Thin Water Layer Research Articles

The Cooling Potential of Sky Radiation With Variations in System Parameters

This study evaluated the building cooling capacity of sky radiation, which was previously identified to have the greatest cooling potential among common ambient sources for climates across the U.S. A heat pipe augmented sky radiator system was simulated by a thermal network with nine nodes, including a thin polyethylene cover with and without condensation, white (zinc oxide) painted radiator plate, condenser and evaporator ends of the heat pipe, thermal storage fluid (water), tank wall, room, sky and ambient air. Heat transfer between nodes included solar flux and sky radiation to cover and plate, wind convection and radiation from cover to ambient, radiation from plate to ambient, natural convection and radiation from plate to cover, conduction from plate to condenser, two-phase heat transfer from evaporator to condenser, natural convection from evaporator to water and from water to tank wall, natural convection and radiation from tank wall to room, and overall heat loss from room to ambient. A thin layer of water was applied to simulate condensation on the cover. Nodal temperatures were simultaneously solved as functions of time using typical meteorological year (TMY3) weather data. Auxiliary cooling was added as needed to limit room temperature to a maximum of 23.9 °C. For this initial investigation, a moderate climate (Louisville, KY) was used to evaluate the effects of radiator orientation, thermal storage capacity, and cooling load to radiator area ratio (LRR). Results were compared to a Louisville baseline with LRR = 10 W/m2 K, horizontal radiator and one cover, which provided an annual sky fraction (fraction of cooling load provided by sky radiation) of 0.855. A decrease to 0.852 was found for an increase in radiator slope to 20 deg, and a drop to 0.832 for 53 deg slope (latitude + 15 deg, a typical slope for solar heating). These drops were associated with increases in average radiator temperature by 0.73 °C for 20 deg and 1.99 °C for 53 deg. A 30% decrease in storage capacity caused a decrease in sky fraction to 0.843. Sky fractions were 0.720 and 0.959 for LRR of 20 and 5, respectively. LRR and thermal storage capacity had strong effects on performance. Radiator slope had a surprisingly small impact, considering that the view factor to the sky at 53 deg tilt is less than 0.5.

Automatic Cooling System for Efficiency and Output Enhancement of a PV System Application in Palembang, Indonesia

Active cooling system technique which flows a thin layer of water on the surface of the PV panels can significantly reduce the panel surface temperature compared to normal use. From experiments conducted in Palembang, Indonesia on August 2018 used two polycrystalline PV panels 100 WP manufactured by Sankelux, can reduce the average surface temperature of PV panels by 18.7°C. The output voltage of the water-cooled PV panels increases by average 1.5 volts when loaded with a 1.5 ohms DC lamp compared to the normal panels. From the experiment, PV panel will work effectively from 8.30 AM to 3.30 PM. Increasing of panel temperature does not significantly affect the output current. The average output power on the water-cooled panel is 31.5 watts, has increased compared to the normal panel 34.2 watts. The overall average output power increase 2.7 watts. The cooling system can also increase the efficiency of the output power. Between 8.30 AM to 3.30 AM, the lowest efficiency enhancement occurred at 9.30 AM is 5.01% and the highest is 13.06 % occurred at 2.30 PM, and overall average efficiency enhancement of output power is 8.6%.

In Situ Supercontinuum Nanopatterning of Silicon Surface by Femtosecond Laser Superfilaments

We for the first time theoretically predict and experimentally demonstrate that selective, low-threshold excitation and interference between short-wavelength plasmons on silicon surfaces photoexcited by infrared femtosecond laser pulses give rise to appearance of permanent sub-diffraction surface gratings with their periods down to 100 nm, dramatically differing from near-wavelength surface gratings succeeding the common “laser photon-surface polariton” interference. Such selectivity was provided in this work by in situ white-light (supercontinuum) femtosecond source generated in a multi-filamentation regime in a thin water layer atop the silicon surface, supporting high-efficiency excitation and dynamic tracking of the transiently tunable plasmon resonance on silicon surface for its easy, robust and ultimate surface nanopatterning.

Influence of interface hydration on sliding of graphene and molybdenum-disulfide single-layers

Humidity influences friction in layered materials in peculiar ways. For example, while water improves the lubricating properties of graphite, it deteriorates those of molybdenum disulfide (MoS2). The reasons remain debated, not the least due to the difficulty in experimentally comparing dry and hydrated interface frictions. Here we show that the hydration of interfaces between a mica substrate and single-layers of graphene and MoS2 with a molecularly thin water layer affects strain transfer from the substrate to the 2D materials. For this, we strain the substrate and detect strain in graphene and MoS2 by changes in Raman and photoluminescence spectra, respectively. Strain relaxation in graphene changes from stick-slip in dry contact, to viscous when hydrated. In contrast, there is no viscous relaxation in MoS2 regardless of hydration. Our work provides a novel approach for better understanding the impact of hydration on friction in layered materials.

