## 1 Introduction  
With the advancement of science and technology, microfluidic technology has also developed rapidly and has received extensive attention in the field of the microchemical industry [1; 2; 3]. Microfluidic technology is widely used in bioengineering [4; 5; 6], drug screening [7; 8; 9], protein crystallization [10], and chemical reactions [11; 12] due to its many advantages, such as high safety, high device integration, low consumption of experimental reagents, strong controllability, and process energy saving. In microfluidic technology, the production efficiency can be greatly improved by obtaining two or more sub-droplets at one time by breaking the droplets in the microchannel [13]. Due to the high controllability and repeatability of this breakup method, it has become an important application in microfluidic technology [14; 15; 16; 17]. Common microchannel structures used to prepare sub-droplets include T-junction [18; 19], Y-junction [20; 21], cross-focusing [22], and microchannels with internal obstacles [23]. Among them, these channels can be designed to be symmetrical [24] or asymmetrical [25]. [PERSON] et al. [18] investigated droplet breakup within multi-order T-junction microchannels of different lengths and proposed a transition rule for droplet breakup and non-breakup behavior. [PERSON] et al. [26] conducted an experimental study on droplet breakup within T-junction microchannels at larger capillary numbers and found two breakup mechanisms: microdroplet tunnel breakup and blockage breakup. [PERSON] et al. [27] studied the breakup process of droplets when a concentrated emulsion flows in a narrow microchannelwith a small pore throat structure. The variation law of droplet divergence with parameters such as flow rate, inlet angle, and droplet size relative to droplet width was studied. [PERSON] et al. [28] used the volume-of-fluid method (VOF) to conduct a 3D simulation of the breakup mechanism of microdroplets in a T-junction microchannel and verified the simulation results through visualization experiments. Through simulations and experimental observations, four breakup modes were discovered and a correlation was proposed to predict the size of the droplet. [PERSON] et al. [29] used a numerical simulation to study the droplet breakup in asymmetric T-junction microchannels with square cross-sections and different pressure gradient ratios. The critical conditions for the breakup of a droplet into two sub-droplets of different sizes are identified and two main asymmetric breakup mechanisms are proposed: permanently blocked breakup and unstable breakup. [PERSON] et al. [30] used experimental methods to study the breakup behavior of bubbles in an asymmetric Y-junction microchannel, and investigated the effects of two-phase flow rate and physical properties on the distribution of broken bubbles. Three different breakup behaviors were found: asymmetric breakup without a gap, asymmetric breakup with a gap, and no breakup. It can be seen that the research in the existing literature mostly focuses on the T-junction microchannel structure and there is little about the droplet breaking behavior in the symmetrical Y-junction microchannel structure. Furthermore, because of its unique bifurcated structure, Y-junction microchannels are similar to blood vessels in living organisms and have ingenious bionic significance. In order to better control the production of sub-droplets, it is of great significance to study the droplet breaking behavior in symmetric Y-junction microchannels. In this paper, using silicone oil as the discrete phase and a 0.5 wt % SDS (sodium dodecyl sulfate) aqueous solution as the continuous phase, the droplet breaking behavior in the symmetric Y-junction microchannel was studied by means of visualization experiments. The effect of the breakup behavior on the flow pattern was studied, and the effect of the physical properties of the two-phase fluid on the size of the sub-droplets was investigated.