## 4 Conclusions  
RIM-based optical visualization measurements have become an important research topic in the field of fluid-solid interactions with a broad application prospect. This paper presents an overview of the application of the RIM method, with a focus on the formulation and prospects. RIM formulations for different fluid-solid interaction application areas are summarized. Furthermore, this paper discusses the advantages and disadvantages of different materials. Finally, the possible research directions on RIM are also given.  
To sum up, the following conclusions can be drawn:  
(i) Compared with the traditional engineering measurements of fluid flow and fluid-solid interaction in hydraulic engineering, RIM-based optical visualization measurements have the great advantages of being intuitive, non-intrusive, low-cost, and convenient. Combined with image processing techniques, this method can obtain and display the flow field information in real-time and quickly, which has a broad application prospect.  
(ii) The selection of suitable solid and liquid materials for optically transparent visualization is crucial. When water is used as the liquid in RIM configurations, it is necessary to identify reliable solids for refractive index matching. Furthermore, the careful selection of RIM-matched liquids, particularly organic ones, for common solid materials is of the utmost importance to ensure the accuracy and reliability of experimental results. Due to the demerits of organic solutions (toxicity and cost, etc) as illustrated previously, one of the future research directions is to develop the RIM technique so that natural water can be used as RIM fluid.  
(iii) Continued advances in imaging and computational analysis techniques are critical to improving the capabilities and applicability of RIM-based methods. As the resolution and sensitivity of imaging systems improve and as the capabilities of computational models and data processing algorithms increase, RIM techniques will achieve higher levels of accuracy and detail in the visualization of complex fluid-solid interactions. Future research should focus on integrating state-of-the-art imaging modalities such as high-speed cameras, 3D scanning, and holography with powerful image processing and machine learning tools to gain deeper insights from RIM-based experiments.  
(iv) The interdisciplinary nature of RIM-based research provides opportunities for collaboration across different scientific and engineering domains. The convergence of materials science, fluid dynamics, optical engineering, and computational physics can yield innovative solutions that address current challenges and open new frontiers in the research and application of RIM methods.  
In general, the accuracy of RIM-based visualization measurement methods is steadily improving with the continuous advancement of image acquisition and processing technologies. By emphasizing the importance of material selection, technology integration, and interdisciplinary collaboration, we aim to inspire a new wave of innovation that will advance the understanding and application of fluid-solid interaction studies. With this development, it is foreseeable that the RIM method will be applied in many more fields in the future.  
**Author Contributions:** Conceptualization, H.Z. and D.C.; methodology, H.Z. and D.C.; formal analysis, H.Z. and D.C.; investigation, H.Z. and D.C.; writing--original draft preparation, H.Z.; writing--review and editing, D.C., H.C., Q.C. and T.L.; supervision, D.C. and H.C.; project administration, D.C.; funding acquisition, D.C. All authors have read and agreed to the submitted version of the manuscript.  
**Funding:** The authors would like to acknowledge the support provided by the Fundamental Research Funds for the Central Universities (22120240016) and the National Key Research and Development Program of China (2022 YFC3106205).  
**Conflicts of Interest:** The authors declare no conflicts of interest.