Alternating and merged droplets in a double T-junction microchannel

In this work, we report experimental and numerical studies of alternating and merged droplets in a double T-junction microchannel. The microchannel device is fabricated using PDMS substrate and experiments are performed with mineral oil with surfactant as the continuous phase and aqueous glycerol as the discrete phase. Based on the flow rate fraction ϕ and Capillary number Ca, four different flow regimes are identified: merging, stable alternating droplets, alternating droplets with transition and laminar. A numerical model that employs volume-of-fluid formulations is used to predict the alternating droplet generation process. In the stable alternating droplet regime, the effect of the discretephase flow rate ratio α on the droplet diameter ratio β is experimentally studied and compared with that predicted from the simulations. It is observed that the droplet diameter ratio β increases linearly with increase in the flow rate ratio α and a good match between experiments and simulations is observed. The diameters of droplets at different Capillary numbers Ca generated using single and double T-junction microchannels are compared and it is observed that, at low Ca, the double T-junction generates larger droplets as compared to single T-junction. In merged droplet regime, the effect of the continuous phase flow rate Qc and discrete phase viscosity μd on diameter dm and interdistance between the droplets λ of the merged droplets are studied. It is observed that the merged droplet diameter dm is reduced and interdistance between the droplets λ increases with increase in the continuous phase flow rate Qc. As the viscosity of the discrete phase μd increases, the diameter dm and interdistance between the droplets λ of the merged droplets decreases.