Asphaltene Adsorption from Toluene onto Silica through Thin Water Layers.

Asphaltenes in crude oil play a pivotal role in reservoir oil production because they control rock-surface wettability. Upon crude oil invasion into a brine-filled reservoir trap, rock adherence of sticky asphaltene agglomerates formed at the crude oil/brine interface can change the initially water-wet porous medium into mixed-oil wetting. If thick, stable water films coat the rock surfaces, however, asphaltenic-oil adhesion is thought to be prevented. We investigate whether water films influence the uptake of asphaltenes in crude oil onto silica surfaces. Water films of known thickness are formed at a silica surface in a quartz crystal microbalance with dissipation and contacted by toluene-solubilized asphaltene. We confirm that thick water films prevent asphaltene molecular contact with the silica surface blocking asphaltene adhesion. The thicker the water film, the smaller is the amount of asphaltene deposited. Film thickness necessary for complete blockage onto silica is greater than about 500 nm, well beyond the range of molecular-chain contact. Water films of thickness less than 500 nm, sandwiched between toluene and solid silica, apparently rupture into thick water pockets and interposed molecularly thin water layers that permit asphaltene adherence.

Quasi-Liquid Layers Can Exist on Polycrystalline Ice Thin Films at a Temperature Significantly Lower than on Ice Single Crystals

Surface melting of ice crystals proceeds below the melting point (0 °C) and forms thin liquid water layers, called quasi-liquid layers (QLLs), which govern a wide variety of phenomena in nature. Hence, many studies have been performed so far; however, the lowest temperature above which QLLs exist on ice crystal surfaces varied from −90 to −1 °C. To reveal the cause of such significant variations, here we show, by laser confocal microscopy combined with Michelson interferometry, the behavior of QLLs on polycrystalline ice thin films that include a large amount of grain boundaries and defects. We found that the QLLs can exist stably on the polycrystalline ice thin films even at −16.2 °C (the lowest temperature adopted in this study), although the QLLs on ice single crystals disappear at temperature lower than −2.4 ± 0.5 °C. These results emphasize the importance of grain boundaries and defects for the presence of QLLs. In addition, we also found that critical water vapor pressure above which the QLLs can gr...

Non-monotonous Wetting of Graphene-Mica and MoS2-Mica Interfaces with a Molecular Layer of Water.

Hydration of interfaces with a layer of water is a ubiquitous phenomenon, which has important implications for numerous natural and technologically important processes. Nevertheless, at the nanoscale, the understanding of the wetting process is still limited, since it is experimentally difficult to follow. Here, graphene and monolayers of MoS2 deposited on dry mica are used to investigate wetting of the two-dimensional (2D) material-mica interfaces with a molecularly thin layer of water employing scanning force microscopy in different modes. Wetting occurs non-monotonously in time and space for both types of interfaces. It starts at relative humidities (RH) of 10-17% for graphenes and 8-9% for MoS2 and concludes with a homogeneous layer at 25-30 and 15-20%, respectively. Investigation of the process at the graphene-mica interface indicates that up to about 25% RH, initially a highly compliant and unstable layer of water spreads, which subsequently stabilizes by developing labyrinthine nanostructures. Moreover, these nanostructures exhibit distinct mechanical deformability and dissipation, which is ascribed to different densities of the confined water layer. The laterally structured morphology is explained by the interplay of counteracting long-range dipole-dipole repulsion and short-range line tension, with the latter causing at least in part by the mechanical deformation of the 2D material. The proposed origins of the interactions are common for thin layers of polar molecules at interfaces, implying that the lateral structuring of thin wetting layers at submonolayer concentrations may also be a quite general phenomenon.

