Thermal stability of superhydrophobic, nanostructured surfaces Sung-Chul Cha a,1 , Eun Kyu Her b,c,1 , Tae-Jun Ko b,c , Seong Jin Kim b , Hyunchul Roh c , Kwang-Ryeol Lee b , Kyu Hwan Oh c , Myoung-Woon Moon b,⇑ a Advanced Functional Materials Research Team, Automotive, Corporate R&D Division, Hyundai Motor Group, Republic of Korea b Institute for Multidisciplinary Convergence of Matters, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea c Department of Materials Science and Engineering, Seoul National University, Seoul 151-742, Republic of Korea article info Article history: Received 31 July 2012 Accepted 24 September 2012 Available online 2 October 2012 Keywords: Superhydrophobicity Nanostructure Thermal stability Wetting transition abstract The thermal stability of superhydrophobic, nanostructured surfaces after thermal annealing was explored. Flat surfaces coated with hydrophobic diamond-like carbon (DLC) via plasma polymerization of hexamethyldisiloxane (HMDSO) showed a gradual decrease in the water contact angle from 90 o to 60 o while nanostructured surfaces maintained superhydrophobicity with more than 150° for annealing temperatures between 25 and 300 °C. It was also found that surfaces with nanostructures having an aspect ratio of more than 5.2 may maintain superhydrophobicity for annealing temperatures as high as 350 °C; above this temperature, however, the hydrophobicity on surfaces with lower aspect ratio nanostructures gradually degraded. It was observed that regardless of the aspect ratios of the nanostruc- ture, all superhydrophobic surfaces became superhydrophilic after annealing at temperatures higher than 500 °C. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction Functional hard coating materials have been developed to sat- isfy harsh requirements, such as low friction, wear resistance, sticking resistance, and high temperature stability up to 300 °C [1,2]. Stable hydrophobicity has been considered a crucial property of coating materials especially due to the recent demand for high temperature performance in various applications, such as automo- bile parts or cooking wares. There have been reports of hydrocar- bon materials being used as hydrophobic coatings with lower surface energy, comparable to that of polytetrafluoroethylene (PTFE), and with good mechanical performance in terms of wear resistance, low friction, and hardness [3,4]. To lower the surface energy for hydrophobic coatings, chemical modifications of carbon films have been performed by adding a third element, such as Si–O, Si, or F, into an amorphous carbon matrix, resulting in a surface en- ergy as low as 20 mN/m [3–5]. However, the hydrophobicity of these low-energy surfaces with Si–O contained within the hydro- carbon coating was reported to decrease gradually as the annealing temperature increased above 250 °C [3]. It was reported that hydrogen might be debonded from the hydrogen–carbon bonding interactions approximately 260 °C [5] and induce significant degra- dation of the hydrophobic properties in hydrocarbon coatings. Hydrophobic or superhydrophobic surfaces are achieved by applying coatings with low surface energy on textured surfaces, which usually mimics micro-/nano- or hybrid-structures, such as that of the lotus leaf with high water contact angle (CA) and low contact angle hysteresis (CAH) [6]. When a water droplet is re- leased onto a lotus leaf, it forms a nearly perfect spherical shape that rolls off and cleans the contaminated surfaces. This water- repellent behavior is attributed to the leaf surface structure due to its micro-/nano-hierarchy and relatively hydrophobic epicuticu- lar wax crystalloid coating [7,8]. This indicates that surface textur- ing is also essential for high hydrophobicity, in addition to the low surface energy of materials with hydrophobic coatings. Until now, there have been many studies on the wetting and kinetics of drop- let behavior on superhydrophobic surfaces [9,10] but few studies on the thermal stability of the superhydrophobicity of structured surfaces. Saleema and Farzaneh reported that the superhydrophob- icity of ZnO nanotower structures with stearic acid modifications was degraded at certain temperatures due to desorption of hydro- phobic stearic acid [11]. However, the work did not discuss in de- tail the contribution of nanostructure to the thermal stability of the superhydrophobic surfaces by varying its geometric configuration. In this work, the thermal stability of hydrophobicity and super- hydrophobicity was investigated on nanostructured surfaces with different geometries. We used a nanostructured Si wafer coated with a hydrophobic coating of a diamond-like carbon (DLC) with and without Si–O and annealed at temperatures ranging from ambient temperature to 500 °C. Due to the geometric aspect ratio, defined as a ratio of the height over the diameter of a nanopillar structure, superhydrophobic, nanostructured surfaces maintained their hydrophobicity with CAs of 150° and low CAH at tempera- tures up to 350 °C; above which temperature, the wettability 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.09.052 ⇑ Corresponding author. E-mail address: mwmoon@kist.re.kr (M.-W. Moon). 1 These authors contributed equally to this work. Journal of Colloid and Interface Science 391 (2013) 152–157 Contents lists available at SciVerse ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis