volume (or height Hjj) is compared with the ultimately reached value V (or H). A coUoidally 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/V or height H/H, it is possible to distinguish between a clayed and flocculated suspension. 

Test for the Presence of Blood. Gunning has discovered in acetate of zinc a rea ut that precipitates the slightest traces of tee coloring matter of blood from solutions, even where the liquids are so dilate as to be colorless. Blood washed from the hands in a pail of water can readily be detected in this way. The flocculent precipitate thrown down by acetate of zinc must be washed by decantation, and Anally collected on a watch-glass, and allowed to dry, when the microscope will readily reveal ciystals if any blood bo present. (See Ab. 6415.)... 

Non-flocculated suspensions can exist at very much higher concentrations and, at all but the highest volumetric concentrations, are often Newtonian. When such suspensions are sheared, some dilation occurs as a result of particles trying to climb over each other". If the amount of liquid present is then insufficient fully to fill the void spaces, particle-particle solid friction can come into play and the resistance to shear increases. This is just one way in which shear-thickening can occur. 

The upper GI tract examination may show dilated small bowel, thickened mucosal folds and rarely flocculation, as well as segmentation of the barium (Parker 2003). 

They adapted an interfacial shear rheometer (plate/ rod) to measure the shear viscoelasticity of the system with and without dispersant. At an applied shear stress, creep curves for the system were monitored. There were no instantaneous elasticity and viscosity for the Kuwait and Tia Juana crudes with and wifliout dispersant. They attributed this to a network structure of flocculated asphaltenes in the films. They found that there was some dilatancy in their crude oil films, described as a stick/slip flow in their flow curves. However, fliis flow was attributed to thick films of asphaltene particles building up at the interface. Lfsing creep measurements, they examined a model system of as-phaltenes/n-heptane/toluene. They found a retarded elastic deformation, which was different from the response of the crade oils. This suggested to fliem that there was a different type of interfaeial slrueture formed with the model oil, and this may be attributed to die solveney of the medium and not to die lower asphaltenes eontent in the model system. 

Umeya, K. and Kanno, T. (1979) Effect of flocculation on die dilatant flow for aqueous suspensions of titanium dioxides, /. Rheol, 23,123-40. 

When slurries having particles that are either relatively large or have a high ionic charge on their surface (and hence have little tendency to flocculate) settle, the settled bed density approaches about 50 to 70% of the particle density. The bed resuspends only slowly and is not easily deformed rapidly. An example of such a bed is settled sand. In general, such beds may exhibit dilatancy, which means that the bed must expand to be deformed, and the apparent viscosity of the bed increases as the rate of shear increases. Th02 spheres of more than 5 to 10 microns settle to beds of this type. 

In our explanation of dilatancy the condition of stability is essential, because only stable Suspensions can give a sediment which is dense enough (see 5b, p. 357) to show dilatancy. In a flocculated system the concentration of particles remains too low. 

Actually as a consequence of the much denser packing of a stable i stem when compared to a flocculated one (see 4, this page) the dilatancy was only evident after partial settling of the stabilized system and decanting of the supernatant liquid. 

Just as it is usually more difficult to prepare stable sols than flocculated systems, there is more chance of obtaining a plastic or at least a flocculated system in preparing a concentrated suspension than a stable, dilatant one The agents promoting the flocculated state are the same as those for dilute colloidal systems and have been treated extensively in chapter VIII Wc only draw attention again to the influence of electrolytes in aqueous suspensions according to the rule of Schulze and Hardy (e chapter VIII, 10 a, p 335) ... 

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