  We measure the flow of water through mixed packings of glass spheres and soft
swellable hydrogel grains, at constant sample volume. Permeability values are
obtained at constant sample volume and at porosities smaller than random close
packing, for different glass bead diameters $D$ and for variable gel grain
diameter $d$, as controlled by the salinity of the water. The gel content is
also varied. We find that the permeability decays exponentially in $n(D/d)^b$
where $n=N_{gel}/N_{glass}$ is the gel to glass bead number ratio and $b$ is
approximately 3. Therefore, flow properties are determined by the volume
fraction of gel beads. A simple model based on the porosity of overlapping
spheres is used to account for these observations.
