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 str ucture in the hybrid-aligned film characterized by opposing surface fields and in the Freedericksz cell where frustration arises from competing bulk a nd surface fields. We find that frustration gives rise to several interesti ng features of the interaction, including a crossover from attraction at sm all distances to repulsion at large distances. The fluctuation-induced inte raction is enhanced substantially by frustration, the enhancement being pro gressively stronger on approaching the transition from uniform to distorted structure. At the structural transition the interaction diverges and we sh ow that the pretransitional singularity is universal.