Water droplet evaporation on Cu-based hydrophobic surfaces with nano- and micro-structures Chi Young Lee a,b,⇑ , Bong June Zhang b , Jiyeon Park b , Kwang J. Kim b a KAERI (Korea Atomic Energy Research Institute), 989-111 Daedeok-daero, Yuseong-gu, Daejeon 305-353, Republic of Korea b Low Carbon Green Technology Laboratory, Department of Mechanical Engineering, University of Nevada-Reno, Reno, NV 89557, USA article info Article history: Received 28 January 2011 Received in revised form 30 November 2011 Accepted 30 November 2011 Available online 11 January 2012 Keywords: Droplet evaporation Surface structure (Super)hydrophobic surface abstract The characteristics of water droplet evaporation on three different hydrophobic surfaces, PCu (Plain Cop- per, h = 115°), MSCu (Micro-Structured Copper, h = 126°) and NSCuO (Nano-Structured Copper Oxide, h = 159°) with coating of the same SAM (Self-Assembled Monolayer) material, were experimentally inves- tigated. For industrial heat transfer applications, copper material was used as the substrate, and the simple and cost-effective fabrication technique to prepare the superhydrophobic surface, NSCuO, was introduced. Based on the observations, the behavior of droplet evaporation was divided into three stages: Stage I (constant contact area stage), Stage II (constant contact angle stage) and Stage III (mixed stage). When studying the PCu surface, the Stages I, II, and III were observed, consistent with previous reports. For the MSCu surface, Stages I and III appeared without Stage II, and the pinning period of contact line was the longest among the test samples due to the formation of Wenzel state droplet. In the case of the superhy- drophobic NSCuO surface, only Stage III occurred, and the contact line moved freely during the entire evaporation time because of the formation of Cassie state droplet. The total evaporation time of the NSCuO was the longest out of all the samples tested. At the last stage of evaporation, the edge of the droplet shrank at a much faster rate in all surfaces. On the other hand, the shrinking velocity of the droplet height drasti- cally increased only on the NSCuO, which was considered as the unique behavior of superhydrophobic surface. In this experiment, it was found that the surface structure determines the motion of the contact line on the surface, which, in turn, strongly influences the characteristics of the droplet evaporation. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction The natural evaporation of a droplet on various surfaces is a fundamental problem and has been studied intensively for a wide range of industrial and biological applications; e.g., inkjet printing, spray painting, DNA chip fabrication, and cell patterning. In gen- eral, the surface wettability is divided into two groups: the hydro- philic surface which is the surface with a water contact angle below 90°, and the hydrophobic surface when the contact angle is above 90°. The characteristics of droplet evaporation on a hydrophobic surface are more complicated than those on a hydrophilic surface. Picknett and Bexon [1] reported that droplet evaporation on the hydrophobic surface occurs in three distinct stages as shown in Fig. 1. The first stage is the ‘‘constant contact area stage,’’ also known as the pinned contact line stage (Stage I). As the droplet evaporates, the contact angle decreases, while the contact area radius remains constant. The second stage is the ‘‘constant contact angle stage’’ also known as the moving contact line stage. As op- posed to Stage I, the contact area radius recedes in Stage II and the contact angle remains constant. The third stage is the ‘‘mixed stage’’ (Stage III), an unpinned contact line stage where both the contact angle and contact area radius decrease. Birdi and Vu [2] and Birdi et al. [3] investigated the evaporation behavior of water and n-octane droplets on smooth glass and Teflon surfaces. Both studies reported that the evaporation of a water droplet on a glass surface and n-octane on a Teflon surface were stationary processes each with a constant contact radius and linear evaporation rates. However, when the contact angle was over 90°, a constant contact angle with a decreasing contact radius and a non-linear evapora- tion rate was observed. None of these studies [1–3] considered the effect of the surface morphology on the behavior of droplet evaporation, and mentioned the behavior of droplet evaporation on a superhydrophobic surface. Recently, studies on superhydrophobic surfaces, which exhibit a water contact angle larger than 150° [4], have been extensively performed. A superhydrophobic surface is notable as it has the fea- tures of water repellency and low surface energy. These two attri- butes have vast potential in various industrial applications such as anti-sticking, self-cleaning, anti-fouling, anti-corrosion, friction 0017-9310/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2011.12.019 ⇑ Corresponding author at: KAERI (Korea Atomic Energy Research Institute), 989- 111 Daedeok-daero, Yuseong-gu, Daejeon 305-353, Republic of Korea. Tel.: +82 42 868 4587; fax: +82 42 8630565. E-mail address: chiyounglee@kaeri.re.kr (C.Y. Lee). International Journal of Heat and Mass Transfer 55 (2012) 2151–2159 Contents lists available at SciVerse ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt