Preparation of a superhydrophobic film with UV imprinting technology

Abstract

Superhydrophobic surfaces with an advancing water contact angle (CA) larger than 150 and a water sliding angle (SA) less than 10 are found on lotus leaves and butterfly wings. These surfaces are interesting because of their various applications for water-repellence and anti-sticking properties. The lotus leaf is especially well known for its selfcleaning ability. Rolling water droplets wash off contaminants and dust due to low surface adhesion. According to previous studies, the surface dewetting is governed by the surface energy of coating material and roughness of the surface. Poly(dimethylsiloxane) (PDMS) and fluorocarbon based materials have been used to artificially modify a surface due to their low surface free energies. Nanotubes, nanofibers, nanorods, and porous structures have been introduced to create complex nano structures. Theoretically, two distinct models (Wenzel and Cassie models) were proposed to explain the effect of roughness on a hydrophobic surface. Because roughness impacts contact angles, they introduced the surface roughness factor (r), which is defined as the ratio between the actual and projected surface areas. Wenzel derived a theoretical relationship from the Young equation, which correlates the ‘apparent’ or measured contact angle on a rough surface with a flat substrate (Young’s model). His equation shows that an increased r can enhance surface wettability, allowing water to penerate the surface. Cassie and Baxter, on the other hand, propose that an r above a critical level leads to a great decrease in CA hysteresis, therefore increasing the receding angle. In the Cassie model, water droplets partially sit on surface air pockets and can easily roll off. In the previous study, we showed a simple method to imitate the hierarchical lotus leaf structure using sol-gel technology. Briefly, we used a micro lens array (MLA) pattern to create ordered, micron-sized, large hills and porous silica aerogel to create disordered nano structure, and then chemically modified the surface with a PDMS coating solution. Nano-imprint lithography (NIL) is a promising technique to fabricate patterned structures with high precision and throughput in the micro/nanometer scale region. Additionally, NIL is inexpensive and does not require a complicated apparatus. In conventional NIL technology, hard molds, such as silicon, dielectric material (e.g., silicon dioxide or silicon nitride), and metal material (e.g., nickel) were used. Recently, flexible mold technology was introduced, replacing hard molds, making it easier to release the mold from the polymer surface, providing better conformal contact with the substrate to be patterned, and reducing the pressure needed during imprinting. In a previous study, Kim et al. introduced a simple NIL method to create a superhydrophobic surface with a flexible mold made from anodic aluminium oxides (AAOs). Furthermore, Lee et al. produced well-defined, large, nanostructured polymeric and metallic surfaces with nanoembossing or nanofibers and controlled aspect ratios by employing AAOs or textured Al surfaces as a replication master. In this study, we present a superhydrophobic flexible film preparation with ultraviolet (UV) NIL technology using a flexible master substrate with ordered micronsized patterns and disordered nano structure. The advantage of this study is that it removes the surface treatment step that is required in conventional imprinting. The surface treatment allows for easy demolding due to the low surface energy of the superhydrophobic and modifiedPDMS coating surface.