Monodisperse droplet formation for both low and high capillary numbers in a T-junction microdroplet generator

•Precise control of the droplet size is of significant importance for the applications of droplet microfluidic systems.•In a T-junction microdroplet generator, for both low and high capillary numbers of droplet formation, the relation between the droplet length and the flow-rate ratio of the fluids is measured experimentally.•More importantly, based on the closed-loop control droplet microfluidic system, precise control of the droplet size can be achieved for both low and high capillary numbers of monodisperse droplet formation. In a T-junction microdroplet generator, the relation between the droplet length and the flow-rate ratio of the immiscible fluids is studied experimentally at both low and high capillary numbers. In particular, different geometries of the T-junctions are designed for monodisperse droplet formation, and the droplet length as a function of the flow-rate ratio is measured for various viscosity of the fluids. It is observed that there is a linear relation between the droplet length and the flow-rate ratio at a low capillary number(Ca⩽0.1), while the droplet length is varying nonlinearly with the flow-rate ratio at a high capillary number(0.1⩽Ca⩽1.0). More importantly, by using the feedback control of the droplet size, a closed-loop control droplet microfluidic system is demonstrated. Especially, by choosing specific control method, good agreements are shown between the predicted and the measured droplet size for a wide range of the capillary numbers of monodisperse droplet production.