Hierarchical rough surfaces formed by LBL self-assembly for oil-water separation

Controlled assembly of nanoscale building units to form special micro-/nanostructures is of interest for achieving desired properties in many practical applications. The raspberry- or strawberry-like hierarchical structure with multi-level dimensions is one example of special surface wettability design. In this work, a series of coatings with hierarchical nanostructure and dual roughness are constructed on sintered stainless steel mesh and stainless steel fiber felt via layer-by-layer self-assembly of SiO2 nanoparticles having different sizes. The surface is then chemically treated to obtain the wetting properties needed for intended separation of oil from water. The surface morphology of the coatings is observed using scanning electron microscopy and atomic force microscopy. The surface wetting properties are investigated by measuring the coatings' water and oil contact angle in air and under water. The results show that the stainless steel mesh with such coatings has superhydrophobicity, and thus can efficiently separate regular oil water mixtures. Furthermore, the stainless steel fiber felt treated with similar coatings can also separate oil-in-water emulsions through the non-sieving coalescence mechanism, achieving an oil water separation efficiency as high as 99.4%.