EXPERIMENTAL-DETERMINATION OF THE EFFECTIVE SPLAY-BEND ELASTIC-CONSTANT

The temperature-induced surface alignment transitions in nematic liqui d crystals are analyzed. It is shown that it is possible to interpret the experimental data by means of an elastic model in which the effect ive splay-bend surfacelike elastic constant is taken into account. Thi s effective elastic constant is defined as the sum of the usual splay- bend and spontaneous splay elastic constant. In this framework the exp erimental 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 va cuum coated by SiO) and on the surfactant dissolved in the liquid crys tal. The value of the effective splay-bend elastic constant is obtaine d for three different liquid crystals in the nematic phase.