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Off-equilibrium surface tension in colloidal suspensions

Domenico Truzzolillo Laboratoire Charles Coulomb, Montpellier

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.

I present a thorough experimental investigation of the Saffman-Taylor (SF) instability arising when a simple fluid (the solvent of a colloidal suspension) is pushed through a miscible, more viscous one (the colloidal suspension itself). I show 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 particle volume fractions, I successfully test Korteweg’s theory.

I rationalize such findings by showing that, for microgel spheres, composed of cross-linked polymers, the surface tension is dominated by the entropic contribution associated with the internal degrees of freedom of the particles. Furthermore, by studying SF-instabilities employing hard silica particles suspensions, I show that, at odd with the bulk behavior, microgels and hard particle suspensions exhibit vastly different interfacial stress properties.