Ultrathin Water Film Research Articles

Modes of Nanodroplet Formation and Growth on an Ultrathin Water Film.

Using nanobubbles as geometrical confinements, we create a thin water film (∼10 nm) in a graphene liquid cell and investigate the evolution of its instability at the nanoscale under transmission electron microscopy. The breakdown of the water films, resulting in the subsequent formation and growth of nanodroplets, is visualized and generalized into different modes. We identified distinct droplet formation and growth modes by analyzing the dynamic processes involving 61 droplets and 110 liquid bridges within 31 Graphene Liquid Cells (GLCs). Droplet formation is influenced by their positions in GLCs, taking on a semicircular shape at the edge and a circular shape in the middle. Growth modes include liquid mass transfer driven by Plateau-Rayleigh instability and merging processes in and out-of-plane of the graphene interface. Droplet growth can lead to the formation of liquid bridges for which we obtain multiview projections. Data analysis reveals the general dynamics of liquid bridges, including drawing liquids from neighboring residual water films, merging with surrounding droplets, and merging with other liquid bridges. Our experimental observations provide insights into fluid transport at the nanoscale.

Water Skin Effect and Arched Double‐Sided Evaporation for Boosting All‐Weather High Salinity Desalination

AbstractHigh efficiency, long‐term salt rejection, and continuous operation under all‐weather conditions have always been the goals of solar‐driven desalination of high salinity brine. Here, a synergistic strategy of water skin effect and double‐sided evaporation is proposed by an arched evaporator to overcome the above issues. Specifically, a hydrophobic carbon‐fiber cloth (CC) is coated with metal‐organic framework (MOF)‐derived superhydrophilic carbon films, and bent into an arch‐like configuration. Due to unique hydrophobic‐core/hydrophilic‐shell structure, an ultrathin water film (30–150 µm) is confined to the outer layer like skin. Thanks to the water skin effect and coevaporation of outer and inner surfaces, the arched evaporator displays ultrahigh evaporation rate of 3.21 kg m−2 h−1 (3.5 wt% NaCl) and 2.87 kg m−2 h−1 (20 wt% NaCl) under 1 sun illumination. Simultaneously, the free‐flowing water skin with fast convection for salt rejection, ensures stable evaporation performance during the whole day. Additionally, an evaporation rate up to 3.5 kg m−2 h−1 is acquired with only 2.5 V input voltage in a dark environment, attributable to the superior electrothermal effect of CC. The constructed evaporator with all‐in‐one function provides an effective way for commercial, portable photothermal conversion to achieve seawater purification.

Evaporation of ultra-thin water film on hot spot with nanopillar array

For the ultra-thin film evaporation, a hot spot was formed by a local dry out, resulting in the heat transfer deterioration. In this work, the stable evaporation of ultra-thin water film on hot spot with designed nanopillar structure was studied based on the non-equilibrium molecular dynamics (MD) simulation. The variation of molecular motion trajectories, evaporation rate and number of migrating water molecules in different regions were recorded over time in the simulation. The results show that the nanopillar array on the hot spot can attract the surrounding liquid to wet the overheated region due to the strong solid-liquid interaction, both the evaporation stability and heat transfer performance can be improved. By comparing three surfaces with different nanopillar arrays on hot spot, it shows that the evaporation rate is one important factor affecting the amount of liquid replenishment: with the increasing number of nanopillars, both the evaporation performance and liquid replenishment capacity are improved. Moreover, the effect of two designed nanostructure configurations on the evaporation performance was investigated. Compared to the configuration of only nanopillar array on the hot spot, the configuration of nanopillar array on the whole surface has a lower temperature for the nanopillars and delays the occurrence of dry out, while the averaged evaporation rate may be reduced as well. Therefore, there is a trade-off between the surface temperature reduction and evaporation rate enhancement. The findings in this study can provide a guidance for the thermal design for the hot spot in the chip cooling technique under the high heat flux application.

Observation of Interfacial Instability of an Ultrathin Water Film

We observed the instability of a few-nanometer-thick water film encapsulated inside a graphene nanoscroll using transmission electron microscopy. The film, that was left after recession of a meniscus, formed ripples along the length of the nanoscroll with a distance only 20%-44% of that predicted by the classical Plateau-Rayleigh instability theory. The results were explained by a theoretical analysis that incorporates the effect of the van der Waals interactions between the water film and the graphene layers. We derived important insights into the behavior of liquid under nanoscale confinement and in nanofluidic systems.

