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Journal Article

Condensation on Slippery Asymmetric Bumps

TL;DR: A conceptually different design approach is presented—based on principles derived from Namib desert beetles, cacti, and pitcher plants—that synergistically combines these aspects of condensation and substantially outperforms other synthetic surfaces.
Abstract: Controlling dropwise condensation is fundamental to water-harvesting systems, desalination, thermal power generation, air conditioning, distillation towers, and numerous other applications. For any of these, it is essential to design surfaces that enable droplets to grow rapidly and to be shed as quickly as possible. However, approaches based on microscale, nanoscale or molecular-scale textures suffer from intrinsic trade-offs that make it difficult to optimize both growth and transport at once. Here we present a conceptually different design approach—based on principles derived from Namib desert beetles, cacti, and pitcher plants—that synergistically combines these aspects of condensation and substantially outperforms other synthetic surfaces. Inspired by an unconventional interpretation of the role of the beetle’s bumpy surface geometry in promoting condensation, and using theoretical modelling, we show how to maximize vapour diffusion fluxat the apex of convex millimetric bumps by optimizing the radius of curvature and cross-sectional shape. Integrating this apex geometry with a widening slope, analogous to cactus spines, directly couples facilitated droplet growth with fast directional transport, by creating a free-energy profile that drives the droplet down the slope before its growth rate can decrease. This coupling is further enhanced by a slippery, pitcher-plant-inspired nanocoating that facilitates feedback between coalescence-driven growth and capillary-driven motion on the way down. Bumps that are rationally designed to integrate these mechanisms are able to grow and transport large droplets even against gravity and overcome the effect of an unfavourable temperature gradient. We further observe an unprecedented sixfold-higher exponent of growth rate, faster onset, higher steady-state turnover rate, and a greater volume of water collected compared to other surfaces. We envision that this fundamental understanding and rational design strategy can be applied to a wide range of water-harvesting and phase-change heat-transfer applications.

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Citations
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Journal Article
TL;DR: It is revealed that properly designed nanostructures should enable nanometric jumping droplets, which would further enhance jumping-droplet condensers for heat transfer, antifogging, and antifrosting applications.
Abstract: It was recently discovered that condensation growing on a nanostructured superhydrophobic surface can spontaneously jump off the surface, triggered by naturally occurring coalescence events. Many reports have observed that droplets must grow to a size of order 10 μm before jumping is enabled upon coalescence; however, it remains unknown how the critical jumping size relates to the topography of the underlying nanostructure. Here, we characterize the dynamic behavior of condensation growing on six different superhydrophobic nanostructures, where the topography of the nanopillars was systematically varied. The critical jumping diameter was observed to be highly dependent upon the height, diameter, and pitch of the nanopillars: tall and slender nanopillars promoted 2 μm jumping droplets, whereas short and stout nanopillars increased the critical size to over 20 μm. The topology of each surface is successfully correlated to the critical jumping diameter by constructing an energetic model that predicts how large a nucleating embryo needs to grow before it can inflate into the air with an apparent contact angle large enough for jumping. By extending our model to consider any possible surface, it is revealed that properly designed nanostructures should enable nanometric jumping droplets, which would further enhance jumping-droplet condensers for heat transfer, antifogging, and antifrosting applications.

146 citations

Journal ArticleDOI
TL;DR: In this article , an asymmetric amphiphilic surface incorporating self-driven triboelectric adsorption was developed to obtain clean water from the atmosphere, inspired by cactus spines and beetle elytra.
Abstract: Abstract The effective acquisition of clean water from atmospheric water offers a potential sustainable solution for increasing global water and energy shortages. In this study, an asymmetric amphiphilic surface incorporating self-driven triboelectric adsorption was developed to obtain clean water from the atmosphere. Inspired by cactus spines and beetle elytra, the asymmetric amphiphilic surface was constructed by synthesizing amphiphilic cellulose ester coatings followed by coating on laser-engraved spines of fluorinated ethylene propylene. Notably, the spontaneous interfacial triboelectric charge between the droplet and the collector was exploited for electrostatic adsorption. Additionally, the droplet triboelectric nanogenerator converts the mechanical energy generated by droplets falling into electrical energy through the volume effect, achieving an excellent output performance, and further enhancing the electrostatic adsorption by means of external charges, which achieved a water harvesting efficiency of 93.18 kg/m 2 h. This strategy provides insights for the design of water harvesting system.

