Wetting transition in laser-fabricated hierarchical surface structures and its impact on condensation heat transfer characteristics

Abstract

Investigation on the fabrication of superhydrophobic surfaces to maintain dropwise condensation, which possess higher degree of heat transfer than filmwise condensation, has become an ineludible area of research. In this work, we have attained superhydrophobicity in plain copper using picosecond laser treatment and silane coating through vaporisation technique and the effects of the same in condensation heat transfer has been studied. Depth of the grooves attained on the surface was changed by varying the laser power. Contact angle and FESEM images were used to characterise the wettability and surface morphologies respectively. Silane coated laser structured samples attained superhydrophobicity, whereas laser structured samples attained a maximum contact angle value of 143°. Condensation experiment was then carried on the as-prepared samples in a dedicated setup and were compared against the bare copper surface to evaluate its performance. Nusselt's theory was used to validate the obtained results by comparing it aginst bare copper surface. All the modified test specimens produced superior results than the untreated surface, but the silane coated surface with maximum depth resulted in 97% and 88% enhancement in heat flux and heat transfer coefficient values respectively. Microstructure formation during laser structuring and increase in the aspect ratio due to increase in groove depth resulted in the increase of Laplace pressure gradient, pushing the droplet to a critical state, that is partial wetting state known as micro-Wenzel nano-Cassie state. Gravity, buoyancy and dispersive adhesion forces then acts on the droplet and detaches it from the surface at much smaller diameter, increasing the droplet sweeping cycle. Droplet departure diameter reduced nearly 2 times and the departure frequency increased by approximately 2.5 times at high subcooling, lead to the increased heat transfer characteristics.