Concentration-dependent anomalous diffusion of crystal violet dye in agar gel: application of the continuous time random walk model

Abstract

The transport of materials is of fundamental importance, with studies on diffusion being at the forefront. Diffusion in a simple matrix is typically considered Fickian. However, anomalous diffusion in various media is a dominant process. It is well documented that anomalous diffusion results from medium heterogeneity, extreme events, phase transitions, medium surface dynamics, and other factors. The present work demonstrated that the diffusion of a cationic dye (crystal violet, CV) in an agar gel medium was anomalous, as shown by spatial and time-series data of dye movement. We estimated the classical diffusion coefficients using the Einstein–Smoluchowski solution to Fick's law, but these did not yield consistent Gaussianity, stationarity, or non-seasonality. However, anomalous diffusion was confirmed by modelling the experimental data. The exponent values (α) ranged from 0.468 ± 0.027 to 0.883 ± 0.107, indicating anomalous sub-diffusion that depended on the concentration of the CV dye in the same medium. We then evaluated time- and ensemble-averaged mean squared displacement using the dye-spreading data via image processing. The discrepancy in the distribution functions over a long experimental period highlighted the non-ergodic nature of the stochastic process. The dimensionless ergodicity-breaking parameter was evaluated, confirming the non-ergodic nature of the process. We found that the continuous time random walk (CTRW) model was well-suited for describing the anomalous diffusion in this system. We attributed the sub-diffusion of the dye to violations of the assumptions of Brownian motion of the particles and the nature of the diffusing medium.