The present work explores a unique yet unexplored synergy between the properties of laser micropatterned metallic surfaces and the requirements for an autonomous dew water harvesting candidate material. Laser-patterned aluminum surfaces achieved simultaneously high infrared emissivity (up to 0.95 in the atmospheric window) and superhydrophilic wettability (water contact angle of 0°), key properties enabling passive radiative cooling and filmwise condensation dynamics respectively. The generation of micrometric-sized grooves during laser processing plays a fundamental role in both properties, as they provide a broadband enhancement of the emissivity based on multiscale topographies and oxide layers, while limiting the growth of the water film during condensation through strong capillary wicking forces. As a result, the patterned aluminum surfaces display self-cooling capacities under radiative deficit conditions as well as low water retention levels (three times lower than the untreated dropwise condensation counterparts). The promising results obtained lead to the construction and evaluation of a real size outdoors autonomous dew water harvesting system based on those surfaces, demonstrating the scalability of the technology. A 70% improvement in the collected dew water in comparison to a state-of-the-art reference material is consistently measured during 1-year outdoor study, proving the robustness of the surfaces and their performance.

Water condensation on a surface strongly affects its effective emissivity, especially in the atmospheric window, a wavelength range essential for outdoor applications related to energetically passive cooling and heating. The evolution of emissivity of a silicon surface during dropwise and filmwise water deposition is studied. The evolution of the spectral radiative properties shows that the increase in effective emissivity due to the growth of a droplet pattern is steeper than for a growing water film of equivalent thickness. The change of surface emissivity takes place in the first moments of condensation where droplets as small as 10 mu m drastically impact the reflectance of the pristine surface. The upper limit of effective emissivity is reached for a droplet radius or film thickness of 50 mu m. During dropwise condensation, effective emissivity is weighted by the drop surface coverage and then remains within an asymptotic maximum value of 0.8, while in case of filmwise condensation, it is shown to reach 0.9 corresponding to water emissivity. Micrometer-scale spatially-resolved infrared spectral images enable to correlate the spatial variation of spectral properties to the droplet size and localization. Such findings are of interest to the implementation of moisture-controlled emissivity tuning and radiative sky-coolers for dew harvesting. Dynamics of the emissivity of a surface during a water condensation time sequence is studied through spectrally and spatially-resolved emissivity measurements, supported by numerical simulations and calculations. Both water dropwise and filmwise deposition are investigated. image

We report experiments on the transient motion of an oxygen bubble rising in liquid oxygen in a fast varying magneto-gravitational acceleration. In the set-up, a strong magnetic field gradient is applied, which creates in bulk oxygen a body force opposed to its weight. When this gravity compensation is suddenly suppressed, the resulting apparent gravity increases within a fraction of a second from 0 g0 to 0.4 g0 (where g0 is the terrestrial gravitational acceleration). Oxygen bubbles of diameters ranging from 6 mm to 18 mm rise rapidly in the liquid by buoyancy, for values of Galilei and Bond numbers rarely reached with common fluids in terrestrial gravity. The shape of the bubble interface is initially ellipsoidal and evolves toward more complicated shapes during the motion. The time-dependent rising velocity of the bubble is measured during the fast variation of gravity. A comparison of the transient bubble dynamics with classical results obtained in a constant gravity environment enables the time scale of the gravity variation to be precisely quantified.

This paper reveals a phenomenon of memory in dropwise condensation in open air. After a first condensation process and complete evaporation of the condensed droplets, further condensations proceed with droplets nucleating at the very places where former droplets evaporated. The origin of this phenomenon is due to the incorporation of airborne salts during the first droplet condensation and its further concentration during droplet evaporation. Salts act as preferential nucleation sites and humidity sinks. The potential impact of this phenomenon on controlled breath figure patterns and plant metabolism is discussed.

Low-tech, radiative and recyclable plastic foil is considered for the passive collection of dew water. Fashioning micro-grooves at its surface by hot embossing improves by a large factor the collection of even tiny water drops. We show that this improvement is conserved under harsh outdoor conditions. The ability of the micro-grooved foil to collect condensed water is measured by the latency time tl to collect the first drop at the bottom of a vertical substrate. Compared to the same, smooth foil, tl is smaller for the micro-grooved foil by an order of magnitude. Under severe outdoor conditions (dust storm, heat wave) the properties of the micro-grooved foil are preserved after 6 months whereas the collection properties of the smooth foil have decreased by 50%. The basic reason is the insensitivity of the water collection mode to the evolution of the surface wetting and roughness properties. Such low cost, scalable and robust material should find applications in passive water harvesting devices to supply supplementary fresh water in regions where water is, or is becoming, a scarce resource.

A cross-linked silicone elastomer swollen in silicone oil releases at its surface a thin oil layer, whose thickness slowly increases with time as a consequence of surface energy minimization and residual crosslinking reactions within the polymer matrix. Here, we tune and characterize this oil layer thickness (between 0 and 5 μm) in order to show its quantitative influence on the physical mechanisms at play during water condensation: droplet nucleation and growth, coarsening by menisci-mediated coalescences, and droplet shedding by gravity. We show that continuous nucleation is at the origin of enhanced condensation. Spontaneous replenishment of the oil layer occurs thanks to the storage of oil in the bulk of the swollen elastomer, allowing the same sample to be used for repeated long-lasting condensation experiments.

