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This work presents a flexible manipulation solution on droplet formation based on the modified T-junction with a rectangular rib to reduce the droplet size and improve monodispersity. The droplet formation in the ordinary and modified T-junctions is numerically investigated using the verified three-dimensional volume of fluid method. The results reveal that the modified T-junction can significantly enlarge the dripping regime and droplet-generable regimes while decreasing the jetting regime. In the modified T-junction, the droplet detachment is much easier as the detachment driving forces are strengthened, while the resistance forces are weakened. By investigating the droplet formation in the ordinary and different modified T-junctions with change in viscosity, surface tension, and wall wettability, it is found that the dominant geometric factor affecting the droplet formation is the rib height, not the rib width. Based on the rib height, two modified scaling laws are proposed to predict droplet size in squeezing and dripping regimes. The wall wettability can deteriorate the droplet formation in the ordinary T-junction, while the rib in the modified T-junction can weaken this adverse effect. [ABSTRACT FROM AUTHOR]