Enhanced oil-in-water droplet generation in a T-junction microchannel using water-based nanofluids with shear-thinning behavior: A numerical study

Nanofluids are widely used as the continuous phase during droplet formation in microsystems due to their impressive features such as excellent thermal, magnetic, and interfacial properties. Although it is well-known that nanofluids are susceptible to exhibit non-Newtonian behavior even at a low concentration of nanoparticles, effects of non-Newtonian behavior of nanofluids have not been studied on droplet formation thus far. In this study, oil-in-water droplet formation with a relatively high viscosity ratio of the immiscible phases was studied numerically in a T-junction microchannel. To inspect the non-Newtonian effects of aqueous nanofluids on droplet formation, empirical data on the rheological behavior of various types of nanofluids were explored. Finally, two water-based nanofluids with shear-thinning behavior were chosen as the continuous phase for numerical simulations. The numerical procedure was validated against some experimental models. Afterward, droplet length was determined for different capillary numbers, flow rate ratios, and nanoparticle concentrations, and some scaling laws were proposed to predict droplet length in different droplet formation regimes. The results showed that using nanofluids with shear-thinning behavior as the continuous phase results in a decrease in droplet size, and this reduction is more intensified as the concentration of nanoparticles increases. Furthermore, it was observed that the change in the droplet formation regime through manipulating the flow rates does not occur easily when pure water is used as the continuous fluid due to the high viscosity ratio of the immiscible phases. However, when nanofluids are employed as the continuous fluid, the droplet formation regime can be changed more easily due to the enhancement of viscous shear force in the continuous phase. Hence, in addition to the well-known advantages of nanofluids in droplet formation processes, nanofluids can be raised as a new alternative for the continuous phases to administer the droplet size and formation regimes rather than using chemical additives for tuning the rheological properties.