Electrically induced birefringence in nanoparticle dispersions for electrorheological applications

Recently, the observation of an anomalously large electrorheological effect in the dispersion of nanosized particles of titania in octanoid acid has been reported. Such an enhanced effect was not observed in the similar dispersion of micrometric particles or in more conventional suspensions of silica in silicon oil. It was suggested that this effect could be promoted by the formation of a thin layer of solvent molecules on the surface of the titania particles. We propose the measurement of electrically induced optical birefringence as a suitable independent method for testing this working hypothesis. In this paper, we report the results from the investigations of the dilute dispersions of 32 nm TiO2 particles in two insulating fluids: silicone oil and octanoic acid. A comparison of the experimental birefringence data with the theoretical predictions suggests that TiO2 nanoparticles behave like permanent electric dipoles, although induced dipoles are expected in the case of the titania material. The source of such behaviour has been individuated at the particle/solvent interface and the different possibilities of the permanent dipole origin are discussed. The lower value of the dipole moment observed in octanoic acid dispersion is explained in terms of a specific particle/solvent interaction leading to the formation of a solvent coating around the particle. The results highlight that electro-optical properties are related to electrorheological performance and that both methods can be considered as supportive for testing electrically driven phenomena in complex fluids.