## 4 Pore-Scale Visualized Experiments for Two-Phase Flow
### Microfluidic-Based Visualized Experiment  
The development and application of microfluidic chips provide a new approach to overcoming the limitation of not being able to visually observe the internal fluid flow process due to the opacity of rocks. These models use transparent materials to depict interconnected pore networks, enabling optical visualization of complex fluid flow processes occurring at the pore scale. Experiments, utilizing high-precision microscopes and recording equipment, implementing image analysis and experimental measurement techniques to achieve visualized studies of microscopic flow processes and dynamic mechanisms in porous media [53]. With the help of visualized microscopic seepage experiments, micro-models based on real rock core pore structures can achieve qualitative and quantitative research on the microscopic seepage process of reservoir cores, revealing the microscopic seepage characteristics of fluids within reservoirs. Those microfluids manufactured using different methods are shown in Figure 5.  
By using microfluidic-based visualization experiments, there is fruitful research on gas-water two-phase flow mechanisms, which are essential for understanding the migration and trapping mechanisms of fluids within complex porous media, as shown in Figure 6. In previous studies, researchers explored various aspects of hydrogen-water two-phase flow in microfluidic systems. Researchers have investigated the flow behaviour of hydrogen in water-saturated porous media [60]. The bubble dynamics and two-phase flow in proton exchange membrane electrolytes have also been captured and studied by using microfluidics combined with high-speed in situ imaging technique [7]. The storage capacity and residual trapping characteristics of hydrogen during cyclic injections are also investigated in microfluidic channels, which offer insights into underground storage applications [61]. Thanks to the advantage of easy cyclic operation in microfluidic experiments, researchers have conducted multi-parameter cyclic injections and production simulations to explore the key factors influencing hydrogen underground storage capacity and recovery efficiency [62]. Collectively, these studies contribute to a deeper understanding of hydrogen-water flow behavior and its potential applications in energy production and storage systems.  
In microfluidic experiments, the study of hydrogen-water two-phase flow has revealed the flow characteristics of hydrogen in water-saturated porous media. The research shows that hydrogen flow in microfluidic channels is significantly influenced by the pore structure. Larger pore channels facilitate the flow of the gas phase, while smaller pores may lead to blockages or local retention of hydrogen, which is crucial for understanding gas migration and trapping mechanisms in underground hydrogen storage. For instance, cyclic injection experiments demonstrate that the residual storage characteristics of hydrogen in porous media, such as residual gas saturation and gas sequestration capacity, are closely related to the pore structure. These findings are essential for understanding hydrogen's underground storage, recovery efficiency, and storage capacity.  
Figure 5: Schematic diagram of different types of microfluidic chips: (**a**) the glass model prepared by wet etching [54]; (**b**) the silicon wafer model prepared by dry etching [55]; (**c**) the PDMS model prepared by soft lithography [56]; (**d**) the glass model prepared by laser engraving [57]; (**e**) the 3D PMMA model prepared by hot embossing [58]; (**f**) the 3D porous structure prepared by stereolithography [59].