Surface engineering through atomic layer deposition on three-dimensionally structured materials

Abstract

Atomic layer deposition (ALD) is effective in depositing conformal thin films, which is highly favorable for coating various patterned surfaces. These coatings serve as barrier layers in addition to surface modifications to improve wettability of porous structures, such as meshes and membrane channels. However, it has been challenging to conformally deposit hydrophilic thin films on three-dimensionally (3D) designed, more complicated architectures. To understand the effect of surface modifications on 3D structures' surface properties, we deposit thin silica films via ALD on hydrophobic porous media, which is nickel inverse opal structures in this case. The silica thin film is used to improve hydrophilicity without modifying the geometries of the microporous structure such as porosity, pore size, and metal type. We study the consequences of applying silica coatings to the 3D structure in comparison to flat surface counterpart. The hydrophilicity effects of ALD coating on porous structures and flat nickel surfaces are approximately the same with a result of decreasing apparent static contact angle of approximately 30°. In relation, the Fowkes method reveals the surface energy of the ALD silica samples increases by a factor of 1.3. Thermal stability of the coating is tested, revealing a relative degradation with increasing thermal cycling, most likely associated with the adsorption species on the thin film surface. The droplet spreading rate is analyzed in addition to droplet volume loss to estimate the liquid penetration rate into the structure, if any. Condensation rate and condensate growth show that despite having lower droplet nucleation in comparison to a flat surface, the droplet area growth on inverse opal regions is larger. These findings showcase potential improvements to 3D microporous structures by employing ALD coating for fluid transport through the porous media. Copyright © 2020 ASME.