Dew water condensation is due to the radiative cooling of a surface and is hence promoted on surfaces with high emissivity in the IR spectrum. The search for surfaces with enhanced radiative properties is usually carried out without considering the contribution of water condensation. On a condensing substrate, however, water droplets can occupy more than 55 % of the surface. Given the high emissivity of water (similar to 0.98), condensed water is thus expected to modify the condensation efficiency. The present study aims to investigate the interplay between water condensation and mean surface emissivity. Radiative cooling experiments in a humid air environment are performed using two substrates of contrasted emissivities (0.88 and 0.05). Although the condensation yield is initially larger on the high emissivity substrate, after a transient the condensation rate appears essentially the same for both substrates. A simple geometrical model taking into account the surface wetting properties reveals, in addition to the expected effect of droplet surface coverage, the existence of a threshold in drop thickness where water droplets effectively contribute to the radiative cooling process. The results are of conceptual and practical importance and can be generalized to any process involving vapor condensation induced by radiative cooling. (C) 2021 Elsevier Ltd. All rights reserved.

Dew water, mostly ignored until now, can provide clean freshwater resources, just by extracting the atmospheric vapor available in surrounding air. Inspired by silicon-based solar panels, the vapor can be harvested by a concept of water condensing panels. Efficient water harvesting requires not only a considerable yield but also a timely water removal from the surface since the very beginning of condensation to avoid the huge evaporation losses. This translates into strict surface properties, which are difficult to simultaneously realize. Herein, we study various functionalized silicon surfaces, including the so-called Black Silicon, which supports two droplet motion modes—out-of-plane jumping and in-plane sweeping, due to its unique surface morphology, synergistically leading to a pioneering combination of above two required characteristics. According to silicon material's scalability, the proposed silicon-based water panels would benefit from existing infrastructures toward dual functions of energy harvesting in daytime and water harvesting in nighttime.

We describe a radiative cooling chamber which reproduces in the laboratory radiative cooling and subsequent dew formation. Based on radiative exchange with a cold source (at a temperature of nearly -80 degrees C), which acts as a cold black body, cooling power of at least 50 W.m(-2) is achieved. Radiative exchange is quantitively estimated. This original device permits to study under controlled air temperature and humidity the formation of dew on any system (material, biological) for which contact cooling is inefficient, and in particular investigate the influence of radiative and wetting surface properties on dew yield. It is applicable to the study of dew itself and its effects on plants and small animals, as well as dew atmospheric chemistry, and more generally of any natural radiative cooling application. (C) 2021 Elsevier Ltd. All rights reserved.

Hydrogels are known to adsorb a large amount of vapor and liquid water, making them good candidates to enhance the amount of dew condensed from atmosphere. Although water vapor adsorption and liquid invasion in hydrogels have been the object of many studies, water condensation has been only little investigated. We address here the process of dew condensation on hydrogel grains widely used in agriculture (Aquasorb 3005 (TM)). We show that dew condensing on hydrogels is enhanced when compared to a regular bare substrate due to vapor adsorption, which adds to condensation. Hydrogels, which can both capture water by vapor adsorption and condense water vapor with high efficiency, are thus good candidates to harvest water vapor from atmosphere with higher yield than regular bare surfaces.

A model is presented for the bouncing dynamics of a fluid-immersed sphere impacting normally a textured wall with micropillars. By taking into account the hydrodynamic and contact interactions between the smooth sphere and the textured wall, the complete motion of the sphere is recovered when approaching, colliding with and bouncing off the wall. We demonstrate that the critical Stokes number for the bouncing transition, $St_{c}$, is the sum of two contributions corresponding to dissipation prior to and during the collision, both contributions being critically influenced by the geometrical parameters of the model roughness. The experimental data obtained from interferometric measurements are found to be in agreement with the theoretical predictions. In the bouncing regime, the coefficient of restitution is also derived analytically and shows a linear evolution with the Stokes number, $St$, just above the bouncing transition, in agreement with the experimental data obtained very close to $St_{c}$.

The collision of a sphere with a wettable microtextured wall in a viscous fluid is investigated experimentally, focusing on the region close to the contact with the wall. High-frequency laser interferometry is used for measuring small displacements of the sphere in that region. The wall texture consists of an a array of square micropillars, whose geometrical parameters (height, width, and spacing of the pillars) are varied. The wall texture decreases the hydrodynamic resistance, and hence the drag on the sphere, compared to the case of a smooth wall. At small Reynolds and Stokes numbers, this drag reduction is quantified in terms of an equivalent plane boundary, shifted down from the top of the pillars. The shift length depends on the geometrical parameters of the pillars array and is compared to available predictions of effective slip length for a flow over arrays of micropillars in the Wenzel state. At finite Stokes number, below the bouncing transition, the wall texture influences the relative importance of sphere inertia and drag force in the near wall region. As a result, a great diversity of sphere dynamics are obtained by varying the texture geometrical parameters. These dynamics can be captured considering a shift-length-modified drag force in the equation of motion of the sphere.

