Pool Boiling Performance of Surfaces Produced by Femtosecond Laser Surface Processing and Copper Hydroxide Nanoneedle Growth

Copper hybrid functionalization that leads to copper hydroxide nanoneedles grown on top of femtosecond laser surface processed (FLSP) copper microstructures is experimentally studied in saturated pool boiling of distilled water at atmospheric pressure. FLSP was used to fabricate self-organized, hierarchical structures that lead to an inherently superhydrophilic surface. The hybrid scheme involved synthesizing copper hydroxide nanoneedles atop the FLSP microstructures using a dilute solution of sodium hydroxide and ammonium persulfate. The nanoneedles lead to increased surface area and altered cavity dimensions available for nucleation. In addition, a citric acid cleaning process was implemented to remove oxides from the FLSP surface before growing the nanoneedles. The FLSP surfaces with and without citric acid cleaning were tested up to critical heat flux to serve as baselines for comparisons with the hybrid functionalization scheme. Pool boiling results using the hybrid functionalization scheme revealed degraded performance for surfaces which incorporated copper hydroxide nanoneedles due to the increased volume of oxides present from the transition of copper hydroxide to copper oxide during heating. Scanning electron microscope images acquired after boiling tests revealed complete removal of the nanoneedles during testing, illustrating the nanoneedles' lack of durability under aggressive boiling conditions. The best performance was shown for a sample functionalized using FLSP only and without incorporating citric acid etching or nanoneedle growth. This surface achieved a 22% increase in the heat transfer coefficient compared to the polished surface at a heat flux of approximately 95 W/cm2, albeit at a reduced critical heat flux. Additionally, large degrees of pool boiling inversion were observed for surfaces which did not include the citric acid cleaning process. This observation points to a connection between the oxides produced from the FLSP process and boiling inversion for copper, suggesting that the presence of oxides and associated microstructures are responsible for boiling inversion on high thermal conductivity metals.