Surface area, effect viscosity limited

The main attraction is that, because of the high surface area and aspect ratio, these benefits are potentially obtainable at much lower volume fractions than with most other established fillers. This has several potential advantages, notably lower cost, lower density, less opacity, and potentially less reduction in other properties such as impact strength. Because of viscosity effects associated with the plate shape and potential particle damage, and difficulties in obtaining and maintaining exfoliation, the practical loading level is, however, limited to about 10% which severely restricts the absolute property values achievable. These various aspects are discussed further next, with the emphasis on thermoplastic applications, which are currently of most interest. 

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. 

Some of this mobility control and improvement in sweep could be due to the emulsions formed diich exhibit a higher bulk viscosity. Some work by Wasan et al (32) has shown that the sodium silicates will result in emulsions with lower shear or interfacial viscosities than sodium hydroxide. These lower interfacial viscosities at the micro-level help promote oil coalescence so that oil banking can occur and oil droplets are not retrapped and left behind. The oil banking and rapid emulsification on the macro-level or in the bulk help to give good mobility control. Also some recent work by Wasan has indicated that the silicates tend to keep the surfaces more water-wet, thereby inproving recovery. It has been noted that the thickness of the water film on quartz surfaces is thicker when silicates are present. Further work in these areas is currently being done to determine the limits and effects on the oil recovery mechanisms. 

Shearer and Akers [5], Callaghan et al. [114] supposed that the mechanism involves elimination of surface tension gradients (see Section 4.4.3) as indicated by elimination of surface elasticity. These authors studied the effect of PDMSs on the surface elasticity of crude oil. PDMSs are used as antifoams to assist gas-oil separation during crude oil production and are apparently effective at the remarkably low concentration of 1 part per million (which presumably still exceeds the solubility limit). Callaghan et al. [114] find that PDMS diminishes the frequency-dependent dynamic dilational (elastic) modulus e = doAo (0/d In A(t) relative to that found for the uncontaminated oil. Here Oao(0 is the time-dependent air-crude oil surface tension, and A(t) is the area of a constrained element of air-crude oil surface subject to time-dependent dilation. The effect is more marked the higher the molecular weight (or viscosity) of the PDMS. This correlates with an enhanced antifoam effectiveness found with increase in molecular weight. 

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