Closed-loop feedback control of microbubble diameter from a flow-focusing microfluidic device.

Real-time observation and control of particle size and production rate in microfluidic devices are important capabilities for a number of applications, including the production, sorting, and manipulation of microbubbles and droplets. The production of microbubbles from flow-focusing microfluidic devices had been investigated in multiple studies, but each lacked an approach for on-chip measurement and control of microbubble diameter in real time. In this work, we implement a closed-loop feedback control system in a flow-focusing microfluidic device with integrated on-chip electrodes. Using our system, we measure and count microbubbles between 13 and 28  μ m in diameter and control their diameter using a proportional-integral controller. We validate our measurements against an optical benchmark with R 2 = 0.98 and achieve a maximum production rate of 1.4 × 10 5 /s. Using the feedback control system, the device enabled control in microbubble diameter over the range of 14-24  μ m.