Dispersion behavior, colloidal measurements

Use of Ultrasonic Vibration Potential To Monitor Coalescence. The complex chemical nature of crude oils makes it difficult to relate the dispersion behavior to the physicochemical properties at the crude-oil-water interface. In addition, the nonpolar and nontransparent nature of the oleic phase provides significant obstacles for studies of the interactions of the suspended water droplets in real systems. Recent development (28, 29) of electroacoustical techniques has shown considerable promise for electrokinetic measurements of colloidal systems and the direct monitoring of the rate and extent of coagulation (flocculation and coalescence) of water droplets in nontransparent water-in-oil media. The electroacoustic measurement for colloidal systems in nonpolar media is based on the ultrasound vibration potential (UVP) mode, which involves the applica-... 

This section draws heavily from two good books Colloidal Dispersions by Russel, Seville, and Schowalter [31] and Colloidal Hydrodynamics by Van de Ven [32] and a review paper by Jeffiey and Acrivos [33]. Concentrated suspensions exhibit rheological behavior which are time dependent. Time dependent rheological behavior is called thixotropy. This is because a particular shear rate creates a dynamic structure that is different than the structure of a suspension at rest. If a particular shear rate is imposed for a long period of time, a steady state stress can be measured, as shown in Figure 12.10 [34]. The time constant for structure reorganization is several times the shear rate, y, in flow reversal experiments [34] and depends on the volume fraction of solids. The viscosities discussed in Sections 12.42.2 to 12.42.9 are always the steady shear viscosity and not the transient ones. 

The object of this study was to clarify some aspects of the mechanism of shear-induced flocculation in colloidal dispersions. Vinyl chloride homopolymer and copolymer latices were prepared by emulsion polymerization using sodium dodecyl sulphate as emulsifier. Agglomeration behavior in these latices was studied by measuring the mechanical stability using a high speed stirring test. The latex particle size was measured by an analytical centrifuge. Molecular areas of emulsifier in the saturated adsorption layer at the surface of homopolymer and copolymer latex particles were estimated from adsorption titration data. 

A streaming current detector based on a completely different principle than the above instruments is presented in [299], The dispersion is in a narrow space between a vertical cylindrical vessel and a coaxial piston, which moves back and forth along the axis. The potential between two gold electrodes on the wall of the cylinder at different heights is measured, and its zero value is identified with the IEP. The apparatus own response corresponds to the electrokinetic behavior of the piston and cell materials. In the presence of a colloid, the piston and the cell are assumed to be covered with colloidal particles. The above design has been utilized in some commercial instruments ... 

Sometimes when dealing with a fluid that contains a dispersed particle phase that cannot be considered a component, we treat the suspension fluid as a continuum with a constitutive relation that is modified because of the presence of the particles. An example to be discussed in Chapter 5 is Einstein s modification of the Newtonian viscosity coefficient in dilute colloidal suspensions due to hydrodynamic interactions from the suspended particles. As with molecular motions, the modified coefficient may be determined from measurements of the phenomenon itself by using results from analyses of the particle behavior in the fluid as a guide. These ideas are further expanded upon in Chapter 9 where the behaviors of concentrated suspensions of colloidal and non-colloidal particles are examined. 

Particle electrophoresis, also sometimes known as microscope electrophoresis or microelectrophoresis, is one of the easiest and most useful techniques for investigating the electrical properties of colloidal particles. If the system of interest is in the form of a reasonably stable dispersion of particle size observable by light microscopy (say, larger than 200 nm for practical application), the electrokinetic behavior of the system can be observed and measured directly. Several commercial instruments are available for the purpose. For smaller particles, laser scattering instruments are now readily available. 

In the case of non-Newtonian behavior and especially in the case of viscoplastic behavior, such as that typical for moderately concentrated colloidal dispersions, Poiseuille s law gradually loses its validity. This happens because in the shear force-free central region close to the capillary axis, the structure of the concentrated colloidal system remains intact, so that the viscous shear exists only in the peripheral regions of the capillary. This process causes serious issues in the pumping of cement slurries or crude oil containing crystallizing phases. In laboratory practice, it is beneflcial to conduct such measurements in combination with other measurements that utilize uniform states. 

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