Surface tension between immiscible fluids is a well-understood and well-characterized phenomenon. By contrast, much less is known about the effective, off-equilibrium surface tension that arises when a (transient) interface is created between miscible fluids. This effective surface tension plays a key role in many driven phenomena, from jetting and drop formation and coalescence to precipitation and deposition, as it was already recognized more than 100 years ago by Korteweg [1]. However, experimental investigations in this field remain scarce and the theoretical predictions by Korteweg have not quantitatively tested so far, to the best of our knowledge. We present a thorough experimental investigation of the Saffman-Taylor instability arising when a simple fluid (the solvent of a colloidal or polymer suspension) is pushed through a miscible, more viscous one (the colloidal or polymer suspension itself). We show [2] that the resulting interface pattern can be accounted for by the non-linear rheological properties of the suspension and an effective surface tension. By measuring the effective surface tension for suspensions at a variety of polymer volume fractions, we successfully test Korteweg's theory for free and cross-linked polymer suspensions. We moreover find that the effective surface tension increases with the size of polymers, a surprising result at odd with the typical behavior in atomic or colloidal systems at equilibrium, where the surface tension usually decreases as the squared particle size. Finally we present new data on the effective surface tension between suspensions of hard, compact particles and their own solvent showing that polymers and hard particles exhibit vastly different effective surface tension properties. We discuss this surprising result in view of available models for interfacial stresses. [1] D. Korteweg, Arch. Neerlandaises Sci. Exactes Naturelles, 6, 1 (1901). [2] D.Truzzolillo, S. Mora, C. Dupas, L. Cipelletti, Phys. Rev. Lett., 112, 208303 (2014).