Experimental characterization of a micro cross-junction as generator of Newtonian and non-Newtonian droplets in silicone oil flow at low Capillary numbers

In this paper the generation of Newtonian and non-Newtonian droplets in a continuous silicone oil flow through a micro cross-junction has been studied experimentally. The results obtained by using, as dispersed phase, pure water, water with Tween 20 and different concentrations of Xanthan gum aqueous solutions with the addition of Tween 20 are presented and discussed with the aim to put spotlight on the effect of properties of the involved immiscible liquids on the droplet regimes. A commercial micro cross-junction based on two glass microchannels having a stadium-shaped cross-section and inner hydrophobic walls has been employed. A wide range of volumetric flow rates of the continuous and dispersed phase has been imposed at the inlets of the micro cross-junction by two independent syringe pumps. The main droplet regimes have been correlated to the Capillary number linked to the continuous phase (Cac) and the volumetric flow rate ratio (α). The role of the viscosity ratio (λ) and of the shear-thinning nature of Xanthan gum aqueous solutions on the droplet regime is discussed. The results evidence that the tested micro cross-junction is able to generate very uniform monodispersed droplets with low polydispersity values (P < 6%) at low Capillary numbers (Cac < 0.02) in presence of both Newtonian and non-Newtonian dispersed phases.