Micro- and macroscopic aspects of prenematic fluctuations in nanoparticles-doped liquid crystals.

Abstract

This study combines broadband dielectric spectroscopy experiments with molecular dynamics (MD) simulations to investigate the impact of nanoparticle (NP) inclusions on pretransitional phenomena in liquid-crystal (LC) systems, specifically focusing on the relationship between nanoparticles, topological defects, and prenematic behavior. Our experimental results, using SiO_{2}-doped 4-Cyano-4'-pentylbiphenyl composites, demonstrate that while NP additions do not significantly alter the isotropic-nematic transition temperature (T_{c}), prenematic effects exhibit universal behavior, confirmed by identical critical exponents across all samples. This indicates that the fundamental character of prenematic fluctuations remains unperturbed by the nanoparticles at these concentrations. Crucially, the observed systematic decrease in dielectric permittivity with increasing NP concentration is elucidated by MD simulations. These simulations reveal that nanoparticles act as "seeds" for topological defects, specifically forcing the surrounding LC molecules into a "hedgehog" configuration. This static, defect-induced structure leads to a local antiparallel alignment and cancellation of molecular dipoles. This provides a direct microscopic mechanism for the macroscopic dielectric response, successfully bridging the micro-macro scales and highlighting the nanoparticle-induced local ordering as a key factor in modifying the dielectric properties of the composite system.