## 3 Results and Discussion
### Visualisation of Bubble Characteristics  
As detailed in Section 2.4 Experimental Setup, the boiling process and associated bubble characteristics were captured using a high-speed camera. Videos were recorded at 16,000 frames per second (fps) with a shutter speed of 1/200,000 s and a 1024 \(\times\) 1024 pixels resolution. Figures 12-19 show representative images of the boiling process on the heat transfer surfaces of the four test pieces at different heat fluxes. Figure 14: Bubble growth characteristics (\(q\) = 100 kW/m\({}^{2}\)). Figure 13: Bubble growth characteristics (\(q\) = 50 kW/m\({}^{2}\)). Figure 15: Bubble growth characteristics (\(q\) = 250 kW/m\({}^{2}\)). Figure 16: Bubble growth characteristics (\(q\) = 500 kW/m\({}^{2}\)). Figure 17: Bubble growth characteristics (\(q\) = 1000 kW/m\({}^{2}\)). Figure 18: Bubble growth characteristics (\(q\) = 2000 kW/m\({}^{2}\)). At a low heat flux of 50 kW/m\({}^{2}\) (Figure 13a-d), isolated bubbles distributed uniformly across the heat transfer surfaces were observed on all test pieces. The Large Lattice and Small Lattice surfaces exhibited a noticeably higher bubble density than the Sintered Plain. This increased density on the lattice structures can be attributed to bubbles nucleating preferentially within the interconnected porous cavities. In contrast, the Sintered Plain surface showed fewer active nucleation sites, which is consistent with requiring a much higher wall superheat to achieve significant bubble growth, as indicated by the boiling curves (Figure 10). At a heat flux of 100 kW/m\({}^{2}\) (Figure 14a-d), a larger number of nucleation sites were observed on the Large Lattice, Small Lattice, and Staggered surfaces. Bubble formation and departure were stable and frequent, indicating efficient heat dissipation. This vigorous bubble activity directly correlates with the rapid increase in HTC observed for these structured surfaces in this low heat flux region (Figure 12). While the high HTC at this low flux is primarily driven by the increased density of active nucleation sites provided by the structures, capillary-driven liquid supply is a major contributing factor. Figure 15a-d shows the bubble characteristics at 250 kW/m\({}^{2}\). For the Large Lattice, Small Lattice, and Staggered, bubbles were observed to detach from the heat transfer surface and then coalesce into larger vapor structures above the heat transfer surface. The interconnected porous cavities of the heated surface facilitate adequate liquid replenishment through capillary forces, supporting this detachment-coalescence behavior away from the surface. However, bubbles tended to merge at the heat transfer surface on the Sintered Plain before detaching, forming larger vapor slugs. These vapor slugs effectively isolate portions of the heated surface from the bulk liquid, hindering the rewetting of the heating surface. At a heat flux of 500 kW/m\({}^{2}\) (Figure 16a-d), the Large Lattice and Small Lattice surfaces continued to show bubbles detaching from the surface before coalescing into vapor structures above it. The formation of more defined vapor columns was observed for the Staggered surface. Bubble formation on the Sintered Plain coalesced into large, stable vapor slugs that covered a significant portion of the heat transfer surface, isolating the bulk liquid from the heating surface. This extensive vapor blanketing on the Sintered Plain led to a significant increase in wall superheat, ultimately resulting in CHF for the Sintered Plain reaching around 782 kW/m\({}^{2}\), which is consistent with the boiling curve (Figure 10). Figure 17a-c displays bubble characteristics at a higher heat flux of 1000 kW/m\({}^{2}\) on the Large Lattice, Small Lattice, and Staggered surfaces. Bubbles on all three surfaces detached from the surface and coalesced into several slightly larger bubbles. Crucially, these coalesced bubbles did not form a continuous vapor film that covered the heat transfer surface.