Effect of contact angle hysteresis on water droplet evaporation from super-hydrophobic surfaces S.A. Kulinich *, M. Farzaneh CIGELE/INGIVRE, Department of Applied Sciences, Universite ´ du Que ´bec a ` Chicoutimi, 555 University Blvd., Saguenay, PQ, Canada G7H 2B1 1. Introduction There has been significant interest in recent years in develop- ment of super water-repellent surfaces, which exhibit water contact angles (CAs) larger than 1508 [1–10]. Such surfaces show potential in a variety of applications from anti-sticking, anti- contamination and self-cleaning to anti-corrosive, frost- and snow-repellent, low-friction coatings, among others [6–14]. In parallel, studies on the dynamic behavior of such surfaces have been attracting more and more interest [10,11,13–17]. This can partly be explained by the recent belief that the static hydro- phobicity alone is not sufficient to fully characterize the wetting properties of surfaces [10,16–18]. Free evaporation of small water droplets from various surfaces is an apparently simple problem related to the dynamic behavior of such systems, with relevance to both academic and practical interests. However, more complete knowledge of how evaporation influences the CA and water drop shape is still necessary for better understanding numerous dynamic wetting/dewetting processes on surfaces. That knowledge is also very important in the wetting and surface characterization processes as CA, being an important parameter characterizing surface properties, appears to change when inevitable evaporation of water in air occurs. While extensive studies have been carried out on the evaporation of water droplets from solids [19–26], relatively little research has been done on hydrophobic surfaces [20,23,24,26], and even less so on super-hydrophobic surfaces [26–28]. McHale and co-workers studied evaporation on a super-hydrophobic surface formed by regularly patterned polymer and having high CA hysteresis (CAH) [26]. It was shown that water droplets initially evaporated in a pinned contact line (i.e. so-called constant- contact-diameter, CD) mode, before the contact line receded in a stepwise fashion, jumping from pillar to pillar of the patterned surfaces [26]. An abrupt collapse of some droplets into the pillar structure was observed in some cases during evaporation, while other droplets appeared to collapse into the structure only at very late stages of evaporation, which could not be detected by the optics used [26]. Zhang et al. [27] followed sessile water droplets evaporating on super-hydrophobic lotus leaf and biomimetic polymer surfaces. Both hierarchically structured samples demon- strated the constant-CD mode of evaporation, while CAH values were not evaluated. Most recently, Reyssat et al. followed drop evaporation on Si wafers decorated with regular cylindrical micropillars and passivated with a fluoropolymer, where CAH values were 20–258 [28]. They observed mainly constant CA during the first stage of evaporation, while at later stages drops could collapse into the structure (with shorter pillars) or remain suspended to the very end (on taller pillars) [28]. Evaporation from disordered (i.e. opposed to regularly patterned) super-hydropho- bic surfaces with very low values of CAH has not been reported thus far. Applied Surface Science 255 (2009) 4056–4060 ARTICLE INFO Article history: Received 30 June 2008 Received in revised form 1 October 2008 Accepted 26 October 2008 Available online 7 November 2008 Keywords: Super-hydrophobicity Wetting hysteresis Evaporation Surface roughness ABSTRACT Small water drops demonstrate different evaporation modes on super-hydrophobic polymer surfaces with different hysteresis of contact angle. While on the high-hysteresis surface evaporation follows the constant-contact-diameter mode, the constant-contact-angle mode dominates on the low-hysteresis surface. These modes were previously reported for smooth hydrophilic and hydrophobic surfaces, respectively. The experimental data are compared to the previous models describing spherical cap drops that evaporate in different modes, and good fitting is obtained. Crown Copyright ß 2008 Published by Elsevier B.V. All rights reserved. * Corresponding author. E-mail address: skulinic@uqac.ca (S.A. Kulinich). Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc 0169-4332/$ – see front matter . Crown Copyright ß 2008 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.10.109