title: Droplet Breakup in Expansion-contraction Microchannels.
abstract: We investigate the influences of expansion-contraction microchannels on droplet breakup in capillary microfluidic devices. With variations in channel dimension, local shear stresses at the injection nozzle and focusing orifice vary, significantly impacting flow behavior including droplet breakup locations and breakup modes. We observe transition of droplet breakup location from focusing orifice to injection nozzle, and three distinct types of recently-reported tip-multi-breaking modes. By balancing local shear stresses and interfacial tension effects, we determine the critical condition for breakup location transition, and characterize the tip-multi-breaking mode quantitatively. In addition, we identify the mechanism responsible for the periodic oscillation of inner fluid tip in tip-multi-breaking mode. Our results offer fundamental understanding of two-phase flow behaviors in expansion-contraction microstructures, and would benefit droplet generation, manipulation and design of microfluidic devices.
authors.affiliation: Department of Mechanical Engineering, the University of Hong Kong, Hong Kong.|HKU-Zhejiang Institute of Research and Innovation (HKU-ZIRI), 311300, Hangzhou, Zhejiang, China.
authors.forename: Pingan
authors.initials: P
authors.lastname: Zhu
authors.affiliation: Department of Mechanical Engineering, the University of Hong Kong, Hong Kong.|HKU-Zhejiang Institute of Research and Innovation (HKU-ZIRI), 311300, Hangzhou, Zhejiang, China.
authors.forename: Tiantian
authors.initials: T
authors.lastname: Kong
authors.affiliation: Department of Mechanical Engineering, the University of Hong Kong, Hong Kong.|HKU-Zhejiang Institute of Research and Innovation (HKU-ZIRI), 311300, Hangzhou, Zhejiang, China.
authors.forename: Leyan
authors.initials: L
authors.lastname: Lei
authors.affiliation: Department of Mechanical Engineering, the University of Hong Kong, Hong Kong.|HKU-Zhejiang Institute of Research and Innovation (HKU-ZIRI), 311300, Hangzhou, Zhejiang, China.
authors.forename: Xiaowei
authors.initials: X
authors.lastname: Tian
authors.affiliation: Department of Mechanical Engineering, the University of Hong Kong, Hong Kong.|HKU-Zhejiang Institute of Research and Innovation (HKU-ZIRI), 311300, Hangzhou, Zhejiang, China.
authors.forename: Zhanxiao
authors.initials: Z
authors.lastname: Kang
authors.affiliation: Department of Mechanical Engineering, the University of Hong Kong, Hong Kong.|HKU-Zhejiang Institute of Research and Innovation (HKU-ZIRI), 311300, Hangzhou, Zhejiang, China.
authors.forename: Liqiu
authors.initials: L
authors.lastname: Wang