Abstract
Nucleate boiling remains a critical mechanism for high-performance thermal management in advanced energy systems. This study investigates the evolution of pool boiling heat transfer performance across three distinct surfaces: a ground copper substrate as a baseline, ground copper electroplated with nickel (ED-Ni) matrix, and ground copper electroplated with biphilic ED-Ni-PTFE composite. Utilizing synchronized optical surface vapor phase recordings and embedded micro-thermocouples, we established a direct relationship between boiling regime transitions and macroscopic thermal transients across multiple boiling cycles. The results reveal a clear performance hierarchy: the biphilic electroplated Ni-PTFE (ED-Ni-PTFE) surface on copper substrate demonstrated the best boiling performance , reaching a heat flux of 100 W/cm² at a wall superheat of only 6.8 °C, representing a 73% wall superheat reduction compared to unplated copper substrate and a 60% reduction relative to the pure electroplated nickel (ED-Ni) on copper substrate. While the Ni-PTFE provides the highest efficiency, the results show that the ED-Ni matrix inherently provides a more favorable vapor bubble nucleation environment than unplated copper, despite copper's superior bulk thermal conductivity. Systematic analysis via scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) indicates that the surfaces undergo significant degradation (in-situ conditioning) during the boiling cycles. Specifically, the formation of a nanostructured oxide layer, which was confirmed by a measurable rise in the oxygen-to-metal X-ray intensity, resulted in an increase of the density of active nucleation sites and promotes liquid rewetting. This morphological and chemical evolution of the degradation layer likely accounts for the stabilization of the critical heat flux (CHF) observed in subsequent boiling cycles.