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On the fundamental role of substrate compliance and enhanced light absorption on droplet condensation and evaporation

English title On the fundamental role of substrate compliance and enhanced light absorption on droplet condensation and evaporation
Applicant Poulikakos Dimos
Number 179062
Funding scheme Project funding (Div. I-III)
Research institution Institut für Energietechnik ETH Zürich
Institution of higher education ETH Zurich - ETHZ
Main discipline Mechanical Engineering
Start/End 01.10.2018 - 30.09.2022
Approved amount 825'000.00
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Keywords (4)

Phase change heat transfer; Wettability; Surface engineering; Thermofluidics

Lay Summary (German)

Lead
Ziel dieses Projekts ist zu verstehen, wie eine Variation in der Elastizität und Heizen der Oberfläche durch verstärkte Sonnenlicht Absorption die individuelle und kollektive Tropfen-Substrat Interaktion in Kondensations- und Evaporationsprozessen beeinflussen. Wir zielen darauf ab, mithilfe dieser neuen Erkenntnisse, Materialien und Prozesse in den Bereichen der Wasser Sammlung und der Anti-Beschlag Technologien zu entwerfen.
Lay summary

Wir beabsichtigen experimentelle und theoretische Fragestellungen zur Evaporation, Kondensation und Adhäsion von Tropfen auf Oberflächen zu studieren. Fokus liegt dabei auf den, noch nicht zulänglich verstandenen, Einflüssen der Materialelastizität und gesteigerter Sonnenlichtabsorption. Dieses Problem ist relevant für viele wissenschaftliche und praktische Anwendungen in der Natur und der Technik, so etwa in den Bereichen Energie, Fertigung, dreidimensionales Drucken, Sammlung und Entsalzung von Wasser. Ziel dieses Projekts ist zu verstehen, wie eine Variation in der Elastizität und Heizen der Oberfläche durch verstärkte Sonnenlicht Absorption die individuelle und kollektive Tropfen-Substrat Interaktion in Kondensations- und Evaporationsprozessen beeinflussen. Wir zielen darauf ab, mithilfe dieser neuen Erkenntnisse, Materialien und Prozesse in den Bereichen der Wasser Sammlung und der Anti-Beschlag Technologien zu entwerfen. Die systematische Kopplung von physikalischem Wissen und wissenschaftsbasierten Oberflächen/Material Design wird daher durchgeführt, um rational entworfene Materialien zu etablieren, welche fähig sind weit über den bisherigen Stand der Technik hinauszugehen.

Direct link to Lay Summary Last update: 24.04.2018

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Abstract

We propose to investigate open fundamental questions related to evaporation, condensation, and adhesion of droplets on surfaces focusing on the important, but not well understood, effects of compliance and plasmonically enhanced light absorption and heating, through experiments and theoretical considerations. In addition to its fundamental nature, the problem is relevant to many scientific and practical applications in nature and technology ranging from energy to manufacturing, printing, and water collection, including desalination. During the past years, research has shown how by using the appropriate surface topography and chemical composition, interfacial transport phenomena can be beneficially altered by passive means, in order to advance liquid repellency, anti-icing, and phase change heat transfer technologies. In order to do this, generally, the surface should consist of a repeat array of surface texture and chemical features with the characteristic length scale(s) down to the order of the element to be influenced, e.g., nucleation, wetting, etc. While there has been significant progress in surface engineering according to the above design rules to control droplet-substrate interactions, the less obvious, but very important in many applications impact of compliance and natural heating through enhanced light absorption (in particular of sunlight), is still not well understood. Therefore, in this proposal, through theoretical and experimental considerations, we aim at investigating how the presence and variations in compliance, and heating through plasmonically enhanced sunlight absorption, impact individual and ensemble droplet-substrate interactions in condensation and evaporation situations. We also target designing materials and processes with this new understanding, using as application examples improved water collection and anti-fogging technologies. The proposed project consists of two parts:1.Condensation and evaporation on materials with compliance: We plan to investigate single and multiple droplets (range of sizes) interacting with substrates with a range of homogeneous and heterogeneous compliance under static, growth, and shrinking conditions. We will determine the size and distribution of heterogeneous features by relevant elasto-capillary and thermofluidic theories. The environmental conditions will be controlled in order to tune the condensation/evaporation growth rates of the droplets. We will vary these rates such that the contact line motion ranges from much slower to much faster than the elastic relaxation time of the substrate and the mass, viscosity, and surface tension of the droplets will be varied in order to assess the effects of inertia, viscosity, and surface tension on the behavior. We also plan to investigate the elastic behavior of substrates. We will use high-speed and high-resolution optical and infrared imaging, combined with confocal traction force microscopy to assess how viscoelastic substrate effects impact the wetting, condensation, and evaporation behavior of individual and multiple droplets. We will design materials with particular emphasis on the spatial control of their compliance with micrometric-to-macroscopic resolution, coupled with appropriate surface chemistry. 2.Condensation and evaporation on materials with engineered enhanced light absorption heating: For operating conditions with water vapor in the subcooled (metastable) temperature range for prolonged times (of common occurrence in nature and technology), liquid water nucleation is inevitable. This is due to the undercooling of the substrate with respect to the gaseous environment. However, slight changes in surface temperature, such as those due to solar radiation, can have a major impact on nucleation dynamics. Understanding the interaction of such phenomena, inherent in particular to materials with color (light absorbing), with the surface texture at the abovementioned environmental conditions, can play a significant role toward designing surfaces with desired properties with respect to wetting and fogging. To this end we plan to take advantage of plasmonic absorption resonances of nanostructures (metal nanoparticles) and subsequent losses into heat to attain broadband absorption, and engineer respective nanocomposites. A major benefit of this approach is the controllable transparency property of such films, long-term persistence of absorption property (compared to conventional dyes), chemical inertness, and thinness of the film. We will perform a series of experiments to study the physics of condensation nucleation and evaporation on the formation and retention of water droplets on such surfaces, under controlled temperature, humidity, and lighting conditions. Specifically, we aim to exploit the plasmonic light absorbing property of the substrate to induce rapid and significant heating-while maintaining partial transparency-to repel droplets from the substrate. Characterization of this process will be achieved with an environmental chamber, equipped with optical and infrared access to facilitate high-speed imaging of the rapid dynamics.With the above is well-understood and using the developed facilities, we plan to investigate the simultaneous effects of light absorption and compliance on phase change, synthesizing the knowledge gained, to harvest the best of both properties addressing important applications like thermoregulation (textiles) and unforeseen ones.In closing, the goal of this proposal is to advance the fundamental understanding of surface wetting and phase change with respect to compliance and plasmonically enhanced light absorption, as it pertains to markedly improving performance of material systems used in applications exemplified by anti-fogging, water collection, condensation and evaporative cooling. Systematic coupling of the inherent physics at these environmental conditions and science-based surface/material design will be performed hand-in-hand in order to establish well-understood and rationally engineered materials, capable of advancing the current state of the art.
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