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In this paper we study how the steady state morphology during shear of an incompatible blend can be affected by the initial conditions. By means of rheological experiments it is shown that below a critical shear rate multiple steady states can be obtained, the final morphology being then determined by the initial conditions of the blend. Above the critical shear rate the morphology is univocally determined by an equilibrium between break-up and coalescence. The critical shear rate is identified as the value at which the break-up limiting curve crosses the coalescence one. The applicability of different coalescence theories has been investigated by changing the viscosity ratio. The different coalescence theories can all describe the experimental results with a reasonable precision. The accuracy of the fully mobile interface theory seems to increase with decreasing viscosity ratio, whereas the opposite holds for the immobile interface theory. The partially mobile interface theory describes the various results equally well.

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In this work, the microconfined shear deformation of a droplet in an equiviscous non-Newtonian immiscible fluid is investigated by modeling and experiments. A phenomenological model based on the assumption of ellipsoidal shape and taking into account wall effects is proposed for systems made of non-Newtonian second-order fluids. The model, without any adjustable parameters, is tested by comparison with experiments under simple shear flow performed in a sliding plate apparatus, where the ratio between the distance between the confining walls and the droplet radius can be varied. The agreement between model predictions and experimental data is good both in steady state shear and in transient drop retraction upon cessation of flow. The results obtained in this work are relevant for microfluidics applications where non-Newtonian fluids are used.

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