Merged and alternating droplets generation in double T-junction microchannels using symmetrically inserted capillaries

In this work, merged and alternating droplets generated in a microfluidic double T-junction are investigated using experiments and numerical simulations. The double T-junction is constructed by symmetrically inserting two capillaries into a microfluidic chip at specific positions. We explore the effects of the two-phase flow rate fraction, capillary tip distance (30 μm, 60 μm, and 200 μm), and fluid properties on droplet formation phenomena. Detailed observations reveal four distinct regimes during the dynamic evolution of the two-phase interface morphology: merged state, stable alternating droplets, droplet pairs, and jetting. Two phase diagrams are obtained to demonstrate that interfacial tension and dispersed phase viscosity significantly influence these regimes. Moreover, we find that as the flow rate fraction increases from 0.054 to 0.286, the length of generated droplets increases from 156 to 789 μm; we provide a theoretical prediction formula for dimensionless droplet length accordingly. Additionally, our simulations show fluctuating pressure in dispersed flows throughout the process of droplet generation. The simulated pressure in the dispersed flows fluctuates during the droplet generation process. The understanding of the underlying physics of the capillary-based double T-junction contributes valuable insights for various related applications.