Modeling
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The diffusion of particules is based on Fick's first law which is given by:
\begin{equation}
\textbf{j} = -D \, \bf{\nabla} n
\label{eq:fick}
\end{equation}
In this equation, $\textbf{j}$ is the diffusion flux, $D$ the diffusion coefficient and $n$ the concentration of particles.
This equation can be coupled with the continuity equation $\partial_t n = \mathbf{\nabla} \cdot \mathrm{j} \quad (+\sigma)$ expressing the conservation of the total number of diffusing particles. $\sigma$ is the net particle production rate.