Interfacial tension of aqueous mixtures of Na-caseinate and Na-alginate by drop deformation in shear flow

In this work, the interfacial tension of biphasic aqueous mixtures of Na-caseinate and Na-alginate was measured by a methodology based on drop deformation in shear flow. The two phases coexisting at equilibrium in the mixtures were separated by ultracentrifugation. A drop of one phase was injected into the other phase and sheared in a sliding plate apparatus equipped with video-enhanced microscopy. A detailed quantitative characterization of drop shape in several flow conditions, including steady state shear and transient flows, was performed by image analysis techniques. All the results are well described by classical theories of drop deformation in shear flow (which were originally developed for immiscible fluids), the only fitting parameter being the interfacial tension. The methodology used in this work allowed the measurement of very low values of interfacial tension, of the order of 10-5 N/m, and can be applied in general to aqueous mixtures of proteins and polysaccharides.