Enhanced bubble nucleation and liquid rewetting for highly efficient boiling heat transfer on two-level hierarchical surfaces with patterned copper nanowire arrays

Enhancing boiling heat transfer by surface modification is of critical interest for improving the efficiency of many energy systems and for addressing thermal management bottlenecks in electronics. However, the improvement of all boiling heat transfer characteristics including the critical heat flux, heat transfer coefficient and onset of nucleate boiling, usually has conflicting requirements on surface wettability and morphology. In this work, we develop a two-level hierarchical surface with patterned copper nanowire arrays for boiling heat transfer enhancement. By surrounding long nanowire arrays with short nanowires where microcavities are formed between short nanowire clusters, a novel strategy is reported to improve all the boiling heat transfer characteristics through increasing bubble nucleation site density, capillary-induced liquid rewetting, and the separation of liquid-vapor pathways. Compared to boiling heat transfer performance on the plain copper surface, a 71% higher critical heat flux, a 185% higher heat transfer coefficient as well as a 37% lower onset of nucleate boiling are demonstrated on such two-level hierarchical surfaces. In addition, we correctly predict the effect of surface structure on the boiling heat transfer performance by an analytical model. Through distinguishing the role of different structure morphologies including the improved nucleation site by microcavities, enhanced liquid wicking by nanowires, and continuous liquid supply by long nanowire arrays, we have established a comprehensive understanding on the relation between the surface structures and boiling heat transfer characteristics.