MEASUREMENT OF THE SPLAY-BEND ELASTIC-CONSTANT IN LYOTROPIC FERRONEMATIC LIQUID-CRYSTALS - THE INFLUENCE OF THE BOUNDING SURFACES

We report the first experimental investigation devoted to analyze the influence of a ferrofluid on the surface properties of lyotropic liqui d crystals doped with magnetic particles. By means of optical methods, the bulk orientation of the director vs the temperature is determined , The theoretical analysis shows that near the nematic-isotropic trans ition, the thermodynamical model for the temperature surface transitio n works reasonably well. On the contrary, in the low temperature regio n near the lamellar-nematic transition, important deviations from the theoretical behavior are observed. In order to interpret our experimen tal data, we propose a simple phenomenological extension of the thermo dynamical model that takes into account the different contributions to the surface energy and of the residual lamellar order in the nematic phase, We show, furthermore, that the effective splay-bend elastic con stant depends on the ferrofluid doping concentration, in agreement wit h a recently proposed theory. (C) 1997 American Institute of Physics.