Steam generation enabled by a high efficiency solar absorber with thermal concentration

Solar steam generation, a typical solar energy utilization way, has wide applications and attracts many researches, such as the enhancement achieving by numerous nanomaterials and porous membranes. However, some issues need to be solved (e.g., blockage of pore structures and poisoning of film nanoparticles (NPs)), which are critical for developing sustained and efficient evaporation processes. In this sense, we developed herein a novel evaporation method involving a thin water layer and a highly efficient solar absorber for enhanced steam generation. This technology prevented pore blockage and poisoning of NPs by employing a dense surface structure and continuous water flow. A C-Au-TiO2 solar absorber prepared by a sol-gel method with a superior photo-thermal conversion capacity, even for lights with large angles of incidence was the key to enhance the solar steam generation process. By experiments and theoretical calculations for solar evaporation, the steam generation performance and heat change process were investigated. It was found that improving the light absorption of the solar absorber and reducing the thermal loss were effective methods to enhance evaporation rate and efficiency, while reducing the water height could cut down the time needed to reach stable stage. And the C-Au-TiO2 solar absorber achieved a significantly enhanced solar steam generation, which may pave the way for developing new highly efficient solar steam generation paths.

Nanoparticles reveal Extreme Size-Sorting and Morphologies in Complex Coacervate Superstructures

We here provide detailed insight in self-assembled complex coacervate systems exploiting gold nanoparticles for cryoTEM contrast. Nanoparticle-containing dendrimicelles are formed from fifth-generation dendrimer-encapsulated nanoparticles (DENs) and dendrimer-stabilized nanoparticles (DSNs). The complex coacervate structures self-organize in biconcave thin water layers into size-sorted monolayer superstructures. The embedded nanoparticles are a straightforward tool to visualize dendrimicelles and determine the aggregation number and polydispersity. The superstructure shows extreme size-sorting patterns which, contrary to related systems with higher generation dendrimers, consists not only of dendrimicelles but also much bigger complex coacervate nanoassemblies, such as vesicles.

Sensing phases of water via nitrogen-vacancy centres in diamond

Ultra-thin layers of liquids on a surface behave differently from bulk liquids due to liquid-surface interactions. Some examples are significant changes in diffusion properties and the temperature at which the liquid-solid phase transition takes place. Indeed, molecular dynamics simulations suggest that thin layers of water on a diamond surface may remain solid even well above room temperature. However, because of the small volumes that are involved, it is exceedingly difficult to examine these phenomena experimentally with current technologies. In this context, shallow NV centres promise a highly sensitive tool for the investigation of magnetic signals emanating from liquids and solids that are deposited on the surface of a diamond. Moreover, NV centres are non-invasive sensors with extraordinary performance even at room-temperature. To that end, we present here a theoretical work, complemented with numerical evidence based on bosonization techniques, that predicts the measurable signal from a single NV centre when interacting with large spin baths in different configurations. In fact, by means of continuous dynamical decoupling, the polarization exchange between a single NV centre and the hydrogen nuclear spins from the water molecules is enhanced, leading to differences in the coherent dynamics of the NV centre that are interpreted as an unambiguous trace of the molecular structure. We therefore propose single NV centres as sensors capable to resolve structural water features at the nanoscale and even sensitive to phase transitions.

A numerical model of superspreading surfactants on hydrophobic surface

Many contributions significantly on experimental and mathematical studies are made to understand the mechanism of superspreading. Only few numerical methods have been proposed which solve the system of equations with soluble and insoluble surfactants. Among them, we propose a computational fluid dynamics model, based on the volume of fluid technique, with the piecewise linear interface calculation method. Interface reconstruction is applied to simulate the time evolution of the dynamics of drop spreading of surfactants on a thin water layer. We have allowed the occurrence of both the regimes relating to a series of trisiloxane (M(D′EnOH)M), sodium dodecyl sulphate, and Tergitol NP10 surfactants drop on a thin water layer with the influence of Marangoni stress. The numerical data seem consistent with those experimental for both regimes. It validates predictions for the spreading exponent in which the law of the radius of the circular area covered by the surfactant grows as tα, where 0 < α < 1. The comparison of the numerical and experimental predictions by Lee et al. [“Spreading of trisiloxanes over thin aqueous layers,” Colloid J. 71, 365–369 (2009)] is well represented in both regimes. The numerical study confirms that the spreading rates during the first stage increase as the solubility increases. This finding suggests that the model is adequate for describing the spreading of surfactants on thin fluid layers.

