| In the last decade, with the rapid development of the micro-chemical technology, the bubble breakup behavior in microfluidic devices, as an important basis of micro-chemical technology, has received extensive attention. The behavior of bubble breakup in asymmetric microchannels was investigated experimentally and numerically in this work.The bubble breakup and distribution in a T-junction with unequal width bifurcations were observed and investigated by a high-speed camera. Flow patterns including breakup with permanent obstruction(POB), breakup with tunnel(TB) and no breakup(NB) were observed in the experiments.The distribution ratio of two daughter bubbles V2/V1 non-linearly increased at first and then decreased with the increase of the ratio of gas to liquid flow rate due to the feedback effect. Similar tendency was also found in the influence of the dimensionless volume of mother bubble on V2/V1, and there existed a critical dimensionless volume of mother bubble remarkably dependent on the ratio of gas-liquid flow rate, but nearly independent of the liquid viscosity. Two correlations were proposed to predict the distribution rule of daughter bubbles, and the calculated results by present correlations showed a good coincidence with experimental data. The process of permanent obstructed bubble breakup could be divided into two stages: squeezing stage and pinch-off stage. Squeezing stage could further be partitioned into fast squeezing and slow squeezing two stages. During the fast squeezing stage, universal regulation of the minimum width of the bubble neck had not been found uo to now. In slow squeezing stage, the relation between the dimensionless minimum width of the bubble neck and the time could be described by a power-law:(1-wm/w0)?t????. The thinning rate of the bubble neck would be accelerated with the increase of the superficial velocity and the liquid viscosity, but regardless of the initial length of the bubble. In the pinch-off stage, the variation of the dimensionless minimum width of the bubble neck with the remaining time could be scaled by a power–law relationship:wm/w0?(T-t)????.The flow patterns and the distribution of bubble breakup in a T-junction with unequal length bifurcations were investigated experimentally. Three regimes of the flow patterns were observed including breakup with permanent obstruction(POB), breakup with tunnel(TB) and no breakup(NB). The transition between TB and NB could be described by a power law dependent on dimensionless initial length of bubble and the Ca number. The ditribution ratio decreased with the increase of the dimensionless initial length of bubble, whereas the mechanism was different for the different dimensionless initial lengths of bubble. For the short bubble, the distribution ratio of daughter bubbles was jointly dominated by both the variation of local interface and the resistances of the downstream branches. For the long bubble, the distribution ratio depended mainly on the resistances of the downstream branches.The processes of the breakup for obstructed bubbles were simulated numerically in an asymmetrical microfluidic T-junction by VOF(Volume of Fluid) method. For the low liquid mean velocity(um=0.062-0.20 m·s-1), the bubble broke up symmetrically in the bifurcation. Both the surface tension and length of initial bubble had negligible influence on the length ratio of daughter bubbles. For the high liquid mean velocity(um=0.25-0.40 m·s-1), the bubble inclined to break up asymmetrically. The asymmetrical degree reduced slightly with the increase of the surface tension and the initial length of bubble. A predicting model was proposed to estimate the length ratio of daughter bubbles and the calculating values by present model showed a good coincidence with simulated results. |
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