The Influence of Van der Waals Forces on the Alignment of Nematic Liquid Crystals in Magnetic Fields

We calculate the van der Waals free energy for a nematic liquid crystal confined between two parallel dielectric half spaces. We consider two configurations: (i) director (direction of anisotropic axis) perpendicular to the interface, (ii) director parallel to the interface. Assuming these energies dominate, their differences are compared for two configurations in the presence of a uniform magnetic field parallel to the interface. For a given specimen thickness a magnetic field Hc is found such that for H < Hc the liquid crystal will be perpendicularly aligned and for H > Hc parallel alignment will occur. The dependence of Hc on thickness is calculated and the experimental evidence is discussed. When one of the dielectric half spaces is replaced by a metal half space, we predict that no critical magnetic field will exist.