Shear-thinning effects on dripping-to-jetting transition and droplet size regulation in a microchannel

In the field of microdroplet-related material or device fabrication, the rheological properties of fluids constitute a critical factor influencing the applications of microdroplets. In this study, the role of shear-thinning effect in governing the motion and deformation of microdroplets are investigated based on experiments and interfacial dynamics theory. The flow pattern maps of droplet formation regimes are summarized, and compared with that of the Newtonian fluid, indicating shear-thinning fluids are more likely to form monodisperse droplets while delaying the dripping-to-jetting transition. In dripping, shear-induced reduction of the dispersed phase equivalent viscosity accelerates the neck breakup process, improving the droplet detachment efficiency. Conversely, in jetting, the dominance shifts to the dispersed phase viscous force, which determines the interface stabilization. Combined with high-speed microscopic particle image velocimetry (µPIV) technology and interfacial tracking technique, the transient velocity gradients and shear rate distributions are quantitatively analyzed. The influences of the shear-thinning behavior on interfacial rupture processes are revealed. By defining characteristic shear rates and the equivalent viscosity, an empirical expression of the scaling law for droplet size is established, providing a theoretical basis for precise microdroplet regulation. The results further enrich the microscale flow theory of complex fluids, and the revealed mechanisms help promote the application and development of shear-thinning fluids in many fields such as energy and biomedicine.