66 citations

Journal ArticleDOI
TL;DR: In this paper , a lubricant-infused heterogeneous superwettability surface (LIHSS) for directional water self-transport is proposed on polyimide (PI) film through femtosecond laser direct writing and lubricant infusion.
Abstract: Directional water self‐transport plays a crucial role in diverse applications such as biosensing and water harvesting. Despite extensive progress, current strategies for directional water self‐transport are restricted to a short self‐driving distance, single function, and complicated fabrication methods. Here, a lubricant‐infused heterogeneous superwettability surface (LIHSS) for directional water self‐transport is proposed on polyimide (PI) film through femtosecond laser direct writing and lubricant infusion. By tuning the parameters of the femtosecond laser, the wettability of PI film can be transformed into superhydrophobic or superhydrophilic. After trapping water droplets on the superhydrophilic surface and depositing excess lubricant, the asymmetrical wetting ridge drives water droplets by an attractive capillary force on the LIHSS. Notably, the maximum droplet self‐driving distance can approach ≈3 mm, which is nearly twice as long as the previously reported strategies for direction water self‐transport. Significantly, it is demonstrated that this strategy makes it possible to achieve water self‐transport, anti‐gravity pumping, and chemical microreaction on a tilted LIHSS. This work provides an efficient method to fabricate a promising platform for realizing directional water self‐transport.

48 citations

Journal ArticleDOI
TL;DR: The fundamental and specific mechanisms of fog harvesting involving the Namib Desert beetle, spider silk, cactus, and Nepenthes alata are described in detail, and a critical analysis of current challenges and future development is presented.
Abstract: The increasing demand for clean water driven by the rapid growth of the population and the pollution of water necessitates the development of direct and rapid water harvesting methods. Fog harvesting is proven as a highly efficient water capture method in dry but foggy areas. Herein, the state‐of‐the‐art developments on the multiple biomimetic fog harvesting devices (FHDs) are presented. First, the fundamental and specific mechanisms of fog harvesting involving the Namib Desert beetle, spider silk, cactus, and Nepenthes alata are described in detail, respectively. Second, an in‐depth discussion on the mechanisms of fog harvesting including fog capture and water transport is proposed. Third, a detailed account of the innovative design strategies and fabrication technologies of FHDs is proposed, from single to multiple biomimetic FHDs with typical examples and the merits and demerits are compared. Finally, a critical analysis of current challenges and future development is presented, aiming to promote next‐generation FHDs from scientific research to practical application.

46 citations

Journal ArticleDOI
TL;DR: In this article , a laser belt processing method was proposed for the preparation of Inconel 718 surfaces with ultra-high adhesion and anisotropic wettability, and the results showed that the width and depth of the microgroove increased with increasing laser power and processing times, whereas the depth decreased when the processing times was increased by eight times.

43 citations

References
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Journal ArticleDOI
TL;DR: In this article, the authors analyzed available information concerning energy consumption in buildings, and particularly related to HVAC systems, and compared different types of building types and end uses in different countries.

5,288 citations

Journal ArticleDOI
22 Sep 2011-Nature
TL;DR: A strategy to create self-healing, slippery liquid-infused porous surface(s) (SLIPS) with exceptional liquid- and ice-repellency, pressure stability and enhanced optical transparency, applicable to various inexpensive, low-surface-energy structured materials (such as porous Teflon membrane).
Abstract: Inspired by the insect-eating Nepenthes pitcher plant, which snares its prey on a surface lubricated by a remarkably slippery aqueous secretion, Joanna Aizenberg and colleagues have synthesized omniphobic surfaces that can self-repair and function at high pressures. Their 'slippery liquid-infused porous surfaces' (or SLIPS) exhibit almost perfect slipperiness towards polar, organic and complex liquids. SLIPS function under extreme conditions, are easily constructed from inexpensive materials and can be endowed with other useful characteristics, such as enhanced optical transparency, through the selection of appropriate substrates and lubricants. Ultra-slippery surfaces of this type might find application in biomedical fluid handling, fuel transport, antifouling, anti-icing, optical imaging and elsewhere. Creating a robust synthetic surface that repels various liquids would have broad technological implications for areas ranging from biomedical devices and fuel transport to architecture but has proved extremely challenging1. Inspirations from natural nonwetting structures2,3,4,5,6, particularly the leaves of the lotus, have led to the development of liquid-repellent microtextured surfaces that rely on the formation of a stable air–liquid interface7,8,9. Despite over a decade of intense research, these surfaces are, however, still plagued with problems that restrict their practical applications: limited oleophobicity with high contact angle hysteresis9, failure under pressure10,11,12 and upon physical damage1,7,11, inability to self-heal and high production cost1,11. To address these challenges, here we report a strategy to create self-healing, slippery liquid-infused porous surface(s) (SLIPS) with exceptional liquid- and ice-repellency, pressure stability and enhanced optical transparency. Our approach—inspired by Nepenthes pitcher plants13—is conceptually different from the lotus effect, because we use nano/microstructured substrates to lock in place the infused lubricating fluid. We define the requirements for which the lubricant forms a stable, defect-free and inert ‘slippery’ interface. This surface outperforms its natural counterparts2,3,4,5,6 and state-of-the-art synthetic liquid-repellent surfaces8,9,14,15,16 in its capability to repel various simple and complex liquids (water, hydrocarbons, crude oil and blood), maintain low contact angle hysteresis (<2.5°), quickly restore liquid-repellency after physical damage (within 0.1–1 s), resist ice adhesion, and function at high pressures (up to about 680 atm). We show that these properties are insensitive to the precise geometry of the underlying substrate, making our approach applicable to various inexpensive, low-surface-energy structured materials (such as porous Teflon membrane). We envision that these slippery surfaces will be useful in fluid handling and transportation, optical sensing, medicine, and as self-cleaning and anti-fouling materials operating in extreme environments.

