M. Rajteri et al., VAN-DER-WAALS INDUCED DISTORTIONS IN NEMATIC LIQUID-CRYSTALS CLOSE TOA SURFACE, Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics, 53(6), 1996, pp. 6093-6100
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.