Diffundoida

Diffundoida is a hypothetical concept used in diffusion theory to describe a class of non-Fickian diffusion processes in which the spread of a substance deviates from Fick's laws due to time- and concentration-dependent mobility and dynamic interactions among diffusing species. The term fuses diffundo, Latin for “to pour out,” with the -oida suffix common in taxonomic naming to suggest resemblance.

In the proposed framework, the diffusion flux is influenced by local structure and history, so diffusivity D becomes a function D(c, t) rather than a fixed parameter. This can produce anomalous diffusion, with the mean square displacement scaling nonlinearly with time, and can model crowded or heterogeneous media such as polymer networks, gels, crowded biological tissue, or nanoporous materials. The model supports regimes of subdiffusion or superdiffusion depending on how mobility evolves with concentration and age of the system.

History and usage: The term diffundoida has appeared in speculative discussions of non-Fickian diffusion in the 2010s and in a small number of theoretical works exploring how diffusion can couple to microstructure evolution. It is not widely adopted as a standard classification and has little to no experimental validation as of now. Critics argue that the term risks conflating distinct phenomena under a single label, while supporters see it as a useful shorthand for a broad nonlinear diffusion class.

See also: diffusion, non-Fickian diffusion, anomalous diffusion, diffusion equation.