## 2 Applications of RIM
### Environmental Engineering: Discrete Particle-Flow Interaction  
As a consequence of urbanization, pollutants generated by industrial production are contaminating land and water, which will continue to impact the ecological environment and public health. The principal soil pollutants are organic compounds (such as benzene, polycyclic aromatic hydrocarbons, organochlorine pesticides, etc.) and heavy metals (including nickel, lead, zinc, chromium, copper, etc.). These pollutants vary in type, morphology, concentration, risk level, and spatial dispersion. Studying the migration patterns of pollutants and implementing appropriate management measures hold significant importance. Many researchers have conducted studies using RIM to investigate the transport of pollutants through soil [61; 62; 63; 64; 65] or in pure water [66; 67]. To understand soil contamination, it is essential to visualize the transportation of pollutants. [PERSON] et al. [61] conducted an experiment on the permeation process of grout inside soil using transparent soil made of fused silica and calcium bromide solution with the same refractive index. The study implemented a combined grouting and optical measurement system to visualize grout permeation, revealing that the grout body radius is proportional to grouting time, thus verifying [PERSON]'s permeation grouting formula. The findings underscore the importance of matching the refractive index for detailed visualization and have implications for improving grouting techniques in soil remediation and reinforcement. [PERSON] et al. [62] provided a sophisticated optical imaging technique for three-dimensional visualization and quantification of nonaqueous phase liquids (NAPL) distribution within transparent porous media. The methodology uses a transparent soil model, color space segmentation, and an innovative 3D carving algorithm to reconstruct NAPL zones from 2D projections. The results demonstrate the superior accuracy of the technique over conventional image analysis, providing an efficient and non-intrusive approach to studying NAPL behavior in geo-environmental research. A similar approach can be applied to research on the treatment of soil contaminants. The study conducted by [PERSON] et al. [63] delves into the migration dynamics of remediation agents in contaminated soils using high-pressure jet injection, a technique that has gained prominence for its extensive applicability and cost-effectiveness. The study meticulously investigates the influence of various factors, such as permeability pressure differences, particle size distributions, and void ratios, on the migration rate and distribution area of the remediation agents. The RIM's significance lies in its ability to provide a real-time, visual, and non-intrusive assessment of the remediation process, which is crucial for optimizing the application of high-pressure jet injection in soil remediation efforts. In their study, [PERSON] et al. (2018) investigated the possibility of visualizing multi-phase flow and surfactant flushing in soil by using transparent Aquabeads and an advanced imaging system. Upon absorption by water, the material becomes transparent and is capable of absorbing up to 200 times its weight in water. Three types of Aquabeads were used in the study to simulate soils with different relative hydraulic conductivities. The study conducted tests on multi-phase flow and surfactant flushing, using mineral oil and motor oil as contaminants. The imaging system deployed provided clear visualizations of the flushing process, highlighting the significant boost in oil recovery rates--95.8% for mineral oil and 88.5% for motor oil--attributed to the prevention of contaminant bypass and the improved efficiency of the surfactant solution. The study demonstrated the benefits of the recently developed system in geoenvironmental research. Furthermore, RIM has also been used to study the transport of pollutants in pure water.