VAN-DER-WAALS INDUCED DISTORTIONS IN NEMATIC LIQUID-CRYSTALS CLOSE TOA SURFACE

By a discrete-layer model, we investigate theoretically the alignment of a nematic liquid crystal close to a hat amorphous substrate. We sho w that the presence Df van der Waals interactions induces a strong dis tortion Idealized in a few molecular lengths. We compare these predict ions with that of a continuum model recently developed by some of us. In the continuum model, we derive approximate analytical expressions f or the distortion profile, showing that, asymptotically, the distortio n decays as the inverse of the distance from the substrate. The amplit ude of the distortion according to the two models is different, but th e main qualitative features remain unchanged. We show that the main di storting mechanism can be attributed to an effective surface field. Th e existence of a subsurface distortion is compatible with some recent measurements of the alignment of the first molecular layer, performed with optical second-harmonic generation techniques.