Studies on coupled effects of roughness and wettability on boiling heat transfer and nucleation via molecular dynamics simulation

Properly designed nanostructured surfaces can effectively promote bubble nucleation and enhance heat transfer during phase change at the microscale. Previous studies have generally attributed the enhanced boiling performance of nanostructures to improved local heat transfer. However, the suppression of nucleation due to strong solid-liquid interactions has been largely overlooked. In this study, molecular dynamics simulations are employed to investigate the bubble nucleation process on three-dimensional nanostructured surfaces under the synergistic effects of wettability and roughness. The surface roughness factor r is defined as the ratio of the actual surface area A actual to the projected area A geometry . The roughness factor r at which the surface performance just surpasses that of an ideal smooth surface is defined as the critical roughness threshold. The results indicate that surfaces with high hydrophilicity exhibit enhanced thermal energy accumulation, which positively influences bubble nucleation. However, the energy barrier that must be overcome for the phase transition is also higher. The competing effects lead to an 8.6 % reduction in critical heat flux (CHF) and a 12.3 % delay in bubble initiation time when the roughness factor r = 1.016 compared to smooth surfaces. Moreover, a critical roughness threshold that varies with surface wettability exists on nanostructured surfaces for enhancing nucleation and heat transfer. For super-hydrophilic surfaces, the critical roughness threshold is 1.052 with maximum CHF enhancement reaching 24.2 %. In contrast, hydrophilic and neutral surfaces display lower critical roughness thresholds of 1.019 and 1.033. Nevertheless, the relatively weak solid-liquid interactions on these two surfaces also constrain the potential for further heat transfer enhancement. Compared to the smooth surface, the CHF improvements on hydrophilic and neutral surfaces are limited to 15.1 % and 4.7 %, respectively, indicating an inherent upper bound to performance enhancement. On each type of surface, the bubble nucleation rate exhibits a distinct power-law correlation with surface roughness.