  We study the fingering instability of the interface between two miscible
fluids, a colloidal suspension and its own solvent. The temporal evolution of
the interface in a Hele-Shaw cell is found to be governed by the competition
between the non-linear viscosity of the suspension and an off-equilibrium,
effective surface tension $\Gamma_e$. By studying suspensions in a wide range
of volume fractions, $\Phi_\mathrm{C}$, we show that $\Gamma_e \sim
\Phi_\mathrm{C}^2$, in agreement with Korteweg's theory for miscible fluids.
The surface tension exhibits an anomalous increase with particle size, which we
account for using entropy arguments.
