  We investigate the development of mobility inversion and fingering when a
granular suspension is injected radially between horizontal parallel plates of
a cell filled with a miscible fluid. While the suspension spreads uniformly
when the suspension and the displaced fluid densities are exactly matched, even
a small density difference is found to result in a dense granular front which
develops fingers with angular spacing that increase with granular volume
fraction and decrease with injection rate. We show that the time scale over
which the instability develops is given by the volume fraction dependent
settling time scale of the grains in the cell. We then show that the mobility
inversion and the non-equilibrium Korteweg surface tension due to granular
volume fraction gradients determine the number of fingers at the onset of the
instability in these miscible suspensions.