Implications of Eddy Cancellation for Nutrient Distribution Within Subtropical Gyres

AbstractThe role of mesoscale eddies within the nutrient budget of subtropical gyres remains poorly understood and poorly constrained. We explore a new mechanism by which mesoscale eddies may contribute to these nutrient budgets, namely eddy cancellation. Eddy cancellation describes the rectified effect of mesoscale eddies acting to oppose the Eulerian‐mean Ekman pumping. We present an idealized axisymmetric two‐layer model of a nutrient in a wind‐driven gyre and explore the sensitivity of this model to variations in its parameter values. We find that the residual Ekman pumping velocity has a substantial impact on nutrient concentration, as does mode water thickness. These results suggest the response to both residual Ekman pumping and mode water thickness is nonmonotonic: for small values of these parameters the nutrient concentration decreases as the parameter increases. However, beyond a critical value, further increases in Ekman pumping or mode water thickness increase nutrient concentration throughout our highly idealized model. A thin mode water layer promotes vertical diffusion of nutrients from the abyss, while a thicker mode water layer increases productivity by reducing the parametrized particulate flux through the thermocline. The impact of mode water thickness is modulated by the residual Ekman pumping velocity: strong Ekman pumping suppresses the influence of mode water thickness on nutrient concentrations. We use satellite and in situ measurements to assess the influence of mode water thickness on primary productivity and find a statistically significant relationship; thicker mode water correlates with higher productivity. This result is consistent with a small residual Ekman pumping velocity.

Origin of the infra-red emission peak in freezing water

When droplets of purified water (1–5 μl) were cooled from the bottom, they slowly and continuously supercooled before releasing latent heat as a transient burst of infra-red (IR) radiation. In order to determine the role of this IR emission, a thin rectangular water layer was cooled unilaterally while imaged from above by an infrared (FLIR) camera. The first noticeable event was an IR burst that rapidly (< 0.1 s) moved through a 5-mm-long path of water. Final solidification of the water layer was recognized by an increase in volume, as the meniscus at the air interface changed from concave to convex. The propagation of the IR burst through the water layer preceded the first visible onset of volume increase and solidification by more than one second. The transient and early appearance of the IR burst belongs to what is called the first stage of freezing. This stage has been linked to the formation of so-called spongy ice. Both IR burst and pinnately shaped spongy ice appear at the same time and share a short transient existence. It is only this early type of ice that is associated with the IR burst. By contrast, the later-occurring solid ice formation parallels a diminishing IR emission.

Effect of Medium on Interaction Forces between Atomic Force Microscopy (AFM) Tip and Gold Nanoparticle

Atomic Force Microscopy (AFM) has become powerful tool not only to study the surface topography but also the interaction forces between the tip and the sample. In this works, the measurement of the forces was determined by using AFM imaging in air and in liquid environment. Then, the force-distance (FD) curve was analysed to determine the interaction forces between the tip and the samples. The forces was computed from the pull-off forces of FD curve. When the FD curve are required in air, a thin layer of water (liquid contaminant layer) adsorbed on the sample surface exerts a meniscus or capillary force. Instead, when working in a liquid, the capillary force is eliminated and other forces becomes relevant. The force present is the van der Waals force and electric double layer force. The net force between the tip and the samples is the vector sum of several forces. In air, the dominant component of the adhesion force between the tip and the sample is believed to be the capillary force while in liquid, the dominant components present is the van der Waals force. The adhesion force ranging from 10 to 100 nN have been reported for imaging in air and the adhesion force of 5 to 104 nN observed in our studies lies well within this range. Meanwhile, in liquid, the adhesion force obtained from this work is in the range of 0.1-40 nN and it is lies within the range reported previously which is as low as 100 pN. Detecting forces is an important step toward developing new analytical and biomedical devices such as biosensors.

Vibrational spectroscopy of hydroxylated α-Al2O3(0001) surfaces with and without water: An ab initio molecular dynamics study.

Using gradient- and dispersion-corrected density functional theory in connection with ab initio molecular dynamics and efficient, parametrized Velocity-Velocity Autocorrelation Function (VVAF) methodology, we study the vibrational spectra (Vibrational Sum Frequency, VSF, and infrared, IR) of hydroxylated α-Al2O3(0001) surfaces with and without additional water. Specifically, by considering a naked hydroxylated surface and the same surface with a particularly stable, "ice-like" hexagonal water later allows us to identify and disentangle main spectroscopic bands of OH bonds, their orientation and dynamics, and the role of water adsorption. In particular, we assign spectroscopic signals around 3700 cm-1 as being dominated by perpendicularly oriented non-hydrogen bonded aluminol groups, with and without additional water. Furthermore, the thin water layer gives spectroscopic signals which are already comparable to previous theoretical and experimental findings for the solid/(bulk) liquid interface, showing that water molecules closest to the surface play a decisive role in the vibrational response of these systems. From a methodological point of view, the effects of temperature, anharmonicity, hydrogen-bonding, and structural dynamics are taken into account and analyzed, allowing us to compare the calculated IR and VSF spectra with the ones based on normal mode analysis and vibrational density of states. The VVAF approach employed in this work appears to be a computationally accurate yet feasible method to address the vibrational fingerprints and dynamical properties of water/metal oxide interfaces.

