Evaporation Kinetics of Sessile Water Droplets on Micropillared Superhydrophobic Surfaces Wei Xu, † Rajesh Leeladhar, † Yong Tae Kang, ‡ and Chang-Hwan Choi* ,†,‡ † Department of Mechanical Engineering, Stevens Institute of Technology, Hoboken, New Jersey 07030, United States ‡ Department of Mechanical Engineering, Kyung Hee University, Yong In, Gyeong-Gi, 446-701, Korea ABSTRACT: Evaporation modes and kinetics of sessile droplets of water on micropillared superhydrophobic surfaces are experimentally investigated. The results show that a constant contact radius (CCR) mode and a constant contact angle (CCA) mode are two dominating evaporation modes during droplet evaporation on the superhydrophobic surfaces. With the decrease in the solid fraction of the superhydrophobic surfaces, the duration of a CCR mode is reduced and that of a CCA mode is increased. Compared to Rowan’s kinetic model, which is based on the vapor diffusion across the droplet boundary, the change in a contact angle in a CCR (pinned) mode shows a remarkable deviation, decreasing at a slower rate on the superhydrophobic surfaces with less-solid fractions. In a CCA (receding) mode, the change in a contact radius agrees well with the theoretical expectation, and the receding speed is slower on the superhydrophobic surfaces with lower solid fractions. The discrepancy between experimental results and Rowan’s model is attributed to the initial large contact angle of a droplet on superhydrophobic surfaces. The droplet geometry with a large contact angle results in a narrow wedge region of air along the contact boundary, where the liquid-vapor diffusion is significantly restricted. Such an effect becomes minor as the evaporation proceeds with the decrease in a contact angle. In both the CCR and CCA modes, the evaporative mass transfer shows the linear relationship between mass 2/3 and evaporation time. However, the evaporation rate is slower on the superhydrophobic surfaces, which is more significant on the surfaces with lower solid fractions. As a result, the superhydrophobic surfaces slow down the drying process of a sessile droplet on them. ■ INTRODUCTION Superhydrophobic surfaces 1-4 have attracted great interest because of their extreme water-repellent surface property for many potential applications including self-cleaning, 5-7 hydro- dynamic friction reduction, 8-10 anti-icing, 11-13 anticorro- sion, 14-16 biotechnology, 17-19 thermal systems, 20-24 and micro- and nanodevices. 25-27 In many applications as such, they mostly deal with droplets that are open to the atmosphere and evaporative. Thus, the study of the evaporation kinetics and wetting behaviors of liquid droplets on superhydrophobic surfaces is critical to the design and applications of such surfaces for proper operation. To date, the droplet evaporation kinetics on hydrophilic or hydrophobic surfaces has been studied experimentally in many works, and theoretical kinetics models have also been developed. 28-41 For example, Rowan et al. developed an evaporation kinetic model for a sessile droplet placed on a substrate based on the diffusion of the vapor across the boundary using Fick’s law. 30 Deegan et al. considered that the vapor diffusing in the evaporation should quickly approach a steady-state concentration profile, which would obey the steady-state diffusion equation. 31 The vapor concentration distribution above an evaporating droplet, which would be mathematically equivalent to that of a charged conductor, was examined to predict the droplet characteristics, such as the contact angle, contact radius, and evaporation rate. 31,33,38 Recently, Nguyen et al. also developed a model for the evaporation kinetics in constant contact radius and constant contact angle modes. 41 Although such models have been extensively studied on hydrophilic and hydrophobic surfaces with experimental verifications, the study of superhydrophobic surfaces and comparison with the theoretical models has still been limited, 19,21,42-49 demanding more extensive and system- atic studies for a clearer understanding. It has typically been observed that three distinct evaporation modes appear sequentially during sessile droplet evaporation on a superhydrophobic surface. As illustrated in Figure 1, they include a constant contact radius (CCR) mode (or a pinning mode) with a gradual decrease of the contact angle (Figure 1a), a constant contact angle (CCA) mode (or a receding mode) with a gradual decrease in the contact radius (Figure 1b), and a mixed mode with simultaneous decreases in both the contact angle and the contact radius (Figure 1c). When the droplet shrinks significantly during evaporation and the internal Laplace pressure of the droplet increases more than the critical capillary pressure in sustaining the liquid-gas meniscus, a wetting transition from a dewetting (Cassie-Baxter) state 50 to a Received: February 4, 2013 Revised: March 25, 2013 Article pubs.acs.org/Langmuir © XXXX American Chemical Society A dx.doi.org/10.1021/la400452e | Langmuir XXXX, XXX, XXX-XXX