Control of the orientation of nematic liquid crystal on iron surfaces: Application to the self-alignment of iron particles in anisotropic matrices

We report on the interactions between nematic liquid crystal (NLC) and iron surfaces. Our aim is to apply, to iron particles, the new colloidal interactions evidenced by Poulin et al. [P. Poulin, H. Stark, T.C. Lubensky, D.A. Weitz, Novel colloidal interactions in anisotropic fluids, Science 275 (1997) 1770–1773]. This approach makes it possible to self-align water droplets suspended in NLCs. For so new type of interactions, the driving forces are related to the NLC distortions induced by the anchoring at the droplet surface. So, to generate self-alignment of iron particles, first, it is essential to control the NLC anchoring at iron surfaces. The anchoring of NLCs on various surfaces has been largely described; however, few results concern iron surfaces. In a first step, cleaned and reproducible iron surfaces were well characterized using reflection adsorption infrared (RAIR) and X-ray photoelectron spectroscopy (XPS). A reproducible hydrated native oxide was formed on iron plates. In a second step, these plates were modified with organic molecules. We describe planar and normal anchorings easily and reversibly obtained from adsorbed molecules such as lecithin and methanol. We also report on very stable planar and normal anchorings induce by the grafting of alkyltriethoxysilane derivatives. The experimental conditions used to form stable polymerised films on the plate surface were successfully adapted to particles. The formation, around isolated particles suspended in homogeneous NLC, of a hyperbolic hedgehog (a point defect) with normal boundary conditions, and the formation of boojums with planar boundary conditions, were evidenced. All this goes to show that these planar and normal anchorings were strong enough to self-align iron particles in homogeneous NLC. For homeotropic anchoring beautiful alignments of particles were evidenced; these chains of particles present the original property of being constituted of particles separated by a distance related to their diameters.