Effect of wetting a wall on the impact of a liquid jet

The influence of a thin layer of water wetting a wall on the impact of a water jet is studied. The jet velocity is 250 m/s. The end of the jet is somewhat blunter than the hemispherical one. The thickness of the wetting layer is 4% of the jet radius so that the level of the pressure on the wall in the center of the impact area is approximately equal to that arising when hitting a dry wall. A numerical technique based on the approach without explicit separation of the interphase boundary is applied. The CUP-CUP method is used for integrating the Euler equations. An adaptive soroban-grid is utilized. It is shown that the presence of the wetting layer mainly manifests itself in a significant decrease of the pressure peaks realized in the narrow peripheral area of the impact zone on the wall.

Stabilization of dry protein coatings with compatible solutes.

Exposure of protein modified surfaces to air may be necessary in several applications. For example, air contact may be inevitable during the implantation of biomedical devices, for analysis of protein modified surfaces, or for sensor applications. Protein coatings are very sensitive to dehydration and can undergo significant and irreversible alterations of their conformations upon exposure to air. With the use of two compatible solutes from extremophilic bacteria, ectoine and hydroxyectoine, the authors were able to preserve the activity of dried protein monolayers for up to >24 h. The protective effect can be explained by the preferred exclusion model; i.e., the solutes trap a thin water layer around the protein, retaining an aqueous environment and preventing unfolding of the protein. Horseradish peroxidase (HRP) immobilized on compact TiO2 was used as a model system. Structural differences between the compatible solute stabilized and unstabilized protein films, and between different solutes, were analyzed by static time-of-flight secondary ion mass spectrometry (ToF-SIMS). The biological activity difference observed in a colorimetric activity assay was correlated to changes in protein conformation by application of principal component analysis to the static ToF-SIMS data. Additionally, rehydration of the denatured HRP was observed in ToF-SIMS with an exposure of denatured protein coatings to ectoine and hydroxyectoine solutions.

Development of high-efficient synthetic electric heating coating for anti-icing/de-icing

One novel multi-layered anti-icing/de-icing coating has been developed by combination of the fronted electric heating coating and top super-hydrophobic coating. Compared with the conventional electric heating method where the elements are embedded in fiber reinforced plastic, this multi-layered coating can reduce the energy consumption by up to 58% for anti-icing. The anti-icing experiments demonstrate that the multi-layered coating not only realizes anti-icing with low-temperature (the anti-icing temperature was about 7 °C), but also possesses high-efficient electric heating and anti-icing performances. From the viewpoint of micro-scale, the experiment of ice drop detaching from substrate was conducted to demonstrate that the thin water layer greatly reduces the ice adhesive force. The electric heating coating has the potential for diverse industrial applications due to its convenient fabrication, damage resistance and high-efficient anti-icing/de-icing properties.

Single-bubble and multibubble cavitation in water triggered by laser-driven focusing shock waves.

In this study a single laser pulse spatially shaped into a ring is focused into a thin water layer, creating an annular cavitation bubble and cylindrical shock waves: an outer shock that diverges away from the excitation laser ring and an inner shock that focuses towards the center. A few nanoseconds after the converging shock reaches the focus and diverges away from the center, a single bubble nucleates at the center. The inner diverging shock then reaches the surface of the annular laser-induced bubble and reflects at the boundary, initiating nucleation of a tertiary bubble cloud. In the present experiments, we have performed time-resolved imaging of shock propagation and bubble wall motion. Our experimental observations of single-bubble cavitation and collapse and appearance of ring-shaped bubble clouds are consistent with our numerical simulations that solve a one-dimensional Euler equation in cylindrical coordinates. The numerical results agree qualitatively with the experimental observations of the appearance and growth of large bubble clouds at the smallest laser excitation rings. Our technique of shock-driven bubble cavitation opens interesting perspectives for the investigation of shock-induced single-bubble or multibubble cavitation phenomena in thin liquids.

The corrosion of copper in irradiated and unirradiated humid air

Copper corrosion in aerated water vapour has been studied as a function of temperature, relative humidity in the presence and absence of a low gamma radiation dose rate. On surface areas covered by a thin water layer corrosion ceased after ≤ 30 days by the formation of CuO on the surface of a Cu2O layer. At the locations of condensed water droplets corrosion was more extensive since the corrosion product developed porosity. Radiation accelerated the formation of Cu2O on corroded areas and increased the number and size of the more extensively corroded locations. The distribution of condensed water was the dominant feature controlling corrosion penetration.