Why do metals become superhydrophilic during nanosecond laser processing? Design of superhydrophilic, anisotropic and biphilic surfaces

This research investigates the impact of the surrounding environment on the characteristics of copper, nickel and tin during nanosecond laser processing. By comparing the results of processes conducted in vacuum and air, we conclude that changes in wetting properties cannot be solely attributed to development of surface microstructure. To elucidate this phenomenon, we conducted model experiments using a tin target. Laser processing parameters for the tin target were established, involving deep cavity melting without ablation, resulting in highly evolved material morphology. Under these conditions, laser processing in air did not lead to metal hydrophilization. Therefore, our study demonstrates that the primary cause of changes in material wetting properties during nanosecond laser processing is the re-deposition of ablation products onto the material surface. These products form a nanoporous layer that enhances wicking capabilities and subsequently serves as a sorbent for various impurities, gradually leading to the hydrophobization of most commonly used materials. The proposed mechanism opens the possibility for the development of new methods to create materials with biphilic and anisotropic wetting properties. The key insight lies in the control of the thickness of the nanoporous layer, as it emerges as a pivotal factor dictating wettability properties and wicking capability.