A buffer microzone for controllable formation of viscous droplets in a T-junction microchannel

A buffer microzone based T-junction is designed for viscous droplets formation. • A non-monotonic effect of the dispersed phase flow rate on the droplet size is observed. • Inherent mechanisms of the viscosity-induced droplet size reduction are revealed. • Two dimensionless prediction models are developed for the viscous droplet size. Generating viscous droplets in microchannels remains a technical challenge. Here, a buffer microzone in T-junction microchannel was designed to balance the liquid–liquid two-phase interaction for the controllable preparation of viscous droplets. The polyvinylpyrrolidone (PVP) and chitosan aqueous solutions with a wide range of viscosity variation were employed as the dispersed phase. The results show that compared with the conventional T-junction microchannel, the modified T-junction effectively suppresses the long jetting regime and broadens the operation window of the squeezing and dripping regimes. The formed viscous droplet could be much smaller and the size distribution is much narrower. Importantly, the influence of the dispersed phase flow rate on the droplet size is revealed to be non-monotonic, depending strongly on both the continuous phase flow rate and the fluid viscosity. With the new finding, a droplet size prediction model incorporating the capillary number, fluid viscosity ratio, and flow rate ratio is proposed.