van der Waals induced distortions in nematic liquid crystals close to a surface

By a discrete-layer model, we investigate theoretically the alignment of a nematic liquid crystal close to a hat amorphous substrate. We show that the presence Df van der Waals interactions induces a strong distortion Idealized in a few molecular lengths. We compare these predictions with that of a continuum model recently developed by some of us. In the continuum model, we derive approximate analytical expressions for the distortion profile, showing that, asymptotically, the distortion decays as the inverse of the distance from the substrate. The amplitude of the distortion according to the two models is different, but the main qualitative features remain unchanged. We show that the main distorting mechanism can be attributed to an effective surface field. The 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.