Rheology free-disperse systems

Structural losses are characteristic of structured disperse systems. In free disperse systems these losses play a role at high concentration of dispersed phase. Structural losses are related to the oscillations of the network of particles, which may be viewed as a number of interconnected oscillators. As pointed out by Dukhin [26,27], structural losses provide a link that bridges acoustics with rheology. At this time there is no adequate theoretical description of this energy loss mechanism. 

Fig. IX-23. Rheological curve of a free disperse system containing anisometric particles cot...

When the contacts between the particles in a free-disperse system are established, the transition of the system into a connected-disperse state takes place. This transition is associated with the development of a spatial network of particles in which the cohesive forces between the particles forming a network are sufficiently strong to resist thermal motion and the action of external forces. As a result of the transition, the system acquires a set of new structnral-mechanical (rheological) properties that characterize the ability of the syston to resist deformation and separation into individual parts. That is, the system acquires mechanical strength, which is the principal and universal characteristic of all solid and solid-like materials. For many materials, their mechanical strength defines the conditions of their use. 

One of the unique rheological features of emulsions is that the apparent viscosity of the emulsion can drop below the viscosity of the continuous phase when the concentration of the dispersed phase is low, normally below 0.1 in volume fraction (194). When solids are added to the emulsion, the apparent viscosity can decrease even further and the volume fraction of the dispersed phase at which minimum viscosity occurs increases with increasing solids content. Figure 30 shows the apparent viscosity of water-and-sand-in-bitumen, pwsh, variation with the solid-free water volume fraction, j8w, for two shear rate values. The experimental data were provided by Yan (private communication), where the system consists of 52 pm sand particles treated with hexadecyltri-methylammonium bromide (HAB) and water droplets of a Sauter mean diameter of 9 pm dispersed in bitumen at 60 °C. The sand particle volume fraction on water-free basis is j8s = 0.193. The range of the water droplet volume fraction, on a solid-free basis, jfrw is between 0 and 0.4. It can be observed that a minimum viscosity is present at a solid-free water droplet volume fraction of about 0.1. For a lower solid concentration, Ps = 0.113, the minimum apparent viscosity is found at /3W = 0.05... 

A suspension of finely dispersed particles is thermodynamically unstable this system is inclined to lower its free energy through flocculation. The stability and rheology of a powder in a suspension depend on the nature of the solid/ solution interface, particularly on the electrical properties of this region [70]. This interface may be described as consisting essentially of two layers (Fig. 3) ... 

The first chapter by J. Jiao and D. J. Burgess discusses the thermodynamic instability of multiple emulsions as a result of the excess of free energy caused by the formation of the emulsion droplets. In multiple emulsions consisting of three distinct liquid phases, counteracting the effect of the Laplace pressure by electrolyte addition to the inner dispersed aqueous phase will increase the destabilization of the system owing to osmotic pressure. In addition the authors discuss the effects of both osmotic and Laplace pressure as well as the interfacial rheological properties of these complex systems and their stability. 

The rheology of HA-Na is extremely sensitive to protein contamination [39, 40]. Comparable results are obtained using protein free samples obtained from bacterial sources. Some of these studies were conducted in saline aqueous medium to reproduce physiological conditions in which dilute and semidilute HA-Na dispersions exhibit Newtonian behavior in a wide shear rate range. Equihbrium and release properties of aqueous systems PE-dmg dispersions can be reasonably... 

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