Effect of entry geometry on droplet dynamics in contraction microchannel

In droplet-based microfluidic systems, the movement and control of liquid droplets are primarily governed by microchannel geometry. The aim of this study is to examine droplet dynamics in contraction microchannels by using three-dimensional simulation and theoretical modeling. This work specifically describes three regimes of the droplet dynamics, including the trap, squeeze, and breakup regimes, and investigates the effects of capillary number (Ca), entry angle ($\alpha$), and contraction channel ratio (C). Additionally, a theoretical model is proposed to describe the transition between squeeze and trap regimes, which depends on the entry angle. The critical value of capillary number (Ca_1c) for this transition is observed to be $C{a}_{1c}=a\left(C^M-b/\alpha \right)$, where a and b are fitted parameters. Meanwhile, the entry angle is found to have no influence on the transition from squeeze to breakup regime. The droplet deformations, retraction, and/or breakup position are quantitatively investigated for a wide range of capillary number and entry angle. The aforementioned findings would provide valuable recommendations for designing contraction micro channels.