We report the first experimental investigation devoted to analyze the influence of a ferrofluid on the surface properties of lyotropic liquid 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 transition, the thermodynamical model for the temperature surface transition works reasonably well. On the contrary, in the low temperature region near the lamellar–nematic transition, important deviations from the theoretical behavior are observed. In order to interpret our experimental data, we propose a simple phenomenological extension of the thermodynamical 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 constant depends on the ferrofluid doping concentration, in agreement with a recently proposed theory.

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We show that a surface field localized near the bounding surface over a mesoscopic length gives rise to a subsurface discontinuity. Our result is compared with a recent elastic theory in which the subsurface distortion is due to the splay-bend elastic constant. By means of our model, the interpretation of continuous surface variations of the average nematic orientation is given and compared with the elastic model. ͓S1063-651X͑96͒51205-5͔ PACS number͑s͒: 61.30.Cz, 61.30.Gd

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