Constrictivity is a dimensionless parameter used to describe transport processes (often molecular diffusion) in porous media.
The resistance to transport in porous media increases because the viscosity of the fluid (which fills the pores) increases in the vicinity of the pore walls (Renkin effect;[1] see also electroviscous effects).
Constrictivity must be distinguished from the effects of Knudsen diffusion.
Knudsen diffusion occurs when the particle interacts with the pore walls more than it does with other particles due to the large free path and narrow pores.
[6][7][8] In practice, the constrictivity together with the porosity and tortuosity are often used in models as purely empirical parameters to establish the effective diffusivities in porous media.