Thin Film Condensation on Nanostructured Surfaces

Abstract

Water vapor condensation is a ubiquitous process in nature and industry. Over the past century, methods achieving dropwise condensation using a thin (<1 µm) hydrophobic 'promoter' layer have been developed, which increases the condensation heat transfer by 10 times compared to filmwise condensation. Unfortunately, implementations of dropwise condensation have been limited due to poor durability of the promoter coatings. Here, we develop thin film condensation which utilizes a promoter layer not as a condensation surface, but rather to confine the condensate within a porous biphilic nanostructure, nickel inverse opals (NIO) with a thin (<20 nm) hydrophobic top-layer of decomposed polyimide. We demonstrate filmwise condensation confined to thicknesses <10 µm. To test the stability of thin film condensation, we performed condensation experiments to show that at higher supersaturations (0.975 < S < 2.05), droplets coalescing on top of the hydrophobic layer are absorbed into the superhydrophilic layer through coalescence induced transitions. Through detailed This article is protected by copyright. All rights reserved.3 thermal-hydrodynamic modeling, we show that thin film condensation has the potential to achieve heat transfer coefficients approaching ≈100 kW m -2 while avoiding durability issues by significantly reducing nucleation on the hydrophobic surface. The work presented here develops an approach to potentially ensure durable and high performance condensation comparable to dropwise condensation.

How Surface Orientation Affects Jumping-Droplet Condensation

“…However, it is now well known that the impaled Wenzel state is almost always irreversible, [45][46][47] as the energy barrier for pinch-off is lower than that for dewetting. 48 A final scenario of coalescence actually increasing the extent of droplet impalement was ruled out from energetic 45 and physical criteria 3 shown in Section 2 of the Supplemental Information.…”

“…Vapor condenses on a solid substrate either as a liquid film or in the form of discrete droplets, depending upon the surface energy of the substrate. [1][2][3] Dropwise condensation is nearly an order of magnitude more efficient than filmwise condensation [4][5][6] because of the considerable thermal resistance of a continuous liquid film. The heat transfer coefficient for dropwise condensation increases monotonically as the critical size of departing droplets decreases, as this maximizes the dry surface area exposed to steam.…”

Condensation Induced Delamination of Nanoscale Hydrophobic Films

“…However, the long‐term (>5 years) durability of thin (≈100 nm‐thick) hydrophobic surfaces has not been demonstrated. In fact, the main hindrance to their utilization over the past century has been the lack of durability, with the main degradation mechanism during steam condensation being qualitatively described as blister formation . The previously observed blistering was characterized by liquid water forming beneath the hydrophobic polymer film, the gradual growth of blisters over time, and eventually the complete removal of the film.…”

Development of automated angle-scanning, high-speed surface plasmon resonance imaging and SPRi visualization for the study of dropwise condensation

“…Dropwise condensation occurs on non-wetting surfaces or partially wetting surfaces (Nouri et al 2012). After nine decades of research on dropwise condensation (Cho et al 2017;Emonsi 1939;Enright et al 2014;Miljkovic and Wang 2013;Schmidt et al 1930), design and fabrication of a surface that can sustain this mode of condensation for a long term have not been successful (Ahlers et al 2018;Ma et al 2019;Oh et al 2018). The main reason for this is that there is no united understanding of dropwise condensation phenomena because of the complexity of the physics involved.…”