Formation of bubbles in a simple co-flowing micro-channel

Bubble generation in a simple co-flowing micro-channel with a cross-sectional area of 1.69 × 0.07 mm2 was experimentally and numerically investigated. Air and water were used as the gas and liquid, respectively. Mixtures of water–glycerol and water–Tween 20 were also used to obtain the effects of viscosity and surface tension. The experimental data show that the break-up process is periodic under certain operating conditions. The break-up dynamics are also examined using three-dimensional incompressible two-phase flow numerical simulation based on the volume of fluid (VOF) method. The simulation successfully predicts the flow behavior and provides a more detailed examination of the bubble shape. The physics can be further explained by the detailed micro-PIV measurements, which show that the bubble is formed due to the velocity component perpendicular to the gas flow created by the sudden change of the liquid velocity distribution around the barrier. The bubble length L is dependent on the liquid flow rate Ql and the gas flow rate Qg, and the ratio of L to the channel width w is a function of the ratio of gas and liquid flow rates Qg/Ql which is similar to that previously used in the T-junction case. The formulation of bubble frequency f is derived under current conditions and it shows a good agreement with the experimental data at the low frequency region. Different bubble shapes can be obtained at different liquid viscosities and surface tensions. The ratio L/w can still be predicted by a modified equation which uses the real bubble width wb or an equivalent bubble length Le.