Tuning of Nanoparticle–Surfactant (NPS) Concentration for Continuous Production of Monodispersed Droplets during Biphasic Flow in Reduced Dimensions

The study explores tuning of nanofluid composition to enhance the range of monodispersed plug formation during biphasic flow in a 2.38 mm diameter conduit. Experiments are performed using toluene and dilute nanofluids, such that nanoparticles influence the density (<1%) and viscosity (<5%) of the aqueous phase only slightly. For fixed surfactant concentration, increasing nanoparticle concentration increases the range of monodispersed plug flow until an optimum concentration, beyond which the trend reverses. Interestingly, the optimum is related to the critical micellar concentration for the corresponding nanofluid. The range of monodispersed plug formation is depicted as a graphical correlation of the aqueous phase capillary number, Caaq, and the organic to aqueous phase flow rate ratio, Qr. In the experimental domain, monodispersity occurs for Caaq <0.01 for Qr <1.5 and Caaq >0.25 for Qr <0.6. For 0.01 ≤ Caaq ≤ 0.025, monodispersity is noted for Caaq ≤ (0.052–0.028 Qr). The study thus provides a generalized framework to optimize nanofluid composition for uniform droplet generation.