Geometry-controlled active transport in smectic textures

Abstract

Self-propelled particles moving through structured anisotropic media such as smectic liquid crystals exhibit transport properties that differ qualitatively from those in isotropic environments. Here we introduce a minimal model of an active Brownian particle whose propulsion axis is coupled to the layer-normal field of a smectic liquid crystal. Using numerical simulations complemented by approximate analytical calculations, we investigate the dynamics of an active particle in three representative two-dimensional smectic microstructures: a uniform planar texture, an isolated toroidal focal conic domain, and a periodic array of toroidal focal conic domains. For the planar and isolated focal-conic geometries, we characterize the crossover from short-time superdiffusion to long-time normal diffusion. In periodic focal-conic arrays, however, transport is controlled by two distinct persistence mechanisms whose relative importance depends on the Péclet number ($\mathrm{Pe}$) and the coupling strength $\varepsilon$ between the particle orientation and the smectic layer-normal field. At small $\varepsilon$, long-time diffusion is governed by the intrinsic persistence time of the active Brownian dynamics. At large $\varepsilon$, repeated scattering by smectic defects generates an emergent persistence time that dominates the transport. These results demonstrate that the competition between the intrinsic persistence of active motion and the mean free time between encounters with smectic defects provides a simple physical mechanism for the emergence of nontrivial diffusive behavior in structured anisotropic media.