## 2 Applications of RIM
### Hydraulic/Mechanical Engineering: PorousMedia-Fluid(Surface or Pipe Flow) Interaction
To validate the fabrication of hydrogel, various concentrations of polyacrylamide (PAC) were conducted under the methodology outlined in the cited reference [PERSON] and [PERSON] (2018), as shown in Figure 2. The results demonstrated great feasibility for transparent visualization experiments. Some other solid materials used in porous media flow include: plastics, resins, and so on. [PERSON] et al. [PERSON] et al. (2018) utilized PMMA and p-cymene in RIM to explore pore-scale turbulence, revealing intricate flow patterns. [PERSON] et al. [PERSON] et al. (2018) investigated inertia-dominated, transitional, and turbulent flow in a randomly packed bed of monosized PMMA spheres using an index-matched fluid with particle image velocimetry method. [PERSON] et al. [PERSON] et al. (2018) used a RIM liquid, consisting of 72.2% tetrahydrofuran and 27.8% anhydrous ethanol, which effectively matches the refractive index of PMMA at 25 \({}^{\circ}\)C. The RIM match minimizes the impact of refractive index discrepancies between the working fluid and the fuel assembly model in flow field visualization experiments. Figure 1: Comparison of optical visualization of glass beads in water and formulated solution from our validation tests following [PERSON] and [PERSON] and [PERSON] (2018). Glassbeads (**a**) fully submerged in the liquids, (**b**) partially submerged in the liquids. [PERSON] et al. [46] detailed the use of RIM fluids, which presents another dimension in the study of porous media flow. The refractive index of dispersed phases was investigated by the authors using KSCN solutions and oil mixtures. This method allows for the selection of solid materials for porous media studies by matching their refractive index. The study emphasizes the importance of RIM in achieving accurate flow measurements. Turbulent flows in natural and urban canopies are important for numerous physical and biological processes. The study by [PERSON] et al. [47] represents a significant advance in the field of canopy flow research. The authors employed PIV in conjunction with index-matching techniques, an innovative approach that utilized transparent fractal tree models to permit unimpeded optical access. This approach yielded detailed insights into the turbulent flow structures within complex canopies. The research elucidated the importance of multi-scale elements in flow dynamics and provided a new perspective on horizontal momentum transport through the eddy viscosity model, challenging traditional views and enhancing the precision of canopy flow modeling. In addition to some typical cases in hydraulic engineering summarized above, several cases of RIM-based porous media flow applications in other fields like mechanical engineering can also bring some insights to the study. [PERSON] and [PERSON] [48] utilized RIM to investigate the flow and temperature fields surrounding spherical fuel pebbles in pebble bed reactors. The study introduced the use of PIV in conjunction with matched refractive index fluid to achieve optical access. This provided valuable data for code validation and enhanced the understanding of the complex flow structure within the bed. [PERSON] and [PERSON] [49] reported a polymer with a refractive index similar to water, simplifying the use of existing facilities to study flows around complex geometries in polymer dynamics. [PERSON] et al. [50] con  
Figure 2: Demonstration in our validation experiments following the procedure given by [PERSON] and [PERSON] [17]. The hydrogels prepared with varying concentrations of PAC exhibited satisfactory optical transparency. The strength of the hydrogels exhibited a positive correlation with the concentration of PAC. ducted refractive index-matched PIV experiments and computational fluid dynamics (CFD) simulations to investigate the mixing processes in a cavity transfer dynamic mixer. The diverse materials and techniques applied in these studies collectively enhance our understanding of porous media flow.