Scaling the formation of slug bubbles in microfluidic flow-focusing devices

The present study aims at scaling the formation of slug bubbles in flow-focusing microfluidic devices using a high-speed digital camera and a micro particle image velocimetry (mu-PIV) system. Experiments were conducted in two different polymethyl methacrylate square micro-channels of respectively 600 9 600 and 400 9 400 mu m. N-2 bubbles were generated in glycerol-water mixtures with several concentrations of surfactant sodium dodecyl sulfate. The influence of gas and liquid flow rates, the viscosity of the liquid phase and the width of the microchannel on the bubble size were explored. The bubble size was correlated as a function of the width of the microchannel W-c, the ratio of the gas/liquid flow rates Q(g)/Q(1) and the liquid Reynolds number. During the pinch-off stage, the variation of the minimum width of the gaseous thread W-m with the remaining time could be scaled as W-m proportional to (Q(g)/Q(1))(-0.15) (T - t)(1/3). The velocity fields in the liquid phase around the thread, determined by mu-PIV measurements, were obtained around a forming bubble to reveal the role of the liquid phase.