Wettability transition and optical properties of laser-textured metallic surfaces

date: 28.03.2020

category: Sporočila za javnost

 

Assist. Prof. Dr. Peter Gregorčič published a comment in ACS Applied Materials & Interfaces (IF = 8.456) clarifying why different research groups have observed contradictory wettability transitions on laser-textured metallic surfaces after low-temperature annealing.

GREGORCICANG

Laser texturing of metallic surfaces represents a promising, straightforward, scalable and chemical-free method for controlling surface wettability. Immediately after the laser texturing, the metallic surfaces are in a superhydrophilic regime. Thus, a water droplet spills over the whole surface and forms a thin water film. However, exposure of such surface to atmospheric air leads to development of superhydrophobic, lotus-leaf like behavior.

In recent years, different authors have published results showing that low-temperature annealing (at 100 °C – 350 °C) of laser-textured surfaces significantly influences their wettability transition. However, these published results led to contradictory conclusions. The experiments that were conducted within the ARRS research project LaMiNaS (J2-1741) clarify that controlled contamination is crucial in obtaining consistent surface wettability alterations after low temperature annealing. It is shown that low-temperature annealing in contaminated furnaces promotes superhydrophobicity, while the same annealing conditions in uncontaminated environment turns off the developed superhydrophobicity and returns the surface into initial, superhydrophilic state. This can be used as an advanced approach to obtain the reversibly switchable wettability that is important in cutting-edge applications, including self-cleaning interfaces, tunable optical lenses, microfluidics and lab-on-chip systems.

The achieved experimental results reveal that pre-heating the furnace to temperatures exceeding 350 °C for several hours can provide a contaminant-free annealing environment. It is also shown that (usually overlooked) contamination may arise from the silicone components of the furnace. In this case, a secondary ion mass spectroscopy (ToF-SIMS) analysis revealed the formation of a thin (only several molecular layers thick) and homogeneous hydrophobic polydimethylsiloxane (PDMS) film over the whole annealed surface, which can be also recognized by the appearance of the X-ray photoelectron spectroscopy (XPS) photopeak at 101.8 eV binding energy. However, heating to 350 °C leads to PDMS oxidation and consequently returns the surface into the initial, superhydrophilic state.

The theoretical calculations of the reflectivity of a metallic surface covered with thin oxide layer clearly demonstrate that the interference between the light reflected from the air-oxide, and the oxide-metal interfaces significantly influences surface optical properties. Moreover, the presented theoretical calculations also clearly show that controlling the thickness of the laser-induced oxide layer or even the thickness of an additional transparent liquid layer may lead to advanced applications utilizing the switchable optical properties of the laser-textured metallic surfaces.

The original scientific paper is available under CC BY license at: https://doi.org/10.1021/acsami.9b23462

 

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