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Dimos Poulikakos1

1, Mechanical and Process Engineering, ETH Zurich, Zurich, Zurich, Switzerland

Thermal processes and materials involving phase change, are practically omnipresent in nature and technology. Viewed in the direction of decreasing temperature, the encountered phase transitions are condensation, freezing and de-sublimation. All share significant scientific challenges in terms of material/surface engineering to control nucleation and growth of the generated phase on a surface, and assure its continuous and facile, passive removal from the surface, to maintain surface functionality and robust performance at a high level, depending on the application of interest. Here, I will focus on the rational, physics-derived, surface nanoengineering, for condensation applications, ranging from power generation targeting high efficiency, to sunlight-driven fogging retardation and rapid defogging of transparent materials.

Enhancing the thermal efficiency of a broad range of condenser devices requires means of achieving sustainable dropwise condensation on metallic surfaces, where heat transfer can be further enhanced by facilitating a reduction in the droplet departure diameter. I will present a rationally driven, hierarchical texturing process of metallic surfaces, guided by the collaborative action of wettability and coalescence, which achieves controlled droplet departure, also under challenging vapor flow conditions significantly enhancing heat transport. The textures are attained by both etching processes generating a random but hierarchical re-entrant landscape, as well as by fabricating an array of 3D laser-structured truncated microcones on the surface, covered with papillae-like nanostructures and a hydrolytically stable, low surface energy self-assembled-monolayer coating. Passive droplet departure on this surface is achieved through progressive coalescence of droplets generated and continuously arising from microcavities, resulting in robust depinning and subsequent departure of the condensate, through vapor shear or gravity, Refs. 1,2.

Condensation is also responsible for fogging, a common phenomenon that can have detrimental effects on visibility through otherwise tramnsparent surfaqes. Fogging affects the performance of a wide range of everyday applications including windshields, visors, displays, cameras, and eyeglasses. I will present a novel approach, based on sunlight absorbing metasurfaces, which goes well-beyond state-of-the-art anti-fogging methods such as superhydrophilic coatings. We rationally nanoengineer such transparent metasurfaces, by varying the concentration of embedded plasmonically enhanced light absorbing nanoparticles in an ultra-thin titania film to achieve broadband absorption with tunable transparency. Such surfaces upon illumination induce significant heating at the air-substrate interface where fog is most likely to form and can rapidly de-fog or completely inhibit fog nucleation altogether, Ref. 3. For the same environmental conditions, we demonstrate that such metasurfaces are able to reduce defogging time by up to four-fold compared to reference samples and markedly outperform the most widely implemented solution in anti-fogging, namely, superhydrophilic surfaces. This approach work paves the way for large-scale, low-cost manufacturing that can be applied to a versatile range of materials, including polymers and other flexible substrates, which can further be combined with state-of-the-art technology to overcome remaining impracticalities, safety, and energy-related costs related to fogging.

References:
1. C. S. Sharma, J. Combe, M. Giger, T. Emmerich and D. Poulikakos. ACS Nano, 11 (2): 1673-1682, DOI: 10.1021/acsnano.6b07471
2. C. S. Sharma , C Stamatopoulos, R. Suter, P. Rudolf von Rohr and D. Poulikakos. ACS Appl. Mater. Interfaces, 2018, 10 (34), pp 29127–29135. DOI: 10.1021/acsami.8b09067
3. C. Walker, E. Mitridis, T. Kreiner, H. Eghlidi, T. Schutzius and D. Poulikakos, Transparent Metasurfaces Counteracting Fogging by Harnessing Sunlight, 2018, in review.

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