## 4 Conclusions  
This work experimentally investigated the effects of the anisotropic properties of microgrooved surfaces on the heat transfer during pool boiling. The ultrafast femtosecond-laser texturing technique was employed to create microgrooves on flat copper surfaces. Novec-7100 was used as the working fluid during the experiments. Through the visual study of the bubble dynamics, the nucleation ability and bubble evolution cycle on the surfaces were analyzed and discussed. The mechanisms of the anisotropic wettability and roughness were used to explain the difference in the heat transfer performances for all the test conditions. The conclusions of the present study can be summarized as follows:  
* All the microgrooved surfaces exhibited HTCs superior to that of the S1 surface, partially owing to the lower surface wettability in the direction parallel to the grooves. Superior nucleation ability was achieved on these surfaces. Moreover, the S2 surface, with a groove spacing of 100 \(\upmu\)m, increased the HTC by a maximum factor of 1.37 compared with that of the plain surface (S1). This is because the S2 surface exhibited the highest anisotropy for all the test conditions;
* The effect of the anisotropic surface roughness facilitated the formation of more bubble nucleation sites on microgrooved surfaces. The roughness of the microgrooved surfaces in the normal direction was much higher than that of the microgrooved surfaces in the parallel direction, suggesting more space for bubble nucleation. As  
Figure 16: Comparison of the CHF values in the present work with those in related prediction models [59; 61; 60]. The references of prediction models are from [PERSON] [59], [PERSON] [60] and [PERSON] et al. [61]. (The dotted lines represent the \(\pm 10\%\) error in [PERSON]’s prediction model).  
a result, the combination of the higher anisotropy and roughness on microgrooved surfaces resulted in a preferable condition for bubble nucleation and a superior heat transfer performance;
* The CHF values of the microgrooved surfaces were slightly decreased compared with that of the S1 surface. This is because the over-intensified bubble aggregation, due to the exceptional nucleation ability on microgrooved surfaces, deteriorated the heat transfer performance, forming a vapor film on the surfaces. Furthermore, the CHF values in the present work were evaluated using [PERSON]'s empirical correlations, ensuring a good agreement.  
In summary, microgrooved surfaces with anisotropic properties facilitated the improvement in the boiling heat transfer performance. These characteristics hold the potential for diverse industrial applications in the future. Further investigation of the effect of the dimensions of microgrooves on the boiling heat transfer will be conducted in future work.  
Conceptualization, P.-H.C.; Methodology, H.-C.C.; Investigation, H.-C.L., C.-H.K., H.-C.C. and T.-L.C.; Resources, T.-L.C.; Data curation, C.-H.K.; Writing--original draft, H.-C.L.; Supervision, P.-H.C.; Project administration, P.-H.C. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the Ministry of Science and Technology, Republic of China (Taiwan) (grants number MOST 109-2221-E-002-200-MY3 and MOST 109-2221-E-002-199-MY3).  
Not applicable.  
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to patent filing.  
All the procedures, data acquisition, and experimental work were carried out at the MEMS Thermal Control Laboratory, National Taiwan University, Taipei.  
The authors declare no conflict of interest.