LAUSR

Abstract The coalescence process of microdroplets in the expanded convergence of symmetrical T-junction was investigated. Five types of droplet coalescence regimes were observed. The existence of expanded convergence slows down the moving speed of droplets, which accordingly allows a certain time difference between the two microdroplets arriving at the convergence for coalescence, and the maximum allowable arriving time difference of the two droplets was predicted. The passive coalescence of two droplets is mainly described by the liquid film draining model. The critical capillary number (Cac), which determines whether symmetrical colliding droplets would coalesce, decreases with the increase of the viscosity of the dispersed phase, it means that the increase of the viscosity of dispersed phase is not conducive to coalescence. A correlation equation was proposed to predict the critical capillary number, which may provide valuable guidance for the design and application of microdevices.