diffusionsusceptibility

Diffusionsusceptibility is a descriptor used to express how strongly a diffusive transport process responds to external driving forces. The term is not part of a single standardized framework and may be used variably, but it provides a concise way to compare how different materials or conditions modulate diffusion.

Formally, definitions vary. In a simple concentration-driven diffusion described by Fick's law J = -D ∇C, the response to a small change in the gradient is set by the diffusion coefficient D. More broadly, diffusionsusceptibility χdiff can be defined as the derivative of the diffusive flux with respect to the driving force, which may be ∇C, ∇μ, temperature gradients, or electric fields. In anisotropic or heterogeneous media, χdiff becomes a tensor and can depend on composition and microstructure.

Interpretation and dependence: Higher χdiff means the system's diffusive response is more sensitive to the applied drive. Susceptibility is influenced by defects, porosity, grain boundaries, phase distribution, solvent viscosity, and interactions with traps or binders, leading to nonuniform and nonlinear diffusion in many materials.

Measurement and modeling: χdiff is typically inferred from small perturbation experiments that measure J as a function of a controlled change in the driving field, using tracer diffusion, concentration transients, impedance spectroscopy, or NMR diffusometry. Theoretically, it aligns with linear-response concepts such as Onsager coefficients and mobility relations, and is encompassed in models where D varies with C, T, and structure.

Relation to existing concepts: Diffusionsusceptibility is related to the diffusion coefficient, chemical mobility, interdiffusion coefficients, and transport tensors. It provides a compact way to compare diffusion sensitivity across materials and conditions, though it is less formalized than standard transport coefficients.

See also: diffusion coefficient, Fick's laws, diffusion, Onsager reciprocal relations, transport phenomena.