Distortion in the surface layers on nematic liquid crystals: an elastic constant approach

A strong boundary distortion of the director field [ncirc](R) near the boundary surface of nematics is evidenced and theoretically discussed in terms of elastic constants. The sources of the distortion are the diffuse nature of the surface tension, the spontaneous splay, which is different from zero in a boundary layer of thickness comparable with the range of molecular interactions, and the surface-like free energy term k 13 div([ncirc] div [ncirc]). In the distorted layer a drastic change of [ncirc](R) with respect to the surface normal [kcirc] occurs. As a consequence, the surface alignment may appear homeotropic from a macroscopic point of view, even in the case where the short range nematic-substrate interactions give an obliquely oriented surface layer, as suggested by recent experiments of P. Guyot-Sionnest, H. Hsiung and Y. R. Shen.