## 3 Experimental Results and Discussion
### Flow Pattern  
Figure 5 shows the flow pattern according to the breakup mode in the symmetrical Y-junction microchannel. Take the dispersed phase flow \(q_{\rm d}\) as the abscissa and the continuous phase flow \(q_{\rm c}\) as the ordinate. Clearly, the way the droplets break up changes with the two-phase flow. When \(q_{\rm d}\) is constant, with an increase of \(q_{\rm c}\), the breaking method of the droplet changes from breaking to not breaking. This is due to the fact that the length of the droplet becomes smaller as the \(q_{\rm c}\) increases and it is easier to choose to enter one of the branch channels such that breakup does not occur. Clearly, when the two-phase flow rates are relatively small, the droplets are more prone to no-breakup behavior. This is because as the capillary number of the two phases is small, the shear force on the droplet is small, which is not enough to overcome the effect of surface tension and the droplet is not easy to break. When the \(q_{\rm d}\) is relatively small, the droplet breakup mode is only gapless breakup and no breakup. When the \(q_{\rm d}\) is relatively large, the droplet will experience breakup with permanent obstruction, breakup with gaps and no breakup with the increase of \(q_{\rm c}\).  
Figure 4: Changes in the width of the droplet neck during the breakup stage.  
In the study of two-phase flow, the flow pattern is the basis of all problems. Different application backgrounds have different requirements for flow patterns, so it is of far-reaching significance to study the effect of breakup on flow patterns. It is worth noting that the division of the flow pattern at this time is based on the length of the droplet. In general, the microchannel cross-sectional size has an effect on the droplet length. In order to reduce the influence of different microchannel cross-sectional dimensions on the droplet length, a dimensionless droplet length \(L_{\text{d}}/W\) is defined, where \(L_{\text{d}}\) refers to the droplet length and \(W\) refers to the channel width. Parallel flow refers to the flow pattern in which the dispersed phase does not break, slug flow refers to droplets with dimensionless length \(L_{\text{d}}/W>1.5\), and droplet flow refers to droplets with \(L_{\text{d}}/W<1.5\). As shown in Figure 6a, when \(q_{\text{d}}\) is relatively large and \(q_{\text{c}}\) is relatively small, parallel flow is likely to be generated. This is because the disperse phase has a large inertial force when the disperse phase flow is large. When the inertial force of the dispersed phase plays a dominant role in the two-phase flow, the dispersed phase can maintain continuous flow in the microchannel without being sheared by the continuous phase. At this time, the dispersed phase flows continuously in the main channel, and the continuous phase flows around the dispersed phase near the wall of the microchannel. In the two-phase operating flow range in this paper, the vast majority of flow patterns in the microchannels without breakup behavior are slug flow. As shown in Figure 6b, parallel flow cannot be observed in the microchannel after the droplet is broken, and a new flow pattern: droplet flow appears. In addition, the area occupied by the slug flow in the flow pattern becomes smaller. Clearly, the droplet breakup behavior has a significant effect on the flow pattern.  
Figure 5: Flow pattern diagram based on breakup mode.  
Figure 6: Effect of breakup behavior on droplet flow pattern: (**a**) flow pattern before the droplet breakup; (**b**) flow pattern after the droplet breakup.