Molecular diffusion is ubiquitous in nature and responsible for many biological processes and production technologies. In liquid isotropic systems, the random thermal motion of the molecules in equilibrium conditions, in the absence of concentration gradient or flow, is commonly described by a particle random walk characterized by a mean square displacement (MSD) proportional to the elapsed time, and a Gaussian molecular displacement probability distribution. Fickian diffusion occurs when the MSD
linearly varies with the elapsed time (MSD∝Dt), in which case the diffusion coefficient, D, is a constant to be determined experimentally.
However, for more complex systems, such as gels or porous materials due to molecular confinement or supramolecular interactions, the particle MSD exhibits a power law time dependence (MSD ∝ tα ). In such cases, non-Fickian (often referred to as anomalous) diffusion takes place and the transport regime is characterized by the diffusion exponent α. An α value in the range 0 < α < 1 indicates subdiffusive motion, while for 1< α < 2 superdiffusive motion is observed.
Deviations from Fickian behavior have been observed in a variety of heterogeneous systems such as porous media living biological cells, plasma, solidi-liquid interfaces and hydrogels.
People:
Mosè Casalegno, Franca Castiglione, Guido Raos, Andrea Mele
Selected publications:
1) M. Casalegno, F. Castiglione, G. Raos, G. B. Appetecchi, S. Passerini, A. Mele, E. Ragg, Magnetic Resonance Imaging and Molecular Dynamics Characterization of Ionic Liquid in Poly(ethylene oxide)-Based Polymer Electrolytes, ACS Appl. Mater. Interfaces 12 (21), 23800-23811 (2020). DOI: 10.1021/acsami.0c01890
2) M. Casalegno, G. Raos, G. B. Appetecchi, S. Passerini, F. Castiglione, and A. Mele, From Nanoscale to Microscale: Crossover in the Diffusion Dynamics within Two Pyrrolidinium-Based Ionic Liquids, J. Phys Chem Lett. 8, 5196-5202 (2017). DOI: 10.1021/acs.jpclett.7b02431
3) M. Casalegno, F. Castiglione, M. Passarello, A. Mele, S. Passerini, G. Raos, Association and Diffusion of Li+ in Carboxymethylcellulose Solutions for Environmentally Friendly Li‐ion Batteries, ChemSusChem 9, 1804-1813 (2016). DOI: 10.1002/cssc.201600160