Particle dielectric property

The ER effect is obviously induced by an external electric field, and the polarization definitely plays an important role. The dielectric properties of both the dispersed particle and the dispersing medium should be critical to the ER effect. The dielectric tool thus is frequently used for investigating how dielectric property influences the ER effect. 

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I he ER efTecl depends on the applied electric field slrenglli, the frequency of the electric field, the particle conductivity, the particle dielectric properties, the particle volume fraction, temperature, water content, the liquid medium, even the electrode pattern, etc. Those parameters are reviewed in this chapter. 

The Hamaker constant A can, in principle, be determined from the C6 coefficient characterizing the strength of the van der Waals interaction between two molecules in vacuum. In practice, however, the value for A is also influenced by the dielectric properties of the interstitial medium, as well as the roughness of the surface of the spheres. Reliable estimates from theory are therefore difficult to make, and unfortunately it also proves difficult to directly determine A from experiment. So, establishing a value for A remains the main difficulty in the numerical studies of the effect of cohesive forces, where the value for glass particles is assumed to be somewhere in the range of 10 21 joule. 

Equations for calculating van der Waals interaction forces/energies between macromolecules or colloidal particles are quite well established (Israelachvili, 1992 Dickinson and McClements, 1995 McClements, 2005). (For example, see equations (3.35) and (3.36) in chapter 3). The interactions between nanoparticles are potentially more complicated, however, because the nanoparticle size and interparticle separation are comparable in magnitude, precluding the use of the asymptotic forms of the equations also nanoparticles are commonly anisotropic, and their dielectric properties are often not known (Min et al., 2008). 

Finally, an area which is in need of much further research is that of the dielectric properties of two-phase systems such as frozen foods, emulsions, whips and foams. It is well known that the dielectric behavior of particles of one dielectric property imbedded in a substrate of another, behave very differently from a distributive mixture of both. Fricke (1955) developed a model for randomly oriented oblate spheroids suspended in a continuous medium. It is expected that this model may be used successfully to model two-phase food systems, but to date there is very little literature reporting such studies. 

Owing to the Lorentz factor in formula (4.54), when v approaches c, we have b etf—meaning that a molecule can be excited by a very distant passing particle, which is in contradiction with reality. This is a consequence of the fact that in our derivation (as in Ref. 150) we made no allowance for the weakening of the interaction between a charged particle and molecule when b> a, which is due to the polarization of the medium. The account of dielectric properties of the medium should lead to finite values of b eff even at v = c. 

Ideally, electrical precipitators generally achieve collection efficiencies of more than 99% for a full range of particle size. The efficiency depends on the ratio of the collector surface area particle size and dielectric properties and the volumetric gas flow rate times the charged particle migration speed induced by the applied electrical field. 

The dielectric properties of oligodimethylsiloxanes and their dependence on temperature point to good dielectric characteristics of PMS liquids. Taking into consideration that PMS do not form conductive carbon particles in case of electric breakdown or sparking, it is obvious why they are used as liquid dielectrics in transformers and other electric devices. There is a law in Japan and the USA which forbids the use of nonflammable yet toxic pentachlorodiphenyl and leaves room for oligodimethylsiloxanes. This has stimulated a much more active production of PMS liquids in Japan, Germany and other countries. 

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