Micro/nano engineering on stainless steel substrates to produce superhydrophobic surfaces Samuel Beckford, Min Zou ⁎ Department of Mechanical Engineering, University of Arkansas, Fayetteville, AR 72701, USA abstract article info Available online 14 June 2011 Keywords: Micro-scale topography Nano-scale topography Stainless steel Water contact angle Superhydrophobic Creating micro-/nano-scale topography on material surfaces to change their wetting properties has been a subject of much interest in recent years. Wenzel in 1936 and Cassie and Baxter in 1944 proposed that by microscopically increasing the surface roughness of a substrate, it is possible to increase its hydrophobicity. This paper reports the fabrication of micro-textured surfaces and nano-textured surfaces, and the combination of both on stainless steel substrates by sandblasting, thermal evaporation of aluminum, and aluminum-induced crystallization (AIC) of amorphous silicon (a-Si). Meanwhile, fluorinated carbon films were used to change the chemical composition of the surfaces to render the surfaces more hydrophobic. These surface modifications were investigated to create superhydrophobic surfaces on stainless steel substrates. The topography resulting from these surface modifications was analyzed by scanning electron microscopy and surface profilometry. The wetting properties of these surfaces were characterized by water contact angle measurement. The results of this study show that superhydrophobic surfaces can be produced by either micro-scale surface texturing or nano-scale surface texturing, or the combination of both, after fluorinated carbon film deposition. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Water repellant (superhydrophobic) surfaces have many applica- tions such as solving stiction problems in micro-electromechanical systems (MEMS)/nano-electromechanical systems (NEMS) [1], anti- icing [2], and friction and wear reduction between parts in dynamic contact [3]. Water repellency has been a subject of much interest for some time, but in recent years, it has received much attention as a result of new achievements [4]. Barthlot and Neinhuis, for example, discovered that the epidermal surface of water repellant leaves consists of various micro-textures combined with the presence of water repellant wax crystals on these textures [4,5]. Today, the technology exists to create textures on surfaces which are of the same scale as those in biological systems [6,7] and we are able to mimic the effects of water repellant leaves by both physically and chemically modifying a surface. Wenzel in 1936 and Cassie and Baxter in 1944 proposed that by microscopically increasing the surface roughness of a substrate, it is possible to increase its original hydrophobicity [8–12]. The Cassie– Baxter state is ideal for creating superhydrophobic surfaces because water droplets lie in a suspended state resting on the top of the surface textures instead of dropping between each texture. It has also been established that the height and width of the textures, as well as the distance between textures, are determining factors to achieve a Cassie–Baxter state [9,13]. Ashish Mall et al. discovered that pillar- shaped textures with smooth edges and rough edges can both produce equally hydrophobic contact angles by varying the height, width, and spacing of the textures. However, smooth edges can produce these results at a much lower roughness [14]. It is evident that the size and shape of textures created have a significant impact on the resulting wetting properties. The purpose of this study is to fabricate micro-/nano-textures on stainless steel to produce superhydrophobic properties similar to water repellant leaves. The micro-textures were created by sandblasting the substrates. Two types of nano-textures were created, smooth (semi circular) and sharp (agglomerated, cylindrical) textures. Smooth textures were created by thermal evaporation of aluminum on the stainless steel substrate, while sharp textures were created by aluminum-induced crystallization (AIC) of amorphous silicon (a-Si). It should be noted that AIC of a-Si has only recently been used as a method to produce nano- textures [15] and has not previously been used in combination with sandblasting to create a combination of micro- and nano-textures. Stainless steel was chosen as the substrate due to its wide industrial applications and biocompatibility for potential biomedical applications. 2. Experimental details 2.1. Sample preparation Pre-cleaned 0.9 mm-thick polished AISI 316 L stainless steel (Goodfellow Cambridge Limited) was selected as the substrate for Thin Solid Films 520 (2011) 1520–1524 ⁎ Corresponding author. Tel.: + 1 479 575 6671; fax: + 1 479 575 6982. E-mail address: mzou@uark.edu (M. Zou). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.05.081 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf