Formation dynamics of double emulsions with shear-thinning shell fluids in axisymmetric flow-focusing devices

The formation dynamics of double emulsions with shear-thinning shell fluids in an axisymmetric flow-focusing microchannel are numerically investigated. Emulsification mechanisms, interfacial evolution, generation pattern transitions, and size distributions are analyzed and compared between Newtonian-shear-thinning-Newtonian (NsN) and all-Newtonian fluid systems, with emphasis on the effects of matrix fluid Capillary number (Cam) and shell fluid concentration (C) on these processes. Results indicate that shear-thinning behavior induces localized viscosity reduction near the orifice, which modifies necking dynamics and shifts the droplet pinch-off location upstream. An increase in Cam is observed to trigger dripping-to-jetting transitions in both systems; however, a higher Cam is required in the NsN system because of the shear-thinning viscosity effect. Additionally, transitions between generation patterns can be induced by increasing shell fluid concentration, establishing a rheology-based control strategy independent of geometry or flow rate variations. Emulsion size generally decreases with increasing Cam, but exhibits a distinct increase at transition thresholds because of the production of dual-core droplets. Notably, regime-dependent effects on emulsion size are exhibited by shell fluid concentration: smaller droplets are produced at higher C in dripping patterns, while larger emulsions are generated in jetting patterns.