Anisotropic Diffusion in Lyotropic Chromonic Liquid Crystal using Fluorescence Recovery After Photobleaching

TL;DR

This study quantitatively investigates anisotropic molecular diffusion in lyotropic chromonic liquid crystals using fluorescence recovery after photobleaching, revealing how microstructure and molecular interactions influence transport properties.

Contribution

It introduces a Fourier-space FRAP method to independently measure parallel and perpendicular diffusion in LCLCs with different tracers, elucidating microstructure effects.

Findings

Diffusion becomes anisotropic in the nematic phase.

Strongly coupled dyes slow down with increased concentration.

Weakly interacting dyes show enhanced transport along the director.

Abstract

Anisotropic diffusion governs transport in a wide range of soft and biological materials, where microstructure and molecular interactions jointly shape how matter moves. Here, we quantitatively investigate anisotropic molecular transport in lyotropic chromonic liquid crystals (LCLCs) using fluorescence recovery after photobleaching (FRAP). Disodium cromoglycate (DSCG) serves as a model LCLC system, and diffusion is measured across isotropic, nematic, and columnar phases as concentration and temperature are varied. To disentangle the roles of microstructure and molecular interactions, we employ two fluorescent tracers with distinct affinities for the LCLC aggregates: Acridine Orange (AO), which intercalates into DSCG aggregates, and Bodipy, which interacts weakly and remains largely in the aqueous phase. Fourier-space FRAP analysis independently resolves the parallel and perpendicular diffusion coefficients for both dyes relative to the liquid-crystal alignment. In the nematic phase, diffusion becomes anisotropic, with faster transport along the liquid-crystal director. As the DSCG concentration increases, AO dye molecules that are strongly coupled to the aggregates exhibit a slowdown in all directions, reflecting enhanced packing and steric confinement of the LC microstructure. In contrast, weakly interacting Bodipy dye molecules display enhanced transport along the alignment direction as the DSCG concentration increases in the nematic regime, suggesting the emergence of microscopic channels that guide motion, analogous to transport in oriented porous media. These results reveal how the evolving microstructure of LCLCs controls effective diffusion and provide a quantitative framework for understanding and designing anisotropic transport in aligned soft materials.