Dynamics of thin precursor film in wetting of nanopatterned surfaces

The spreading of a liquid droplet on flat surfaces is a well-understood phenomenon, but little is known about how liquids spread on a rough surface. When the surface roughness is of the nanoscopic length scale, the capillary forces dominate and the liquid droplet spreads by wetting the nanoscale textures that act as capillaries. Here, using a combination of advanced nanofabrication and liquid-phase transmission electron microscopy, we image the wetting of a surface patterned with a dense array of nanopillars of varying heights. Our real-time, high-speed observations reveal that water wets the surface in two stages: 1) an ultrathin precursor water film forms on the surface, and then 2) the capillary action by nanopillars pulls the water, increasing the overall thickness of water film. These direct nanoscale observations capture the previously elusive precursor film, which is a critical intermediate step in wetting of rough surfaces.

Interfacial Design to Enhance Photocatalytic Hydrogen Evolution via Optimizing Energy and Mass Flows

Energy and mass transfer in photocatalytic systems plays a significant role in photocatalytic water splitting, but relevant research has long been ignored. Here, an interfacial photocatalytic mode for photocatalytic hydrogen production is exploited to optimize the energy and mass flows and mainly includes a heat-insulating layer, a water-channel layer, and a photothermal photocatalytic layer. In this mode, the energy flow is optimized for efficient spreading, conversion, and utilization. A low-loss path (ultrathin water film) and an efficient heat localized zone are constructed, where light energy, especially infrared-light energy, can transfer to the target functional membrane surface with low loss and the thermal energy converted from light can be localized for further use. Meanwhile, the optimization of the mass flow is achieved by improving the desorption capacity of the products. The generated hydrogen bubbles can rapidly leave from the surface of the photocatalyst, along with the active sites being released timely. Consequently, the photocatalytic hydrogen production rate can be increased up to about 6.6 times that in a conventional photocatalytic mode. From the system design aspect, this work provides an efficient strategy to improve the performance of photocatalytic water splitting by optimizing the energy and mass flows.

The Effect of Adsorbed Volatile Organic Compounds on an Ultrathin Water Film Measurement

Using surface plasmon resonance imaging (SPRi), we have recently shown for the first time the existence of a monolayer water film between droplets during dropwise condensation. This study examines the effect of adsorbed volatile organic compounds (VOCs) on the ultrathin film measurement using SPRi. Further, the work presents the proper surface-treatment process that enables measurements of the ultrathin water layer during high-speed imaging of dropwise condensation at 3000 frame per second. In this study, two methods were applied for cleaning the surface (gold-coated glass)—(1) standard cleaning procedure (SCP) using acetone, isopropyl alcohol, and deionized water and (2) SCP followed by air plasma cleaning. This work discusses the effect of the cleaning procedures on surface roughness, contact angle, and surface chemistry using atomic force microscopy, optical microscopy, and an X-ray photoelectron spectroscope meter. The results showed that SCP before the SPRi is a proper surface-treatment method. The effect of adsorbed VOCs during dropwise condensation on a surface treated with SCP was measured to be 0.0025 (reflectivity unit), which was 70% smaller than the reflectance associated with a monolayer water film. The results of this work confirm a monolayer water film observation during the dropwise condensation, which has been reported before.

Heat transfer characteristics of micron ultrathin shear-driven water film flowing on a horizontal metal surface

The heat and mass transfer characteristics of shear-driven water film is widely investigated. Most reported studies are modeling work with series of assumptions. In this study, an experimental investigation on the heat transfer characteristics of shear-driven water film flowing on a metal surface was carried out. The surface is simplified as flat and horizontal, and the wind speed is fixed at 20.8 ~ 56.6 m/s. Variation of the width and surface temperature gradient of water film and the heat flux on solid-liquid and liquid-gas interfaces were quantitatively analyzed. As indicated, the width of the water film is dominated by the wind speed, at about 80 ~ 85 mm for a wind speed of 32.8 m/s. When the solid-liquid interface is under cooling and heating conditions at the temperature difference of 15 °C, their heat transfer densities are at ranges of -2 ~ 16 kW/m2, and 18 ~ 30 kW/m2, respectively. The collected experimental data could directly validate these existing and developing models in this field. Conclusions of this work also benefit for understanding the glaze ice formation mechanism and optimizing the anti-icing or de-icing system.

Water Confined in Hydrophobic Cup-Stacked Carbon Nanotubes beyond Surface-Tension Dominance.

Water confined in carbon nanotubes (CNTs) can exhibit distinctly different behaviors from the bulk. We report transmission electron microscopy (TEM) observation of water phases inside hydrophobic cup-stacked CNTs exposed to high vacuum. Unexpectedly, we observed stable water morphologies beyond surface-tension dominance, including nanometer thin free water films, complex water-bubble structures, and zigzag-shaped liquid-gas interface. The menisci of the water phases are complex and inflected, where we measured the contact angles on the CNT inner wall to be 68-104°. The superstability of the suspended ultrathin water films is attributed to the strong hydrogen-bonded network among water molecules and adsorption of water molecules on the cup-structured inner wall. The complex water-bubble structure is a result of the stability of free water films and interfacial nanobubbles, and the zigzag edge of the liquid-gas interface is explained by the pinning effect. These experimental findings provide valuable knowledge for the research on fluids under nanoscale confinement.

