Ion-order coupling in nematic liquid crystals

The impact of ionic impurities on the fundamental properties of nematic liquid crystals (LC) has long been observed; however, their influence on fundamental material parameters remains quantitatively unclear. Here we develop and validate a predictive theoretical framework that integrates ionic Coulomb self-energy into the Landau-de Gennes formalism, revealing how free ions nonlinearly suppress the dielectric anisotropy. Additionally, we propose a model that clarifies how electrostatic ion drag contributes to an increase in the rotational viscosity of the nematic phase. We experimentally verify both the dielectric anisotropy model and rotational viscosity model using a dual-frequency nematic LC doped with controlled concentrations of monolayer graphene flakes, which effectively modulates the ionic environment. The experimental results exhibit strong agreement with the theoretical predictions across both positive and negative dielectric regimes, as well as for the ion-modulated rotational viscosity of the LC. Dynamic optical switching measurements further corroborate the rotational viscosity results. This work establishes a self-consistent link between microscopic ionic screening and macroscopic rheo-optic behavior in soft anisotropic media, advancing the fundamental understanding of ion-related electrostatics in complex fluids.