= Porosity
{title2=$\epsilon$}
{wiki}
<Porosity> is the fraction of a representative bulk material volume occupied by voids:
$$
\phi=\epsilon=\frac{V_{\rm pore}}{V_{\rm bulk}},\qquad 0\leq\phi\leq1.
$$
It is dimensionless and measures storage space. A sample with bulk volume $1.0\,\mathrm L$ and pore volume $0.25\,\mathrm L$ has <porosity> $\phi=0.25=25\%$. If all its pores are saturated by one liquid, the liquid occupies $0.25\,\mathrm L$; partial <fluid saturation> $s$ instead gives liquid volume $s\phi V_{\rm bulk}$.
<Porosity> and <permeability of a porous medium> answer different questions: how much <fluid> fits, and how easily it flows. Isolated bubbles or cavities can contribute to total <porosity> while providing no throughgoing path. Even connected passages may be narrow enough to transmit very little discharge. In the simple fully participating saturated model, <Darcy velocity> is $\phi$ times <pore velocity>; changing <porosity> at a stipulated fixed <Darcy flux> changes ideal advective travel time, but <porosity> alone cannot predict the <Darcy flux>.
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