Gravity shedding of droplets is limited by droplet pinning, a major limitation for low condensation processes and in particular passive dew harvesting in its use as an alternative source of water. We present experiments showing that, paradoxically, a simple surface treatment increasing roughness (sand-blasting) favors droplet shedding compared to the original substrate, provided that sand-blasting does not increase too much the surface roughness. Sand-blasting ensures the high density of nucleation sites and enhances drops coalescence and growth at a sub-micron scale, thus lowering the lag-time to obtain drop sliding during condensation. Early nucleation indeed overcompensates the delay increase due to roughness. Edges of the substrate, where drops grow faster, also improve water collection, thanks to the early sliding of edge drops that behave as natural wipers. Combining the effects of sand-blasting and edges increases significantly the rate of collection of dew condensation on a substrate at a given time, gains of about 30% can be commonly obtained.

Gravity-driven drainage of small volumes of condensates, such as natural dew, is a challenge because small drops usually remain pinned to inclined surfaces. We report that submillimetric grooves substantially reduce dew retention by modifying the repartition of liquid: Because of a long-range coalescence mechanism mediated by grooves imbibition, the growth and shedding of large drops are accelerated. Such findings can be applied to increase the passive harvesting of dew as well as to accelerate the drainage of other condensates.

This paper summarizes one year (April 2011 to March 2012) measurements on planar condensing surfaces of dew and rain events and related physico-chemical characteristics in the urban environment of Paris (city center). Yearly collected water was 3.48 mm for dew (63 events) and 593 mm for rain (146 events). The latter value compares well with rain data (547 mm and 107 events) collected within 12 km at Paris-Orly airport. An estimation of dew yield based on meteo data gives 235 mm and 74 events, to be compared with 17.11 mm and 196 events at Paris-Orly. These differences highlight the large reduction in dew events and dew yields in an urban area as compared to a close rural-like area. This reduction is not due to a sky view reduction but to heat island that increases air temperature and decreases relative humidity.Analysis of dew (34) and rain (77) samples were done concerning pH, electrical conductivity (EC), major anions and cations as well as selected trace metals and other minor ions. Mean pH values are found similar for both, dew (6.5) and rain (6.1), rain being slightly more acidic than dew. The mean dew total ionic content (TIC 1.8 meq/I) and EC value (124 mu S/cm) are about four times that of rain (0.45 meq/I; 35 mu S/cm), meaning that total dissolved solids in dew is nearly four times that in rain. Sulfate and nitrate are the most acidifying components, calcium the most neutralizing constituent with ratio of mean total acidity/total alkalinity comparable for dew and rain (similar to 0.9). Sulfate and nitrate have mainly anthropogenic sources, whereas chloride and magnesium are mostly connected with marine air masses. Dew is a considerable factor of wet deposition of pollutants; dew and rain ion concentrations, however, meet the WHO requirements for drinking water. (C) 2017 Elsevier B.V. All rights reserved.

In a context of climate change and increasing need of fresh water in the world, rain and dew water can have a significant impact as new sources of water, especially in arid and semi-arid areas. The aim of the paper is to demonstrate that atmospheric moisture can be harvested and processed into safe drinking water comparable in quality and price to reverse osmosis processed water available in the market. The paper describes the construction and functioning of a water production plant in northwest India (Kothara). Rain and dew are collected; for dew special attention has to be taken. In particular, special condenser architecture (ridges) is designed using Computational Fluid Dynamics simulation and improved condensing surfaces are operated. Dew yields are estimated from the meteo data and using simulation. From the figures an economic model is derived; it comes out that water passively harvested from atmospheric moisture may be cheaper than that from reverse osmosis and does not pollute the environment, supporting the importance of dew and rain resources to provide supplementary supply of potable water in arid and semi-arid environment.

The motion of a millimetric sphere, translating in a viscous fluid towards a wettable textured wall, is investigated experimentally. The textures consist of square arrays of cylindrical or square micro-pillars, the height, width, and spacing of which are varied, keeping the periodicity small compared to the sphere radius. An interferometric device is used to measure the sphere vertical displacement, for distances between the sphere and the base of the pillars smaller than 0.1 sphere radius, and with a resolution of 200 nm. At a given distance from the top of the pillars, the sphere velocity is found to be significantly larger than the corresponding velocity for a smooth solid wall. A squeeze flow model of two adjacent fluid layers is developed in the lubrication approximation, one fluid layer having an effective viscosity that reflects the viscous dissipation through the array of pillars. The pressure field in the gap between the sphere and the textured surface is then used to obtain the drag force on the sphere and hence its velocity. Adjustment of the model to the velocity measurements yields the effective viscosity for a given texture. Finally, a correlation between the effective viscosity and the geometry of the pillar array is proposed.