## 2 Background: Single Cavitation Bubble Near Solid Boundary  
Formation of a bubble near the surface and its subsequent collapse can lead to erosion on the solid surface. To provide further insight into this phenomenon, Figure 1 illustrates the collapse of a bubble near the solid boundary surface, along with an image of the solid boundary before, during, and after erosion. When a complete bubble, with its maximum radius (Rmax), forms near the flat rigid surface of the solid boundary (Figure 1a), the interaction between the boundary and the bubble's wall initiates the collapse process. At the onset of the collapse process, water vortices form and contribute to the shrinking and crumbling of the upper wall of the bubble (Figure 1b). As time elapses, additional flow of water vortices are drawn into thevortices, contributing to the formation of a water jet (Figure 1c). The establishment of the water jet results in a reduction in the volume of the bubble. As the water jet progresses forward, additional water vortices, formed due to the crumbling of the bubble wall, join the water jet and further reduce the remaining volume of the bubble. At this juncture, the water jet transforms into a water hammer and strikes the flat surface, resulting in surface erosion and the formation of a semi-spherical pit (Figure 1d). After erosion, the remaining water vortices form microbubbles (Figure 1e). Over time, the volume of the microbubbles diminishes as a result of their rotation and interactions between the molecules on the microbubble wall and the molecules in the surrounding matrix. These processes decrease the radius of the microbubble, eventually generating nanobubbles (Figure 1f). Generally, nanobubbles persist within the matrix, and are capable of migrating freely. Upon collision with the flat surface, they contribute to nano-erosion. In the case of riblets, different scenarios unfold. Riblets influence the flow of water vortices and consequently, the formation of the water jet and hammer. When the bubble collapses near a surface with riblets, the water vortices are diverted and separated. Consequently, the convergence of water vortices weakens, and they fail to merge into a cohesive water jet. Under these conditions, the resulting water jet is less powerful and lighter, leading to a weaker impact on the surface. Therefore, it can be inferred that erosion near surfaces with riblets may be reduced compared to that near flat surfaces. In this regard, an experimental approach is employed herein to elucidate the effects of riblets on induced erosion during bubble collapse. In the case of erosion, it can expressed that, the cavitational destruction process of erosion [22; 30; 31] starts with plastic strain effects on the surface of the slab. At this stage, nano-cracks appear and gradually grow into micro-cracks [22]. During this process, the rate of material loss is very low. However, with repeated bubble collapse near the surface at a constant location, the micro-cracks grow and lead to the formation of pits on the surface. Figure 1: The schematic illustrates the single bubble collapse process near a flat solid boundary, depicting the maximum radius (R\({}_{\text{max}}\)) of the bubble and the distance (h) between the center of the bubble and the surface of the solid boundary. **(a)** The complete single bubble near a flat solid boundary. **(b)** The collapse process initiates, and the upper wall of the bubble deteriorates more rapidly than the lower wall near the solid surface. This process is attributed to the formation of water vortices. **(c)** The number of water vortices increases and converges into an arc shape, thereby decreasing the volume of the bubble. This phenomenon leads to the creation of a water jet. **(d)** The water jet forms, and its volume increases as new water vortices are added, slicing through the volume of the bubble. Consequently, the water hammer is created and collides with the solid surface, resulting in erosion. **(e)** After erosion, the flow of water vortices leads to the creation of microbubbles.