FORMATION OF POLARIZED CONTACT LAYERS AND THE GIANT ELECTRORHEOLOGICAL EFFECT.

The discovery of the giant electrorheological (GER) effect poses a challenge to the theorists for an explanation of its physical underpinnings. This is particularly the case as the GER effect breaks the yield stress upper bound predicted on the basis of induced polarization mechanism. The GER effect was observed in suspensions of coated nanoparticles, each consisting of a core particle of barium titanyl oxalate (20-50 nm), coated with a 5-7 nm layer of urea. The suspending oil is also very important, as the "wrong" oil can mean no electrorheological effect at all. Our prior work has shown the GER data, on the field dependence of yield stress, to be quantitatively accountable by the formation of aligned molecular dipolar layers in the region of particle-particle contact. Here we justify this picture through microscopic statistical mechanic considerations. We present a model of hydrogen bonding which can give rise to both electrowetting between the particles and the oil, as well as inducing the formation of aligned surface molecular dipolar layers. Monte Carlo simulation is used to account for both the entropy and energetic effects. It is shown that the formation of aligned surface dipolar layers can indeed be a robust phenomenon. [ABSTRACT FROM AUTHOR]