Résumé
Aqueous colloidal suspensions of goethite have attracted much attention by their outstanding magnetic properties [1]. In both the isotropic and the nematic phase of these suspensions the goethite particles align either parallel to the field (at low field strength) or perpendicular to it (in high fields). The goethite particles orient easily also in electric fields [2], showing strong induced birefringence and short response times, making them attractive for potential applications.Here we study the electric field induced order in the isotropic phase of the goethite suspensions in water and other polar solvents. In our recently developed electro-optic setup [3] we measure the birefringence induced in the sample by short pulses of high frequency electric field. However, due to the strong absorption of the goethite particles and their high specific birefringence, the precision of the measurements is not satisfactory in the case of high volume fractions, >1%. To resolve this problem, we study also the field induced dichroism of the suspensions, using the same sample, sealed in a flat optical glass capillary, and the same experimental setup, mounted on a polarizing microscope. The good agreement between the induced order parameter values measured by the two techniques (see Figure 1) demonstrates that the dichroism is a useful alternative when a direct measurement of the birefringence is impractical. Moreover, in some cases the dichroism gives more information, because the two independent components of the absorption are measured separately, and not only their difference (as in the birefringence case). Indeed, the isotropic part of the absorption is a useful local probe for the volume fraction of the particles in inhomogeneous samples, and also it enables to measure the induced order parameter even when the light propagates parallel to the field (in polarised or unpolarised light). We discuss potential technological applications of the field induced order in the isotropic phase of colloidal suspensions and the required improvement of the materials for their practical realization.Acknowledgement: This work is supported by the ANR (France) through the grant NASTAROD. Figure 1: Field-induced order parameter in 1.0 % aqueous goethite suspension.References: [1]B. J. Lemaire, et al., Phys. Rev. Lett. 88, 125507 (2002).[2]B. J. Lemaire, et al., Eur. Phys. J. E 13, 309-319 (2004).[3]I. Dozov, et al., J. Phys. Chem. B 115, 7751-7765 (2011).