Full-Field Dynamic Characterization of Superhydrophobic Condensation on Biotemplated Nanostructured Surfaces Emre O ̈ lç eroğ lu, † Chia-Yun Hsieh, ‡ Md Mahamudur Rahman, † Kenneth K. S. Lau, ‡ and Matthew McCarthy* ,† † Department of Mechanical Engineering and Mechanics, and ‡ Department of Chemical and Biological Engineering, Drexel University, 3141 Chestnut Street, Philadelphia, Pennsylvania 19104, United States * S Supporting Information ABSTRACT: While superhydrophobic nanostructured surfaces have been shown to promote condensation heat transfer, the successful implementation of these coatings relies on the development of scalable manufacturing strategies as well as continued research into the fundamental physical mechanisms of enhancement. This work demonstrates the fabrication and characterization of superhydrophobic coatings using a simple scalable nanofabrication technique based on self-assembly of the Tobacco mosaic virus (TMV) combined with initiated chemical vapor deposition. TMV biotemplating is compatible with a wide range of surface materials and applicable over large areas and complex geometries without the use of any power or heat. The virus- structured coatings fabricated here are macroscopically superhydrophobic (contact angle >170°) and have been characterized using environmental electron scanning microscopy showing sustained and robust coalescence-induced ejection of condensate droplets. Additionally, full-field dynamic characterization of these surfaces during condensation in the presence of noncondensable gases is reported. This technique uses optical microscopy combined with image processing algorithms to track the wetting and growth dynamics of 100s to 1000s of microscale condensate droplets simultaneously. Using this approach, over 3 million independent measurements of droplet size have been used to characterize global heat transfer performance as a function of nucleation site density, coalescence length, and the apparent wetted surface area during dynamic loading. Additionally, the history and behavior of individual nucleation sites, including coalescence events, has been characterized. This work elucidates the nature of superhydrophobic condensation and its enhancement, including the role of nucleation site density during transient operation. ■ INTRODUCTION Condensation heat transfer is found in a wide range of real- world applications and industries including power generation, thermal management, chemical processing, water purification, and HVAC. Additionally, condensation plays a critical role in the performance of new applications and materials such as biomimetic surfaces for self-cleaning, antifouling, and water harvesting. It has been shown that coatings composed of high- surface-area micro/nanostructures can be used to substantially enhance condensation, 1,2 as well as a variety of other phase- change heat transfer processes including boiling, evaporation, and freezing. 3-6 Superhydrophobic nanostructured coatings drastically reduce surface wettability and demonstrate extreme water repellency, where near-spherical droplets rest on top of the surface structures with contact angles approaching 180°. They have been fabricated using a wide array of techniques including direct etching, 5,7,8 oxidation and growth of nano- structures, 1,9,10 molding, 11 biotemplating, 12 and electro- deposition. 13 Such surfaces have received attention for applications in self-cleaning, 12 reducing heat transfer during freezing to create anti-icing coatings, 5,14 as well as increasing heat transfer efficiency during condensation. 1,15 Dropwise condensation onto hydrophobic surfaces (where condensate forms into millimeter-scale droplets) is much more efficient than filmwise condensation (where condensate forms into liquid films), due to the shedding of condensate by gravity. 16,17 “Jumping-mode” superhydrophobic condensation has been demonstrated more recently, where microscale droplets undergo coalescence-induced ejection. 2 When one or more near-spherical droplets condense onto a superhydro- phobic surface and coalesce with each other, the excess energy due to decreased surface area is converted into kinetic energy, leading to droplet ejection. This mechanism delays the formation of an insulating liquid layer and shows great promise for increasing efficiencies in condensation heat transfer systems. Boreyko and Chen demonstrated self-ejecting microscale droplets using two-tiered hierarchical structures composed of carbon nanotubes and etched silicon pillars. 2 This observed phenomenon has led to extensive research into in situ imaging of nanoscale condensate droplets using environmental electron scanning microscopy (ESEM), 18-21 as well as various Received: March 20, 2014 Revised: May 28, 2014 Published: May 31, 2014 Article pubs.acs.org/Langmuir © 2014 American Chemical Society 7556 dx.doi.org/10.1021/la501063j | Langmuir 2014, 30, 7556-7566