Journal of Colloid and Interface Science 326 (2008) 465–470 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis UV and thermally stable superhydrophobic coatings from sol–gel processing Yonghao Xiu a,b , Dennis W. Hess a,∗ , C.P. Wong b,∗ a School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, GA 30332-0100, USA b School of Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst Drive, Atlanta, GA 30332-0245, USA article info abstract Article history: Received 11 April 2008 Accepted 15 June 2008 Available online 28 June 2008 Keywords: Silica Thin film Superhydrophobicity Fluoroalkyl silane UV stability Thermal stability Accelerated UV weathering test A method for the preparation of inorganic superhydrophobic silica coatings using sol–gel processing with tetramethoxysilane and isobutyltrimethoxysilane as precursors is described. Incorporation of isobutyltrimethoxysilane into silica layers resulted in the existence of hydrophobic isobutyl surface groups, thereby generating surface hydrophobicity. When combined with the surface roughness that resulted from sol–gel processing, a superhydrophobic surface was achieved. This surface showed im- proved UV and thermal stability compared to superhydrophobic surfaces generated from polybutadiene by plasma etching. Under prolonged UV tests (ASTM D 4329), these surfaces gradually lost superhydro- phobic character. However, when the as-prepared superhydrophobic surface was treated at 500 ◦ C to remove the organic moieties and covered with a fluoroalkyl layer by a perfluorooctylsilane treatment, the surface regained superhydrophobicity. The UV and thermal stability of these surfaces was maintained upon exposure to temperatures up to 400 ◦ C and UV testing times of 5500 h. Contact angles remained >160 ◦ with contact angle hysteresis ∼2 ◦ .  2008 Elsevier Inc. All rights reserved. 1. Introduction The water repellent properties of many biological surfaces, es- pecially plant leaves, have prompted great research interest since, when water drops fall on certain leaf surfaces, they can easily pick up contamination, roll off the surface, and thereby clean the leaves. Because the first observation of this self-cleaning phenomenon was on lotus leaves, the effect is generally termed “lotus effect.” De- tailed inspection of the leaves showed that micron and submicron structures as well as hydrophobic materials exist on the leaf sur- face [1]. The combination of these factors leads to superhydropho- bicity. Wenzel [2,3] showed that surface roughness produces a physical enhancement of hydrophobicity which was described by cos θ A = r cos θ Y . (1) In Eq. (1), θ A is apparent contact angle, θ Y is the contact angle on a flat surface and r is the ratio of the actual to the projected area. This expression predicts that the wetting behavior of a sur- face is enhanced by roughness; thus, creation of roughness on a flat surface with an equilibrium contact angle θ Y (flat) > 90 ◦ in- creases the contact angle, while the same roughness on a surface with θ Y (flat) < 90 ◦ decreases the contact angle. In practice, inti- mate contact is not usually maintained between liquid and solid on * Corresponding authors. Faxes: +1 404 894 2866 (D.W. Hess), +1 404 894 9140 (C.P. Wong). E-mail addresses: dennis.hess@chbe.gatech.edu (D.W. Hess), cp.wong@mse.gatech.edu (C.P. Wong). very rough surfaces with θ Y (flat) > 90 ◦ ; rather, the droplet effec- tively resides on a composite surface. In this case Cassie’s equation applies [4]. cos θ A = f 1 cos θ Y − f 2 , (2) where f 1 and f 2 are the fractions of the surface occupied by solid/ liquid and air/liquid, and f 1 + f 2 = 1. Self-cleaning surfaces have many potential applications, for example, surface decontamination on microelectronic equipment or devices, water repellent coatings, biocompatible surfaces, and friction reduction. Such surfaces have been created on a vari- ety of materials such as silicones [5], alumina [6–8], silicon [9], silica [10–14], polyelectrolytes [15,16], carbon nanotubes [17], polystyrene [18], PTFE (polytetrafluoroethylene) [19] and fluorocar- bons [20], and polybutadiene [21]. Several criteria have been used to characterize these surfaces including contact angle (>150 ◦ ), contact angle hysteresis (<10 ◦ ) and roll off angle. However, limita- tions exist when polymeric materials are exposed to atmospheric conditions where degradation in superhydrophobic properties oc- curs due to UV irradiation, and to exposure to impurities, O 2 , and moisture present in the environment. Also, because organic poly- mers are easily deformed when a force is applied, the surface structure is not stable when the surface undergoes abrasion from friction, handling, or dust particles. Sol–gel processes offer a method to fabricate porous glass films under ambient conditions. The mild preparation conditions offer the ability to incorporate a wide range of labile organic species into a glass composite. In addition to the mild processing condi- 0021-9797/$ – see front matter  2008 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2008.06.042