Fickean diffusion coefficient

D is the Fickean diffusion coefficient, or transport diffusion coefficient, which is the proportionality constant... 

This equipment can be used for the study of a single-component diffusion, and the measurement of the corresponding Fickean diffusion coefficient made using a solution of Fick s second law for a geometry appropriate for the experimental setup [87-92], In this case, the flow rates were adjusted to get the desired partial pressure (6.7 Pa, P/Pn = 0.006) [90],... 

Scharfer et al. set up a multi-component transport model to describe the diffusion driven mass transport of water and methanol in PEM [170]. For a PEM in contact with liquid methanol and water on one side and conditioned air on the other, the corresponding differential equations and boundary conditions were derived taking into account the polymers three-dimensional swelling. Phase equilibrium parameters and unknown diffusion coefficients for Nafion 117 were obtained by comparing the simulation results to water and methanol concentration profiles measured with confocal Raman spectroscopy. The influence of methanol concentration, temperature and air flow rate was predicted by the model. Although there are indications for an influence of convective fluxes, the measured profiles are ascribed to a Fickean diffusion. Furthermore, the assumption to describe the thermodynamic phase equilibrium as liquid-type equilibrium also at the lower surface of the membrane, which is in contact with a gas phase, can be confirmed by their results. 

A review of the theoretical basis, finite-element model, and sample applications of the program NOVA are presented. The updated incremental Lagrangian formulation is used to account for geometric nonlinearity (i.e., small strains and moderately large rotations), the nonlinear viscoelastic model of Schapery is used to account for the nonlinear constitutive behavior of the adhesive, and the nonlinear Fickean diffusion model in which the diffusion coefficient is assumed to depend on the temperature, penetrant concentration, and dilational strain is used. Several geometrically nonlinear, linear and nonlinear viscoelastic and moisture... 

By plotting M/M = f (F ) (Pignre 2), for Fickean sorption kinetics the slope of the linear portion of the obtained curve provides direct access to the diffusion coefficient. But no direct access to the diffusion coefficient is possible for sigmoid-type and two-steps sorption kinetics. The diffusion coefficient can be determined by fitting the experimental curves. 

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