Closed-loop feedback control of droplet formation in a T-junction microdroplet generator

To precisely control the size of monodisperse droplets for droplet microfluidics, we here establish a closed-loop control system with real-time feedback of droplet length during droplet generation. Particularly, the microvalves are integrated into the system to control the flow rates of the two immiscible fluids. From experimental measurements, we observe that the droplet length is a linear function of the flow-rate ratio for low capillary numbers (C_a ≤ 0.1). More importantly, the coefficients of the linear relation are determined by the geometry of the microchannel, and independent of the viscosity of the fluids. Based on MATLAB Simulink, the deviation between the predicted and measured droplet length is quantitatively studied. By using a PI controller, both the response speed and control accuracy of the droplet length can be greatly improved for the closed-loop control system. As a result, high monodispersity and uniformity of droplet generation can be achieved in a T-junction microdroplet generator.