Diffusive chemical transport and mass continuity
In an isotropic Fickian transport model, diffusive mass flux is proportional to the negative local mass-density gradient.
Key knowledge
In an isotropic Fickian transport model, diffusive mass flux is proportional to the negative local mass-density gradient. Conservation converts this constitutive relation into a diffusion equation when the diffusion coefficient is uniform. This describes spreading down a gradient within its assumptions; reactions, advection and coupled multicomponent transport add further terms.
Values and equations
Jm = −D ∂ρ/∂x; ∂ρ/∂t = −∂Jm/∂x = D ∂²ρ/∂x²
- Jm
- diffusive mass flux in the positive x direction kg m⁻² s⁻¹
- D
- constant diffusion coefficient m² s⁻¹
- ρ
- mass density of the diffusing component kg m⁻³
- x
- distance m
- t
- time s
Conditions and scope
- The displayed one-dimensional equation assumes a positive constant D and no sources or advection.
- A scalar mass-density gradient model need not describe concentrated interacting mixtures or electrochemical cross-effects.
Sources · 1
References
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Evidence and review
Read Fick mass flux and continuity derivation; excluded approximate kinetic-theory prefactors and erroneous Gaussian sign in the notes.
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Entry details
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- uni-chem-fickian-diffusion
- Revision
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- Structured reference record
- University core expansion