Formation characteristics of Taylor bubbles in a microchannel with a converging shape mixing junction

Abstract The bubble formation in a square microchannel with a converging shape mixing junction has been investigated under gas–liquid Taylor flow using a high-speed camera. A typical bubble formation process was found to consist of two steps including the expansion and rupture steps. The bubble length could be approximated as the product of the rupture time and two-phase mixture velocity. Significant influence of liquid viscosity and two-phase mixture velocity on the bubble length was observed, although a linear dependence of the bubble length on gas–liquid flow ratio is present for a given two-phase mixture velocity or liquid viscosity. This indicates that shear stress plays an important role in determining the bubble length in the current microfluidic device even at low Capillary numbers where the squeezing regime is expected to predominate. An empirical correlation expressing the bubble length as a function of gas–liquid flow ratio, liquid viscosity and two-phase mixture velocity was developed to describe the experimental results. The bubble frequency was found to reach a maximum as gas–liquid flow ratio is increased from 0.5 to 1. The cross-sectional shape of Taylor bubble was close to be square at low capillary numbers, which is in agreement with the literature results.