Pseudo-Casimir effect in nematic liquid crystals in frustrating geometries

We study theoretically the fluctuation-induced structural force in nematic systems frustrated by external fields. We focus on the uniform director structure in the hybrid-aligned film characterized by opposing surface fields and in the Freedericksz cell where frustration arises from competing bulk and surface fields. We find that frustration gives rise to several interesting features of the interaction, including a crossover from attraction at small distances to repulsion at large distances. The fluctuation-induced interaction is enhanced substantially by frustration, the enhancement being progressively stronger on approaching the transition from uniform to distorted structure. At the structural transition the interaction diverges and we show that the pretransitional singularity is universal.