= Mass transport
<Mass transport> is movement of material through space, whether as whole-fluid motion, relative motion of constituents or migration driven by an external field. Bulk <advection> carries a constituent with the local <velocity field>; <diffusion> moves it relative to that mean motion because of microscopic fluctuations and spatial variations in its thermodynamic driving quantities.
For a dilute passive constituent with concentration $c$ and constant <diffusion coefficient> $D$, a simple model writes total flux $\mathbf J=c\mathbf v-D\nabla c$. The first term transports concentration with the flow, while the second smooths concentration differences. Charged constituents can also migrate under an <electric field>, requiring an additional flux term and appropriate electrochemical constraints. Combining the flux with a local conservation equation predicts how concentration evolves. Measuring only total material crossing a boundary does not by itself distinguish which transport mechanism supplied it.
Back to article page
