## 3 Experimental Results and Discussion  
### Droplet Breakup Process  
Figure 3 shows the droplet breakup process in a symmetric Y-junction microchannel. The operating flow range of this paper is \(q_{\text{d}}\) = 2-18 mL/h, \(q_{\text{c}}\) = 2-22 mL/h, and it is found that the behavior of the droplet at the symmetric Y-junction bifurcation is divided into two types: breakup and no breakup. In the breakup behavior, it is divided into two breakup modes: breakup with permanent obstruction and breakup with gaps. No matter what kind of breakup mode, it can be divided into three stages: entry stage, deformation stage, and breakup stage. The droplet breakup behavior inside the microreactor was captured using a high-speed camera and the frame rate was set to 2000 fps. The time interval of each picture can be calculated according to the frame rate. By calculating the product of the number of pictures and the interval time of each picture, the instantaneous time represented by any picture can be calculated. In all processes, the droplet head just reached the bifurcation as the start time of the droplet breakup process, and the moment when the droplet breaks as the two sub-droplets as the end time of the droplet breakup process. Taking the breakup with a permanent obstruction as an example, from 0-13.7 ms, the head of the droplet completely fills the Y-junction bifurcation and this moment is the sign of the end of the entry stage. From 13.7-43.7 ms, after the end of the entry stage, the droplet gradually expanded and deformed, eventually filling the two branch channels with the \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Fluid System** & **\(\mu\)/(Pa-s)** & **\(\rho\)/(kg:m\({}^{-3}\))** & **\(\gamma\)/(N:m\({}^{-1}\))** \\ \hline Continuous phase & 0.5 wt \% SDS & 0.00135 & 971.53 & – \\ Discrete phase & Silicone oil & 0.01 & 901.4 & 0.0118 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Fluid properties. Figure 1: Schematic diagram of the experimental system and microchannel. Figure 2: Definition of parameters related to droplet breakup. passage of time, thus forming the neck of the droplet. The appearance of the droplet neck is a sign of the end of the deformation phase. From 43.7 to 52.7 ms, the droplet neck gradually becomes thinner until the neck width is 0, at which time the droplet breaks into two sub-droplets. The sign to distinguish gapless breakup from gapped breakup is whether the droplet separates from the microchannel wall during the process of entering the branch channel until breakup. As shown in Figure 3a, if the droplets always contact the wall of the microchannel, there is breakup with permanent obstruction. As shown in Figure 3b, after 28 ms, there is a gap between the droplet and the wall of the microchannel, which is a breakup with gaps. As shown in Figure 3c, the droplet selectively entered the unilateral branch channel and did not break into two sub-droplets, which is the case of no breakup. Figure 4 shows the variation of the droplet neck width during the breakup stage. Among them, \(t_{\text{P}}\) refers to the total time of the droplet breakup process, that is, the droplet breakup period. It is defined as the time interval from the droplet head just reaching the bifurcation to the droplet breaking into sub-droplets, with \(t\) being a certain moment in the breakup process. It is defined as the time interval from the droplet head just reaching the bifurcation to the droplet breaking into sub-droplets. Figure 3: Breaking process of liquid drop in symmetric Y-junction microchannel: (**a**) breakup with permanent obstruction; (**b**) breakup with gaps; (**c**) no breakup. It can be clearly seen that there are two distinct stages in the rate of change of the dimensionless neck width \(W_{\rm m}/W\) with the dimensionless remaining time throughout the breakup process. In the first stage, \(W_{\rm m}/W\) decreased with an increase of time, and the decreasing rate gradually became slower, which was defined as the rapid breakup stage.