Asymmetric flow-focusing: A novel microreactor operation procedure for making smaller microbubbles

• A novel asymmetric operation procedure for the flow-focusing microreactor is proposed. • Bubble size reduces 1 fold when flow-focusing translates from symmetric to asymmetric. • Mechanism of bubble formation in asymmetric and symmetric flow-focusing is revealed. • A much smaller microbubble can be easily prepared via the asymmetric flow-focusing. • The unified semi-empirical models for microbubble formation frequency are developed. Production of smaller bubbles benefits the promotion of the mass transfer performance in microreactors, but the existing methods mainly focus on designing complex structures. This work proposes a novel asymmetric operation procedure of the flow-focusing microreactor to directly prepare smaller microbubbles without the structure modification. It shows that under a given total liquid volumetric flux, the microbubble size could be obviously reduced with the increase in the liquid volumetric flux deviation between two side inlets. Importantly, the microbubble size can be decreased by 1 fold and the minimal bubble diameter is lower to 25% of the channel width, which breaks the microbubble size bottleneck from the standard symmetric operation procedure of the flow-focusing microreactor. Besides, the inherent reason behind the excellent performance of the asymmetric operation procedure is revealed from the velocity field profile and the interface evolution behavior. Finally, semi-empirical prediction models are proposed for the bubble size. This work breaks the symmetric operation procedure of the flow-focusing microreactor that lasted for two decades and provides a new process intensification strategy for the microbubble preparation.