Diffusional

Diffusional is an adjective describing processes and phenomena related to diffusion, the spontaneous movement of particles from regions of higher concentration to lower concentration due to random thermal motion. Diffusional transport is a passive mechanism that typically does not require external energy input and is described, in simple cases, by Fick's laws of diffusion.

Fick's first law expresses the flux of particles as J = -D ∂C/∂x, where J is the diffusive flux, D is the diffusion coefficient, and ∂C/∂x is the concentration gradient. Fick's second law describes how concentration changes in time: ∂C/∂t = D ∂²C/∂x². The diffusion coefficient D depends on temperature, medium, and the diffusing species, and often follows Arrhenius-type behavior D = D0 exp(-Ea/RT).

In solids, diffusional processes include vacancy- and interstitial diffusion, contributing to alloying, phase transformations, and sintering. In liquids and gases, diffusion governs mixing and transport of solutes. In biology, diffusional transport underpins the movement of small, nonpolar molecules across membranes; for polar molecules and ions, diffusion is frequently aided by facilitated diffusion via channels or transporters, a process distinct from active transport.

Applications of diffusional principles span drug delivery, membrane separation, corrosion science, geochemistry, and environmental engineering. Diffusion rates set limits on reaction kinetics, biological signaling distances, and material aging. Measurement techniques for diffusion coefficients include tracer methods, pulsed-field gradient NMR, and various flow and time-lapse diffusion analyses.

Overall, diffusional phenomena describe how substances spread due to concentration differences, constrained by medium properties, temperature, and structural factors, and they are foundational to many scientific and engineering disciplines.