In the present research, a visualization study is carried out to understand the flow patterns and break-up formation in flow-focusing microfludic device, which usually involve gas-liquid two-phase flow patterns such as bubbly flow, slug flow, and annular flow. High-speed camera and fluorescence microscopy are used to the characterize microscale gas-liquid two-phase flows in 191.64 μm × 200 μm rectangular PDMS microchannel which are fabricated by using soft lithography technique. N2 and Silicone oil serve as the gas and liquid phases, respectively. The influence of gas and liquid flow rates and the viscosity of the liquid phase on the bubble size, flow patterns, and generating frequency are experimental studied. This study is focused on viscosity of liquid phase. In order to influence of viscosity of liquid phase, various viscosity silicone oils are used in this study. The breakup mechanism, bubble generating rate, and gas bubble making time are investigated at the cross junction. The gas bubble size and space of between bubbles are dependent on the flow rates of the liquid and gas, and viscosity of liquids. The slug length L is also dependent on the flow rates of the liquid and gas in such a manner that the ratio of slug length to the channel width is a function of the flow rate ratio of gas to liquid.

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