## 3 Experimental Setup
### Experimental Methodology  
Figure 3a,b illustrates the arrangement of the experimental setup utilized to generate a single cavitation bubble near a rigid surface, without and with riblets, in separate experiments, respectively. The primary flow control in the cavitation experiment is achieved using a laser delay generator and a laser controller with an Q-switch Nd:YAG Litron laser (Litron Lasers, Rugby, UK). The laser beam is then deflected and focused by a deflector and lenses, respectively, inside a cuvette filled with water to create a single bubble. Finally, the bubble collapses, and the collapse dynamics are recorded using a high-speed camera and monitored by a computer. The wavelength and pulse duration of the laser are 1.064 nanometers (nm) and 4 nanoseconds (ns), respectively. Moreover, the laser produces a maximum output energy of 400 millijoules (mJ) and a maximum repetition rate of 10 hertz (Hz).  
Figure 2: The application of the shark-skin trapezoidal riblet pattern to the surfaces of Al and brass (CuZn37 alloy; 63% Cu and 37% Zn). (**a**) Schematic of a shark (the portrait of the shark was generated utilizing Adobe Firefly [33; 34; 35]), (**b**) magnified shark skin, (**c**) designing bio-inspired trapezoidal riblets based on the shark skin, (**d**) the contact angles of a droplet of water with surface, with and without riblet, (**e**) aluminum surface without riblets, (**f**) brass surface without riblet, (**g**) aluminum surface with trapezoidal riblets, and (**h**) brass surface with trapezoidal riblets.  
A single bubble, with a \(\mathrm{R_{max}}\) of approximately 1.5 millimeters (mm) and a standard spatial deviation of 14 micrometers (\(\mathrm{\upmu m}\)), can be generated with a laser beam energy level of approximately 14.6 mJ, obtained using an adjustable laser controller. This magnitude of energy helps to create bubbles without causing significant turbulence in the water. Inside the cuvette at the focal region of the lens, the energy and convergence of the laser beam generate high-temperature plasma close to the solid boundary. This causes the water in the plasma region to evaporate, leading to the creation of a single cavitation bubble with wall distance of \(\gamma=1.8\) (dimensionless). This value was selected because, at this distance, the bubble collapse time was reduced in the presence of riblets compared to that of a flat surface, helping to clearly reveal the effects of riblets [9]. The growth and collapse processes of the bubble were recorded using a Phantom v9.1 high-speed camera (Adept Turnkey, Osborne Park, Australia), with a sampling rate of 20 kHz and a resolution of 288 \(\times\) 192 pixels. An internal high-speed controller and an external pulse/delay generator were coupled to trigger the camera with an exposure time of 10 \(\upmu\)s.  
After the growth of the bubble, its volume shrinks, and the collapse phenomenon occurs near the rigid surface due to the bubble-rigid surface interaction. The collapse process results in the formation of a water jet and hammer, which impact the rigid surface and lead to erosion. To obtain visible erosion, the cavitation-induced erosion generated by the collapse of the bubble was repeated five hundred times near the flat and ribleted surfaces. All experiments were performed at a temperature of 20 degrees Celsius and a pressure of 101 kilopascals (kPa), which are well-known atmospheric conditions. After performing all five hundred bubble collapses, the damage on the surfaces of Al and brass, with and without riblets, was analyzed using a confocal microscope. Subsequently, three-dimensional microscope maps of the damaged surface, optical microscope images, three-dimensional topographies of the damaged surface, and depth versus width diagrams were determined.