Regulation of the size and flow pattern of yield stress droplets in flow-focused microchannels by orifice constraints

Although droplet microfluidics have attracted increasing research interest, the regulation of complex polymeric droplets remains incomplete in chemical industry. In this study, we investigate the formation of yield stress droplets in flow-focused microchannels with orifice constraints by means of numerical simulations using a volume-of-fluid method coupled with an adaptive mesh refinement technique. The results show that different orifice sizes induce various flow patterns, including dripping, quasijetting, quasijetting with droplet coalescence, and jetting. A predictive model is established for the extension length of the dispersed phase during the necking stage, considering orifice dimensions interplay. Furthermore, higher yield stress influences the droplet size and formation frequency by expanding the high viscosity distribution in the dispersed phase. The proposed predictive correlations for droplet size, velocity, and formation frequency, accounting for the effects of orifice dimensions and yield stress, are believed to have significant impacts on the preparation of polymeric droplets in microreactors.