Scalably manufactured textured surfaces for controlling wettability in oil-water systems

Abstract

Competitive wettability in oil-water systems influences applications such as oil-water separation and enhanced oil recovery. Here, we study the wettability of water (in oil) and oil (in water) on sub-millimeter/micro/nano textured surfaces fabricated on a variety of substrates (metals, polymers, elastomers). Importantly, all the fabrication processes employed involved non-cleanroom-based scalable techniques. Metal surfaces were fabricated via wet etching processes and polymer/elastomer surfaces were fabricated via laser etching. These fabrication techniques can enable texturing with sub-millimeter, micron and sub-micron feature sizes. Wettability was characterized by measuring static contact angle and dynamic contact angle (roll-off angle).

Several insights into wettability are obtained from this work. Firstly, textured metal surfaces with low energy surface chemistry showed the lowest adhesion to water and oil droplets. Textured metal surfaces coated with Teflon AF were superhydrophobic (in oil) with very low roll-off angles (4°– 7°). Uncoated textured metal surfaces were superoleophobic (in water) with roll-off angles of 3°– 9°. Secondly, textured polymer and elastomer surfaces exhibited utrahydrophobicity (in oil); however not all textured elastomers exhibited superoleophobicity (in water). Thirdly, no droplet roll-off was observed on any textured elastomer and polymer surface, despite very favourable contact angles. This indicates that high contact angles do not always translate to superhydrophobicity/oleophobicity. Fourthly, it is seen that competitive wettability of a surface can be understood by analysing the corresponding water and oil wettability of that surface in an air environment. Additionally, the initial state of the surface can be important, as the first fluid in contact with the surface fills the porous textures, and dictates subsequent wettability. All these findings and insights position this work as the foundation for more detailed studies on the development of surfaces for specific applications.