3,084 citations

Journal ArticleDOI
TL;DR: In this paper, the basic principles involved in designing hierarchical biological materials, such as cellular and composite architectures, adapative growth and as well as remodeling, are discussed, and examples that are found to utilize these strategies include wood, bone, tendon, and glass sponges.

2,274 citations

Journal ArticleDOI
01 Nov 2001-Nature
TL;DR: It is shown that these large droplets form by virtue of the insect's bumpy surface, which consists of alternating hydrophobic, wax-coated and hydrophilic, non-waxy regions, and may find application in water-trapping tent and building coverings, for example, or in water condensers and engines.
Abstract: This insect has a tailor-made covering for collecting water from early-morning fog. Some beetles in the Namib Desert collect drinking water from fog-laden wind on their backs1. We show here that these large droplets form by virtue of the insect's bumpy surface, which consists of alternating hydrophobic, wax-coated and hydrophilic, non-waxy regions. The design of this fog-collecting structure can be reproduced cheaply on a commercial scale and may find application in water-trapping tent and building coverings, for example, or in water condensers and engines.

1,725 citations

Journal ArticleDOI
04 Feb 2010-Nature
TL;DR: Artificial fibres are designed that mimic the structural features of silk and exhibit its directional water-collecting ability by tapping into both driving forces.
Abstract: Many biological surfaces in both the plant and animal kingdom possess unusual structural features at the micro- and nanometre-scale that control their interaction with water and hence wettability. An intriguing example is provided by desert beetles, which use micrometre-sized patterns of hydrophobic and hydrophilic regions on their backs to capture water from humid air. As anyone who has admired spider webs adorned with dew drops will appreciate, spider silk is also capable of efficiently collecting water from air. Here we show that the water-collecting ability of the capture silk of the cribellate spider Uloborus walckenaerius is the result of a unique fibre structure that forms after wetting, with the 'wet-rebuilt' fibres characterized by periodic spindle-knots made of random nanofibrils and separated by joints made of aligned nanofibrils. These structural features result in a surface energy gradient between the spindle-knots and the joints and also in a difference in Laplace pressure, with both factors acting together to achieve continuous condensation and directional collection of water drops around spindle-knots. Submillimetre-sized liquid drops have been driven by surface energy gradients or a difference in Laplace pressure, but until now neither force on its own has been used to overcome the larger hysteresis effects that make the movement of micrometre-sized drops more difficult. By tapping into both driving forces, spider silk achieves this task. Inspired by this finding, we designed artificial fibres that mimic the structural features of silk and exhibit its directional water-collecting ability.

1,584 citations


Additional excerpts

  • ...In summary, we have achieved unprecedented droplet growth and transport by designing surfaces covered with slippery asymmetric bumps based on quantitative models that integrate three mechanisms— (1) optimization of focused diffusion flux by the radius of curvature and shape of the apex, (2) asymmetric topography that guides the droplet off the bump by optimizing the free-energy profile, and (3) positive feedback between capillary transport and continued growth by coalescence along the slope....

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