Multimode Multidrop Serial Coalescence Effects during Condensation on Hierarchical Superhydrophobic Surfaces Konrad Rykaczewski,* ,† Adam T. Paxson, † Sushant Anand, † Xuemei Chen, ‡ Zuankai Wang, ‡ and Kripa K. Varanasi* ,† † Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States ‡ Department of Mechanical and Biomedical Engineering, City University of Hong Kong, Hong Kong, China * S Supporting Information ABSTRACT: The prospect of enhancing the condensation rate by decreasing the maximum drop departure diameter significantly below the capillary length through spontaneous drop motion has generated significant interest in condensation on superhydrophobic surfaces (SHS). The mobile coalescence leading to spontaneous drop motion was initially reported to occur only on hierarchical SHS, consisting of both nanoscale and microscale topological features. However, subsequent studies have shown that mobile coalescence also occurs on solely nanostructured SHS. Thus, recent focus has been on understanding the condensation process on nanostructured surfaces rather than on hierarchical SHS. In this work, we investigate the impact of microscale topography of hierarchical SHS on the droplet coalescence dynamics and wetting states during the condensation process. We show that isolated mobile and immobile coalescence between two drops, almost exclusively focused on in previous studies, are rare. We identify several new droplet shedding modes, which are aided by tangential propulsion of mobile drops. These droplet shedding modes comprise of multiple droplets merging during serial coalescence events, which culminate in formation of a drop that either departs or remains anchored to the surface. We directly relate postmerging drop adhesion to formation of drops in nanoscale as well as microscale Wenzel and Cassie-Baxter wetting states. We identify the optimal microscale feature spacing of the hierarchical SHS, which promotes departure of the highest number of microdroplets. This optimal surface architecture consists of microscale features spaced close enough to enable transition of larger droplets into micro-Cassie state yet, at the same time, provides sufficient spacing in-between the features for occurrence of mobile coalescence. ■ INTRODUCTION Development of passive methods to enhance water con- densation rate could dramatically improve the energy efficiency of power generation, 1-3 air conditioning systems, 4 water desalination, 5,6 and water harvesting. 7-12 The rate of this phase change process is largely limited by how quickly condensate departs the surface. 13-19 For example, the steady state condensation rate on hydrophobic surfaces, which promote formation of easily shedding droplets, is significantly higher than that on hydrophilic surfaces, which favor water film formation (i.e., dropwise vs filmwise condensation mode). On vertically inclined flat hydrophobic surfaces, hemispherical drops grow via coalescence until reaching a diameter equal to the water capillary length of about 2.7 mm. 13,16 Subsequently, the drops slide off the surface due to gravity. Recently, Chen et al. 20 and Dorrer and Ruhe 21 observed that during condensation on properly designed superhydrophobic surfaces (SHS) 22 drops with sizes significantly below the capillary length can depart the surface via spontaneous droplet motion. 2,20-30 The prospect of enhancing the condensation rate by decreasing the maximum departure diameter, which could double the heat transfer rate, 2,13,16,31,32 has generated much interest and research effort in condensation on SHS. Boreyko and Chen 23 initially reported that hierarchical SHS, which consist of both nanoscale and microscale topological features, are necessary to promote spontaneous drop motion. 23 More recently this mode of condensation has also been observed on solely nanostructured surfaces. 2,21,22,25,28,29,33,34 Rykaczewski et al. 22,35 found that the primary role of the nanostructure is confinement of the base area of nucleating droplets, which leads to formation of nearly spherical microdroplets through contact angle increase. In a subsequent work, Enright et al. 34 reached a similar conclusion by studying the effect of the nanoscale SHS architecture on later stages of microdroplet growth. Similarly, Liu et al. 36 found in their theoretical study that the nanostructure is crucial for formation of high contact angle microdroplets. Rykaczewski 35 demon- strated that the forming droplets grow in purely constant base area mode until reaching a contact angle of 130° to 150°. The Received: October 27, 2012 Revised: December 13, 2012 Published: December 21, 2012 Article pubs.acs.org/Langmuir © 2012 American Chemical Society 881 dx.doi.org/10.1021/la304264g | Langmuir 2013, 29, 881-891