Off-equilibrium surface tension in colloidal suspensions

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.