## 3 RIM Formulation Summary, Discussion, and Prospects
### Discussion  
Each combination of solid and liquid materials used in RIM visualization experiments has its own advantages and disadvantages. Next we will discuss their relative merits and demerits from the perspective of solids and liquids, respectively. (1) Solid  
Glasses, such as those used by [PERSON] and [PERSON] [1], [PERSON] et al. [46], are readily available and exhibit a broad range of RIs, spanning from 1.45 to 1.51. They demonstrate excellent chemical stability, allowing for the matching of a diverse range of aqueous and organic fluids within the system. Additionally, glasses demonstrate robust mechanical properties suitable for stress experiments. Conversely, glass is brittle and susceptible to damage, and its shaping and molding are constrained by numerous limitations. The glass most commonly utilized is spherical or cylindrical in shape. Plastics are easily obtainable, cost-effective, and can be conveniently produced in various shapes and sizes for RIM experiments. Extrusion, casting, 3D printing, and other processes make plastics widely applied. Commonly used plastics include the following: fluorinated ethylene propylene (FEP), polyvinyl acetate (PVA), PMMA, nylon, and polystyrene (PS). It is crucial to emphasize that plastics are organic materials, and selecting RIM-matching fluids necessitates careful attention to compatibility and stability. Otherwise, plastics and organic liquids dissolve and react, causing changes in refractive index, which makes experiments fail. Resins, which are widely used on eyeglasses in daily life, demonstrate the distinctiveness of being RI-adjustable. A relatively accurate RI ranging from 1.49 to 1.74 can be modulated after the solidification of a mixture of different liquids. However, the cost of utilizing resins is relatively high. Hydrogels, a novel and convenient material in RIM experiments, exhibit the superior characteristic of being almost RI-equivalent with water, which can be expected to have a broad application prospect. Hydrogels significantly reduce the difficulty of seeking RIM fluids; however, the relatively low strength of hydrogels limits their applications to a few specific scenarios. Also, a salient edge effect appears when hydrogel particles are millimetres in size. The two drawbacks above for hydrogels still require continuous research efforts. (2) Liquid  
The liquids used in the RIM visualization experiments are generally divided into two categories: water and organic liquid. Water is a convenient and accessible medium, while solid materials with a refractive index matching that of water are scarce and must be sought out. Organic liquid matches are usually configured for solids that have already been identified in experiments. In previous studies, numerous organic liquids and mixtures \begin{table}
\begin{tabular}{l l l l} \hline \hline
**Topic** & **Solid** & **Liquid** & **References** \\ \hline
1. Visualization of microbial biofilm architecture and transport dynamics & Nafion 495786 & (Sigma-Aldrich) & Water & [68] \\
2. Biomedical fluid dynamics using optical flow visualization & Clear photopolymer resin (FLGPCLXX) & A fluid composed of 66\% NH4 SCN and 34\% deionised water & [69] \\
3. Vortex phenomena in sidewall aneurysm hemodynamics & Urethane rubber & A solution of sodium iodide, glycerin, and water by weight percentages of 54.3\%, 20.8\%, and 24.9\% & [70] \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Biological engineering and other applications. thereof have been tested for their suitability as working materials. Organic RIM solutions tend to have several common problems. (i) Toxicity. Some organic liquids are both toxic and volatile, which necessitates the implementation of appropriate ventilation and security measures during experimentation. (ii) Cost. Whether they are purchased off the shelf or prepared in the laboratory, the cost of the organic RIM solutions is often quite high, compared with the easily obtained natural fluid like water. (iii) Differences in the physical properties of the fluid.