## 1 Introduction  
In the open literature, various experimental techniques have been adopted to study the problems of fluid-solid interaction. Conventional fluid sensors are versatile but mostly intrusive setups that produce undesirable physical disturbances to the flow, which may lead to erroneous measurements of the flow.Optical techniques, including laser doppler velocimetry (LDA) (All the abbreviations in the article are summarized in Abbreviations), particle tracking velocimetry (PTV), and particle image velocimetry (PIV), are advantageous for their nonintrusive nature and have been extensively employed in experimental research concerning fluid dynamics. The PIV method, which is commonly used to observe fluid flow fields in transparent glass tanks, has been highly successful in pure fluid experiments. However, the prevalence of opaque flowing media in natural environments poses significant challenges for studying fluid-solid interactions using non-invasive optical measurement techniques. Over recent decades, researchers have begun to use transparent substances and liquids to facilitate their studies, and have achieved some remarkable results. One notable example is the use of RIM in hydraulic engineering to visualize the complex interactions within porous media. In a study by [PERSON] and [PERSON] [1], the RIM technique was employed to align the refractive index of glass beads with that of the interstitial fluid, allowing for detailed flow measurements within a porous matrix using PIV. This approach not only provided insights into the intricacies of flow behavior within such media but also demonstrated the practical applicability of RIM in enhancing our understanding of fluid-solid dynamics. Another case in point is the application of RIM in geotechnical engineering, where it has been instrumental in visualizing the displacement field of granular media. [PERSON] and [PERSON] [2] developed transparent porous media with permeabilities and porosities similar to natural soils, enabling the observation of fluid flow and soil interaction without disrupting the soil structure. This has significant implications for the design and assessment of geotechnical structures. A significant challenge arises when alternative materials are used [3]. Optical distortions are a consequence of the reflection and refraction of light as it traverses device materials and fluids with disparate RIs. The bending of light at a boundary is attributable to the disparity in the velocity of light in the two adjacent media, which results in a change in the direction of the light's trajectory as it propagates. According to [PERSON]'s law (also referred to as the law of refraction), when there are significant relative RI mismatches between the two transmission media, the optical path transmission undergoes a significant change. Consequently, the use of optical technologies for the illumination of a specific area to observe flows through interfaces (such as fluid-liquid) with different refractive indices can result in significant uncertainties and inaccuracies in flow measurement. Therefore, the researchers attempted to use materials with the same refractive index for their experiments. The RIM method, which entails maintaining a uniform refractive index (RI) across all fluid and solid components within an experiment, enables the avoidance of imaging distortion caused by refraction when employing optical techniques such as PIV, PTV, and PLIF. This method is particularly advantageous because it allows the observation of flow within complex geometries and through opaque media, which is not always possible without using RIM. The unique advantage of the RIM method is its ability to provide highly accurate and undistorted flow measurements, making it a versatile tool for a wide range of fluid-solid interaction studies. The RIM method has been applied in a diverse range of fields, including hydraulic engineering, geotechnical engineering, and environmental pollution control, among others.