Computer simulations are presented of the isotropic-to-nematic transition in
a liquid crystal confined between two parallel plates a distance H apart. The
plates are neutral and do not impose any anchoring on the particles. Depending
on the shape of the pair potential acting between the particles, we find that
the transition either changes from first-order to continuous at a critical film
thickness H=Hx, or that the transition remains first-order irrespective of H.
This demonstrates that the isotropic-to-nematic transition in confined geometry
is not characterized by any universality class, but rather that its fate is
determined by microscopic details. The resulting capillary phase diagrams can
thus assume two topologies: one where the isotropic and nematic branches of the
binodal meet at H=Hx, and one where they remain separated. For values of H
where the transition is strongly first-order the shift DT of the transition
temperature is in excellent agreement with the Kelvin equation. Not only is the
relation DT~1/H recovered but also the prefactor of the shift is in
quantitative agreement with the independently measured bulk latent heat and
interfacial tension.Comment: To appear in Phys. Rev.
