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

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.

Figures (3)

• Figure 1: FRAP measurements of anisotropic diffusion. A) Fluorescence image taken immediately after photobleaching ($t =$ 0s) in 18 DSCG with acridine orange (AO); the dashed line indicates the bleached region. B) Fluorescence image at $t =$ 20s during recovery. C) Time-dependent intensity decay of single Fourier modes used to extract the diffusion coefficients, the mode at $(u, v) = (0.014, 0)$µm for the parallel component $D_{\parallel}$, and the mode at $(u, v) = (0, 0.014)$µm for the perpendicular component $D_{\perp}$. Open symbols are experimental data, solid lines are fits to Eq. \ref{['eq : fit']}. Black and red correspond to parallel ($D_\parallel$) and perpendicular ($D_\perp$) diffusion, respectively.
• Figure 2: Normalized UV-vis absorption spectra of two dyes dissolved in two different media: DI-water and 2 DSCG solution. A) Acridine Orange. The peak at 465nm in DI-water shifted to 500nm in 2 DSCG solution. B) Bodipy. No shift in the peak wavelength was observed. Black dash lines represent the UV-vis spectrum of dye dissolved in DI-water, and colored lines correspond to the 2 DSCG aqueous solution.
• Figure 3: Diffusivities of Acridine Orange (AO, orange) and Bodipy (green) as a function of DSCG concentration. A) Diffusion coefficients along the aligned director measured at a constant temperature of 25℃ for AO (orange) and Bodipy (green). B) Diffusion anisotropy reported as a ratio of diffusivities perpendicular and parallel to the director, $D_{\perp}/D_{\parallel}$.