Production rate and diameter analysis of spherical monodisperse microbubbles from two-dimensional, expanding-nozzle flow-focusing microfluidic devices

Flow-focusing microfluidic devices (FFMDs) can produce microbubbles (MBs) with precisely controlled diameters and a narrow size distribution. In this paper, poly-dimethyl-siloxane based, rectangular-nozzle, two-dimensional (2-D) planar, expanding-nozzle FFMDs were characterized using a high speed camera to determine the production rate and diameter of Tween 20 (2% v/v) stabilized MBs. The effect of gas pressure and liquid flow rate on MB production rate and diameter was analyzed in order to develop a relationship between FFMD input parameters and MB production. MB generation was observed to transition through five regimes at a constant gas pressure and increasing liquid flow rate. Each MB generation event (i.e., break-off to break-off) was further separated into two characteristic phases: bubbling and waiting. The duration of the bubbling phase was linearly related to the liquid flow rate, while the duration of the waiting phase was related to both liquid flow rate and gas pressure. The MB production rate was found to be inversely proportional to the sum of the bubbling and waiting times, while the diameter was found to be proportional to the product of the gas pressure and bubbling time.