Abstract

Aqueous suspensions of goethite (-FeOOH) nanorods form a lyotropic nematic phase in which the nanorods show orientational long-range order but only short-range positional order. It is actually one of the few examples of mineral liquid-crystals. Despite the fact that bulk goethite is a typical antiferromagnetic material, colloidal suspensions of goethite nanorods display unique magnetic properties [1-3]. Small angle X-ray scattering (SAXS) and birefringence measurements show that the nematic phase aligns in magnetic fields much lower than usual liquid crystals. The nanorods orient along the field direction at field intensities smaller than 350 mT, but they reorient perpendicularly to the field beyond 350 mT. This surprising behaviour is also observed in the isotropic phase which has a very strong magnetic-field-induced birefringence. These effects most probably result from a competition between a nanorod remanent magnetic moment and a negative anisotropy of its magnetic susceptibility. At still larger magnetic fields (around B l 1 T), the nematic suspensions form 2-dimensional columnar phases. This magnetic-field-induced transition, at constant concentration, is reversible. We could record the SAXS patterns of single crystals, which allowed us to determine the organisation of the nanorods in the columnar phase. Most of the experimental techniques and formalism used in these studies may be applied to (or even stem from) the field of suspensions of rod-like biomolecules (TMV, DNA , microtubules, F-actin, chitin, cellulose ….)

[1] Q.Majorana, Comptes Rendus Acad. Sci., 135, 159 (1902) [2] A.Cotton, H.Mouton, Comptes Rendus Acad. Sci., 141, 317 (1905) [3] B.J.Lemaire et al, Phys. Rev. Lett., 88, 125507 (2002). idem, 93, 267801 (2004).