Computational modeling of tactoid dynamics in chromonic liquid crystals

Motivated by recent experiments, the isotropic-nematic phase transition in chromonic liquid crystals is studied. As temperature decreases, nematic nuclei nucleate, grow, and coalesce, giving rise to tactoid microstructures in an isotropic liquid. These tactoids produce topological defects at domain junctions (disclinations in the bulk or point defects on the surface). We simulate such tactoid equilibria and their coarsening dynamics with a model using degree of order, a variable length director, and an interfacial normal as state descriptors. We adopt Ericksen's work and introduce an augmented Oseen-Frank energy, with non-convexity in both interfacial energy and the dependence of the energy on the degree of order. A gradient flow dynamics of this energy does not succeed in reproducing some simple expected feature of tactoid dynamics. Therefore, a strategy is devised based on continuum kinematics and thermodynamics to represent such features. The model is used to predict tactoid nucleation, expansion, and coalescence during the process of phase transition. We reproduce observed behaviors in experiments and perform an experimentally testable parametric study of the effect of bulk elastic and tactoid interfacial energy parameters on the interaction of interfacial and bulk fields in the tactoids.