Breakup dynamics of droplets in an asymmetric bifurcation by μPIV and theoretical investigations

Abstract Dynamics of the three breakup regimes of droplets in an asymmetric bifurcation is experimentally studied by a high-speed microscope system and a micro-particle image velocimetry (μPIV) system. Effects of the droplet length and the capillary number on the evolution of the neck thickness are analyzed. For droplets that obstruct the microchannel, the pressure drop across the droplet during the breakup process is theoretically calculated and its relationship with the neck shrinkage is discussed. Based on mass conservation law, the theoretical prediction on the geometric shape of droplets can be constructed and both the radius of droplet rear cap and the neck thickness agree well with experimental results. It is proven that the theoretical model could also be successfully constructed for asymmetric bifurcations and the model in this paper could be applied to bifurcations with arbitrary angles. If gaps are present between the droplet and channel sidewalls, it is found that the thinning process of the neck and variations of the flow field of droplets are distinctly different from the obstructed breakup. For no breakup regime, the vortex flow accompanies the backflow of the droplet tip and two different vortex distributions within the droplet are disclosed for the first time, which are separated by the droplet length. The critical neck thickness that determines whether droplets break after flowing through the bifurcation is revealed and compared with other works.