Anomalous anisotropy of the elastic constants in the induced smectic O film

Surface tension is known to induce smectic O (S_O) films at the free surface of isotropic droplets of 1-methylheptyl terephthalylidene-bis-4-aminocinnamate (MHTAC). The S_O film has its molecules disposed in a herringbone fashion, with the layers parallel to the free surface. It constitutes a model of a 2D polar nematic liquid crystal without any contact to solid substrates. The film is oriented uniformly when applying a uniform electric field, except along disclination walls. By measuring the width of the disclination walls in the two configurations with the electric field parallel and perpendicular, we determine the anisotropy of the elastic constants in the induced S_o films. We find the ratio of the 2D bend to splay elastic constants K_b/K_s to be ≃ 0.25 which is anomalously small when compared to the corresponding 3D ratio (K₂ + K₃)/2K₁ usually found for the bulk nematic phase. This experimental result is comparable to measurements performed with 2D suspended S_c films with, however, a different physics.