Ordered suspensions structure

For 10-/mn particles suspended in water at room temperature, fR is of the order of 10 min. Hence, it is only in dilute systems of small particles that complete relaxation of a suspension structure can take place within the shear rate range of most rotational viscometers on the market. 

Figure 5. Suspension structures (a) disordered (stable) (b) ordered (stable) and (c) flocculated.

The compressive yield properties are strongly dependent on the suspension structure. Mills et al. (186) showed that extensive preshearing of a flocculated suspension can result in a much denser floe morphology (in fractal terms), which was reflected by a decrease in the compressive yield stress by one order of magnitude. 

The slow settling and consolidation after the suspension, reaching about 30% solids, have been attributed to the presence of unrecovered bitumen (Figure 9) (5) or to the presence of fine clays and amorphous materials, which either may hold amounts of water disproportionate to their concentration or may form an ordered floe structure at 30 wt% solids (4). One explanation emphasizes the possible role of soluble organic surfactants that modify the clay surfaces, the effect of strongly bound organic material on the minerals, asphaltenic components from... 

Thixotropy is a reversible process and in an immobile state of fluid the continuous, progressive ordering of structure occurs. The one flow curve without hysteresis can be obtained, but the share must continue until the equihbrium is attained. The structure of ordinary tixotropic fluids is totally destroyed at high shear stress. After the shear stress elimination they behave like normal fluids until the stracture is rebuilt. There are also the tixotropic plastic fluids (Fig. 5.5) which do not lose totally the features of plastic fluids which is evident from the stable, however, even low yield stress value. Some suspensions show outstanding features the stracture is formed under the shear stress exclusively and without shear it collapses these fluids show a rheopexy. These properties are appear at moderate shear rate only. 

Sood A K 1991 Structural ordering in colloidal suspensions Soiid State Phys. 45 1-73... 

Carbon blacks are synthetic materials which essentially contain carbon as the main element. The structure of carbon black is similar to graphite (hexagonal rings of carbon forming large sheets), but its structure is tridimensional and less ordered. The layers of carbon blacks are parallel to each other but not arranged in order, usually forming concentric inner layers (turbostratic structure). Some typical properties are density 1.7-1.9 g/cm pH of water suspension 2-8 primary particle size 14-250 nm oil absorption 50-300 g/100 g specific surface area 7-560 m /g. 

The critical gel equation is expected to predict material functions in any small-strain viscoelastic experiment. The definition of small varies from material to material. Venkataraman and Winter [71] explored the strain limit for crosslinking polydimethylsiloxanes and found an upper shear strain of about 2, beyond which the gel started to rupture. For percolating suspensions and physical gels which form a stiff skeleton structure, this strain limit would be orders of magnitude smaller. 

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