## 3 Results and Discussion
### Analysis of Bubble Dynamics  
The images of the bubble dynamics at four different heat-flux intervals, as obtained using a high-speed camera, are shown in Figure 14. Owing to the increased surface roughness in the normal direction and the increased CA in the parallel direction, the numbers of bubble nucleation sites on the S2, S3, and S4 surfaces surpassed that of bubble nucleation sites on the S1 surface in the low heat flux regime. Unlike the S1 surface, the S2-S4 surfaces had bubbles that started to nucleate at lower wall superheats (5.56-6.43 K). As shown in the high-speed images in Figure 14, the emerging bubbles were more pronounced on the S2-S4 surfaces compared with those on the S1 surface at a heat flux of \(\sim\)1.79 W/cm\({}^{2}\). The difference in the bubble nucleation abilities can be attributed to the lower wettability in the parallel direction on the S2-S4 surfaces, which was induced by the more significant energy barrier of the microgrooved surfaces in the parallel direction. As stated by [PERSON] et al. [40], this energy barrier prevented liquid droplets from collapsing and penetrating the microgrooves in the normal direction. Therefore, this phenomenon caused a less-wetted condition that lowered the minimum surface energy requirement to activate the bubble nuclei. A preferable bubble nucleation environment was formed with the assistance of a larger directional wettability difference and increased surface roughness in the normal direction. With an earlier onset of nucleate boiling and an increased number of nucleated bubbles, as presented in Figure 14, the S2-S4 surfaces could transfer more heat with the help of the liquid-vapor phase change. After the ONB, a large number of bubbles emerged and started to dominate the test surface at a low heat-flux interval (1.79-4.21 W/cm\({}^{2}\)). In this heat flux regime, the bubble departure frequency was noticeably increased, making it challenging to examine the bubble behavior. As the process continued, the interaction between the bubbles intensified, causing smaller bubbles to merge and form larger ones at the moderate heat-flux interval  
Figure 13: Mechanisms of structure-induced anisotropic properties: (**a**,**b**) surface wettability effect; (**c**,**d**) surface roughness effect. (4.21-12.27 W/cm\({}^{2}\)). The rapid aggregation and coalescence of bubbles propelled the boiling to the high heat-flux interval (12.27-21.59 W/cm\({}^{2}\)). When the heat flux reached \(\sim\)22.44 W/cm\({}^{2}\) on the S4 surface, the generated bubbles no longer had sufficient time to depart from the test surface. Consequently, these numerous bubbles coalesced to form a large vapor film that covered the entire surface. As depicted in Figure 14, larger vapor columns rapidly developed on the S1-S4 surfaces in the high heat flux regime because of over-intensified bubble coalescence, resulting in a vapor film. Owing to the low thermal conductivity of the vapor film, effective heat transfer was hindered, leading to the occurrence of film boiling. The bubble evolution cycle is another critical factor affecting the boiling heat transfer performance. As shown in Figure 15, the bubble detachment rate of the S2 surface was much faster than those of the S1, S3, and S4 surfaces in the low heat flux regime (\(\sim\)1.79 W/cm\({}^{2}\)). This is because the wettability in the normal direction for S2 was higher than those in the normal direction for the other test surfaces (Table 4). Enhanced liquid replenishment would accelerate the bubble evolution cycle. This efficient bubble detachment could facilitate heat dissipation. As presented in Figure 15, S3 also exhibited a high bubble departure rate. As a result, the differences in the directional wettability and surface roughness played a crucial role in reducing the surface wall superheat and achieving a higher heat transfer rate for S3 compared with S1.