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Bubble formation process in a T-junction microchannel was investigated numerically.The diameter of microchannel was 100μm and the operation conditions were limited in the squeezing regime.Generally,bubble formation was divided into three stages:expansion,collapse and pinching-off.Forces acting on gas thread in the whole process were analyzed and compared quantitatively.It was observed that the pressure in inlets and surface tension are 1~2order higher than shear stress.Shear stress participates in dragging gas thread downstream for only a part time of the expansion stage but the dragging action can influence the collapse stage to some extent.Collapse stage was considered as the most important for bubble formation and investigated in detail.It was found that collapse process was mostly influenced by liquid superficial velocity.Collapse rate can be represented with a function of liquid superficial velocity and Capillary number while the collapse time is also a function of Reynold number.