Anomalous Diffusion: Physical Origins and Mathematical Analysis

Diffusion is the physical process of particles or molecules gradually become uniformly distributed in an ambient medium, e.g., the dispersal of an ink drop in water. Anomalous diffusion refers to instances when the process deviates significantly from its usual behavior, and it manifests in two distinct ways: subdiffusion when solute particles move much slower than expected, and superdiffusion when solute particles cover larger distances in shorter timescales, exhibiting more erratic behavior. Diffusion dictates important physical properties of materials (e.g., electrical or heat conductivities) and phenomena in complex systems (e.g., human body). By studying the physical origins and mathematical intricacies of anomalous diffusions, the project will enhance the fundamental understanding of diffusion processes in diverse fields, ranging from physics and chemistry to biology and environmental science. The scientific goal is to establish the quantitative relationship between the microstructural and physical properties of the particle and ambient medium and the measurable generalized diffusivity and its scaling laws. The project will provide research training opportunities for graduate students. The investigator will consider factors that are observed to affect diffusion processes, such as anisotropy, heterogeneity, and deformability of particles, and the viscoelasticity of the medium and externally applied electromagnetic fields and aim to scale up from microscopic stochastic differential equations to macroscopic fractional differential equations, using a systematic multiscale analysis approach. The goal is to extend the classical Stokes-Einstein framework and address technical challenges related to power laws, non-standard Brownian motions, and anomalous diffusions. On a microscopic level, the project considers the complete set of multiphysics field equations governing the behavior of particles and the ambient medium. On a macroscopic level, the aim is to derive effective evolution equations for the concentration or probability distribution function of particles at different time scales. This involves coarse-graining the microscopic equation of motion and identifying distinct diffusional behaviors within different time regimes. Multiscale analysis methods are employed to uncover the mathematical and physical origins of fractional differential equations. By exploring how these equations emerge and how their solutions impact anomalous diffusions, the project will result in a deeper understanding of the underlying physical mechanism.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.