Cycle-Dependent Evolution of Critical Heat Flux in Pool Boiling in Water on Copper Driven by Oxidation-Induced Surface Modification

Boiling heat transfer enables extremely high heat fluxes at small temperature differences, but the underlying mechanisms remain difficult to resolve due to the dynamic interplay of microscale interfacial processes and macroscopic heat transfer. In this work, synchronized optical-thermal measurements of pool boiling on mechanically ground and polished copper surfaces were conducted to directly correlate near-wall temperature transients with boiling regime transitions. Embedded micro-thermocouples captured repeatable near-wall temperature transients associated with boiling regime transitions, while synchronized video recording confirmed regime transitions from natural convection to nucleate and film boiling. Critical heat flux(CHF) values of 105.1-132.0 W/cm² were measured at transition wall superheats of 25.3-31.4 {degree sign}C for all three tested samples. The largest performance change occurred after the first boiling cycle, with wall superheat and CHF increasing significantly, whereas subsequent cycles produced smaller shifts. On cooling, film-to-nucleate collapse occurred at wall superheats around 4-17 {degree sign}C higher than in the forward transition, confirming a pronounced hysteresis effect. Scanning electron microscopy (SEM) analysis revealed substantial roughening after three boiling cycles, while energy dispersive X-ray spectroscopy (EDS) showed the oxygen to copper X-ray intensity ratio nearly doubled (0.08 to 0.17), indicating oxide layer growth. These surface modifications(conditioning) explain the observed performance evolution: roughening enhanced CHF, while oxidation introduced thermal resistance, elevating wall superheat. Taken together, these results demonstrate that boiling hysteresis and cycle-dependent CHF evolution are governed by coupled morphological and chemical transformations of the surface. The integrated optical, thermal and microstructural approach provides direct evidence linking interfacial dynamics to surface conditioning in pool boiling.