Colloidal suspensions relative viscosity

For monodisperse or unimodal dispersion systems (emulsions or suspensions), some literature (28-30) indicates that the relative viscosity is independent of the particle size. These results are applicable as long as the hydrodynamic forces are dominant. In other words, forces due to the presence of an electrical double layer or a steric barrier (due to the adsorption of macromolecules onto the surface of the particles) are negligible. In general the hydrodynamic forces are dominant (hard-sphere interaction) when the solid particles are relatively large (diameter >10 (xm). For particles with diameters less than 1 (xm, the colloidal surface forces and Brownian motion can be dominant, and the viscosity of a unimodal dispersion is no longer a unique function of the solids volume fraction (30). 

With increasing shear, Pe — the relative viscosity of suspensions — usually decreases (see Fig. 6.19). This shear thinning effect is quite moderate in colloidally stable suspensions, which actually can behave as nearly Newtonian up to... 

Here, is termed the fine relative viscosity and represents the contribution of the colloidal size particles. It is defined by the ratio of the apparent viscosity of the mixture of suspending liquid plus fine particles, to the viscosity of the suspending liquid, ju,. The quantity, is termed the coarse relative viscosity and is the contribution of the coarse particles to the net relative viscosity. It is defined by the ratio of the apparent viscosity of the coarse suspension, 17, to the apparent viscosity of the fine fraction, 17. ... 

Chapter 8 briefly introduced the concept of supercritical fluids in the context of undersea thermal vents. The supercritical point for water occurs at a temperature of 705°F (374°C) and a pressure of 222.3 bar (atmosphere). Above this temperature, no pressure can condense water to its liquid state. For carbon dioxide (CO2), the critical temperature (88.0°F or 31.1°C) and critical pressure (73.8 bar) are much lower. Above the supercritical point, CO2 behaves as a liquidlike gas liquidlike densities, gaslike viscosities. The solubility properties of supercritical CO2 are mnable by varying temperature and/or pressure. Density and dielectric constant increase with increasing pressure and decreasing temperature. Water and ionic substances are insoluble in supercritical CO2. The ability of supercritical CO2 to dissolve and extract relatively non-polar substances has been known for decades. The range may be extended by adding polar solvents such as methanol or acetone. The addition of surfactants helps to disperse microscopic particles to form colloidal suspensions. Carbon dioxide is nonflammable, nontoxic, and inexpensive. 

In summary, ultrasonication facilitates the dispersion of colloidal suspensions at relatively high stress intensities. It relies on the presence of cavitation and on a deep penetration of the sound field into the suspension. The latter is guaranteed by low suspension viscosities (e.g. as for aqueous media with low solid content). A particular problem is the abrasion of the sonotrode tip, which depends on the solid phase, the particle size and shape, the sound intensity, as well as on the abrasion history of the tip. Abrasion particles from sonotrodes are coarse particles in the micrometre range and may spoil the quality of colloidal suspensions. 

Figure 17 The calculated relative viscosity of a spherical colloidal suspension with Eq. (91). vs. xr at different particle volume fractions (large scale)...

Since the colloidal particle is dispersed in a viscous fluid, the relative motion between the particle and the viscous liquid medium plays an important role in the flow behavior of a whole colloidal suspension. For a single spherical particle of radius r in a state of relatively moving in a Newtonian liquid of viscosity r, the frictional force F exerted on the particle can be expressed by Stokes law ... 

The bimodal model has also been applied to polydisperse suspensions (Probstein et al. 1994), which in practice generally have particle sizes ranging from the submicrometer to hundreds of micrometers. In order to apply the bimodal model to a suspension with a continuous size distribution, a rational procedure is required for the separation of the distribution into fine and coarse fractions. Such a procedure has not been developed so that an inverse method had to be used wherein the separating size was selected which resulted in the best agreement with the measured viscosity. Again, however, the relatively small fraction of colloidal size particles was identified as the principal agent that acts independently of the rest of the system and characterizes the shear thinning nature of the suspension viscosity. 

Albert Einstein (1879-1955). .. was a German-bom theoretical physicist who is mainly renowned for his special theory of relativity and its extension to the general theory of relativity. In addition to this, he worked on statistical mechanics and quantum theory and investigated the thermal properties of light. At the beginning of his scientific career he also set important landmarks for colloid science. This applies particularly to his explanation of Brownian motion, but is also valid for the calculation of suspension viscosity as well as his theory of critical opalescence. In 1921, he was given the Nobel Prize in Physics Tor his services to theoretical physics, and especially for his discovery of the law of the photoelectric effect . 

The list of experimentally accessible properties of colloid solutions is the same as the list of accessible properties of polymer solutions. There are measurements of single-particle diffusion, mutual diffusion and associated relaxation spectra, rotational diffusion (though determined by optical means, not dielectric relaxation), viscosity, and viscoelastic properties (though the number of viscoelastic studies of colloidal fluids is quite limited). One certainly could study sedimentation in or electrophoresis through nondilute colloidal fluids, but such measurements do not appear to have been made. Colloidal particles are rigid, so internal motions within a particle are not hkely to be significant the surface area of colloids, even in a concentrated suspension, is quite small relative to the surface area of an equal weight of dissolved random-coil chains, so it seems unlikely that colloidal particles have the major effect on solvent dynamics that is obtained by dissolved polymer molecules. 

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