Experimental determination of the effective splay-bend elastic constant

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Abstract
The temperature-induced surface alignment transitions in nematic liquid crystals are analyzed. It is shown that it is possible to interpret the experimental data by means of an elastic model in which the effective splay-bend surfacelike elastic constant is taken into account. This effective elastic constant is defined as the sum of the usual splay-bend and spontaneous splay elastic constant. In this framework the experimental data seem to indicate that the easy axis characterizing the nematic-substrate interaction is independent of the liquid crystal. It depends only on the evaporation geometry (we use flat glass plates vacuum coated by SiO) and on the surfactant dissolved in the liquid crystal. The value of the effective splay-bend elastic constant is obtained for three different liquid crystals in the nematic phase.