Fabrication of disordered arrays of micropillars for anomalous diffusion of colloidal particles

Diffusion, the process by which particles undergo random motion and spread over time, is a fundamental phenomenon in physics, chemistry, and biology. In classical diffusion, often referred to as Brownian motion, particle displacements are random, and the mean square displacement (MSD) grows linearly with time. However, in many complex and heterogeneous environments, particle transport deviates from this classical behavior, giving rise to anomalous diffusion, which is characterized by nonlinear MSD growth or non-Gaussian displacement distributions. Understanding such deviations is essential for describing transport in crowded, confined, or disordered systems, such as cellular environments, porous media, and engineered microstructures. This study investigates the 2D diffusion of micrometer-sized colloidal particles in aqueous suspension within disordered pillar-like microstructures. These structures were fabricated by maskless photolithography, an advanced microfabrication technique whose exposure parameters were optimized and properly characterized. These disordered pillar-like microstructures provide a controlled platform for studying anomalous diffusion in confined colloidal systems, linking structural heterogeneity to transport dynamics. Particle tracking revealed three dynamical regimes. At short lag times, the MSD grew linearly with Gaussian displacements. Intermediate lag times showed subdiffusive behavior that became increasingly pronounced for higher structure area fractions. At long lag times, MSD growth remained linear, but displacement distributions deviated from Gaussianity, indicative of non-Gaussian Fickian diffusion. Diffusion coefficients decreased monotonically with area fraction, while the Non-Gaussian Parameter captured transitions across the three regimes.