The aim of this study is to investigate the bubble formation mechanism in a microfluidic flow-focusing device using a high-speed digital camera and a micro-particle image velocimetry (V-PIV) system. Experiments were conducted in a PMMA square microchannel with 600 Pin wide and 600 Pin deep. Gas bubbles were generated in glycerol-water mixtures with several concentrations of surfactant sodium dodecyl sulfate (SDS). Various flow patterns were obtained at the cross-junction by changing gas and liquid flow rates. The formation mechanism of slug bubble at the cross-junction was investigated to gain insight into the effects of liquid and gas flow rates, and viscosity of the liquid phase on the breakup rate of the gas thread, and on the collapse time. The velocity fields in the liquid phase around the thread were determined by mu-PIV measurements. The experimental data of the breakup rate and the collapse time of the gas thread were described as a function of the liquid superficial velocity u(l), the ratio of the gas and liquid flow rates Q(g)/Q(l) and Reynolds number Re = rho ul/mu. (C) 2009 Elsevier Ltd. All rights reserved