Vapor condensation is a crucial phenomenon governing the efficiency of many processes. In particular, dropwise condensation on hydrophobic thin films (≈100 nm-thick) has the potential to achieve remarkable heat transfer. However, the lack of durability of these thin films has limited applications for a century. Although degradation due to steam condensation has been described as "blistering," no satisfactory insight exists capable of elucidating the driving force for film delamination. Here, it is shown that nanoscale pinholes in hydrophobic films are the source of blister formation. By creating artificial pinholes via nanoindentation on thin (30 to 500 nm-thick) fluorinated hydrophobic films, it is demostrated that water blisters can be initiated at the pinholes during condensation. It is experimentally demonstrated that vapor is transferred to the blister through the nanoscale pinhole, and the driving force for delamination is capillary pressure generated at the pinhole by the pinned liquid-vapor interface. The techniques and insights presented here will inform future work on polymeric thin film and enable their durable design for a variety of applications.

Dropwise condensation (DWC) on non-wetting surfaces has remarkable potential to enhance heat transfer performance compared to filmwise condensation on wetting substrates. In this article, we discuss important recent developments and challenges in the field of DWC, including durability of DWC-promoting coatings, DWC of low surface tension fluids, physical mechanisms governing DWC, unconventional methods to achieve DWC, and promising metrology techniques for DWC. We end the article by providing a road map detailing where we believe the community should direct both fundamental and applied efforts in order to solve the identified century-old challenges that limit DWC implementation.

Durable hydrophobic materials have attracted considerable interest in the last century. Currently, the most popular strategy to achieve hydrophobic coating durability is through the combination of a perfluoro-compound with a mechanically robust matrix to form a composite for coating protection. The matrix structure is typically large (thicker than 10 μm), difficult to scale to arbitrary materials, and incompatible with applications requiring nanoscale thickness such as heat transfer, water harvesting, and desalination. Here, we demonstrate durable hydrophobicity and superhydrophobicity with nanoscale-thick, perfluorinated compound-free polydimethylsiloxane vitrimers that are self-healing due to the exchange of network strands. The polydimethylsiloxane vitrimer thin film maintains excellent hydrophobicity and optical transparency after scratching, cutting, and indenting. We show that the polydimethylsiloxane vitrimer thin film can be deposited through scalable dip-coating on a variety of substrates. In contrast to previous work achieving thick durable hydrophobic coatings by passively stacking protective structures, this work presents a pathway to achieving ultra-thin (thinner than 100 nm) durable hydrophobic films.

Polymeric coatings having micro-to-nanoscale thickness show immense promise for enhancing thermal transport, catalysis, energy conversion, and water collection. Characterizing the work of adhesion (G) between these coatings and their substrates is key to understanding transport physics as well as mechanical reliability. Here, we demonstrate that water vapor condensation blistering is capable of in situ measurement of work of adhesion at the interface of polymer thin films with micrometer spatial resolution. We use our method to characterize adhesion of interfaces with controlled chemistry such as fluorocarbon/fluorocarbon (CF n /CF m , n, m = 0–3), fluorocarbon/hydrocarbon (CF n /CH m ), fluorocarbon/silica (CF n /SiO2), and hydrocarbon/silica (CH n /SiO2) interfaces showing excellent agreement with adhesion energy measured by the contact angle approach. We demonstrate the capability of our condensation blister test to achieve measurement spatial resolutions as low as 10 μm with uncertainties of ∼10%. The outcomes of this work establish a simple tool to study interfacial adhesion.

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