Investigation on wavy characteristics of shear-driven water film using the planar laser induced fluorescence method

Aircraft icing poses a serious threat to flight safety. Unfrozen impinging water on the surface of the aircraft will run back under the effect of high-speed airflow, altering liquid distribution and heat transfer characteristics. In this paper, a series of experiments were conducted over a wide range of air speed (Ua = 20.8–56.6 m/s), film Reynolds number (Ref = 26–128) to investigate the dynamics of ultrathin water film on an Aluminum substrate. The analyses of water film thickness and wavy characteristics were conducted through the planar laser-induced fluorescence method, in which the variations of transient characteristics of water film thickness with both the air speed and flow rate were analyzed. The statistics of water film thickness characteristics indicates that, the ratio of surface roughness of water film to its mean thickness is 0.4–0.8. The variation of the wave parameters of gas-liquid interface with both the air speed and the Reynolds number of the water film are also presented and analyzed in detail, including the main frequency of wave, interface wave height, surface wave velocity, interface wave curvature, and wave length parameters. This work is helpful to fundamentally understanding the wavy characteristics of shear-driven water film, as well as optimize the de-icing system for aircraft.

Ultrafast Water Permeation in Graphene Nanostructures Anomalously Enhances Two‐Phase Heat Transfer

AbstractThe unique property of ultrafast water transport in graphene nanoplatelets (GNPs) coating is attributed to the frictionless interaction between the smooth hydrophobic carbon nanostructures and the well‐ordered hydrogen bonds of water molecules, whereas the driving force originates from the randomly distributed hydrophilic functionalities. The anomalous two‐phase heat transfer enhancement that results from this distinctive property of GNPs‐coating is investigated. The fast water permeation property of graphene enables the formation of ultrathin water film that synergizes the evaporation and the circulation of water in a two‐phase closed thermosyphon (TPCT). In addition, the GNPs‐coating which performs as a hydrophilic surface, improves the droplet nucleation during condensation and its fast water permeation property augments the circulation of condensate back to the evaporator. Ultimately, the ultrafast water permeation property of GNPs‐coating enhances the efficiencies of all the three main processes taking place in a TPCT, namely, evaporation, condensation, and circulation of condensate, leading to a maximum enhancement of 80.7% in the effective thermal conductance and 116.3% in the evaporative heat transfer coefficient. The experimental results are supported by the molecular dynamics simulations. A great potential for the application of GNPs‐coating in the phase‐change heat transfer devices is envisaged.

High-Speed Measurements of Steel–Ice Friction: Experiment vs. Calculation

An ultra-thin water film plays the decisive role in steel–ice friction in bobsleighing. The water film has a thickness on the order of nanometers and results from the superposition of an existing quasi-liquid layer and additional surface water generated by frictional heat. When friction is measured as function of sliding velocity, the coefficients decrease according to the typical Stribeck behavior. However, for highest sliding velocities, it is still unknown whether friction decreases further or shows an increase due to viscous drag. Both tendencies are essential for the construction of safe bobsleighs and bobsleigh tracks. This contribution presents results of high-speed experiments up to 240 km/h for a steel slider on a disk of ice at different ice temperatures. In addition, using the friction model of Makkonen, friction coefficients were calculated as function of sliding velocity and ice temperature. The significant correlation between experimental results and model calculation supports the model conception of frictional melting and viscous shearing.

Superstable Ultrathin Water Film Confined in a Hydrophilized Carbon Nanotube.

Fluids confined in a nanoscale space behave differently than in the bulk due to strong interactions between fluid molecules and solid atoms. Here, we observed water confined inside "open" hydrophilized carbon nanotubes (CNT), with diameter of tens of nanometers, using transmission electron microscopy (TEM). A 1-7 nm water film adhering to most of the inner wall surface was observed and remained stable in the high vacuum (order of 10-5 Pa) of the TEM. The superstability of this film was attributed to a combination of curvature, nanoroughness, and confinement resulting in a lower vapor pressure for water and hence inhibiting its vaporization. Occasional, suspended ultrathin water film with thickness of 3-20 nm were found and remained stable inside the CNT. This film thickness is 1 order of magnitude smaller than the critical film thickness (about 40 nm) reported by the Derjaguin-Landau-Verwey-Overbeek theory and previous experimental investigations. The stability of the suspended ultrathin water film is attributed to the additional molecular interactions due to the extended water meniscus, which balances the rest of the disjoining pressures.

Water at Interfaces.

The interfaces of neat water and aqueous solutions play a prominent role in many technological processes and in the environment. Examples of aqueous interfaces are ultrathin water films that cover most hydrophilic surfaces under ambient relative humidities, the liquid/solid interface which drives many electrochemical reactions, and the liquid/vapor interface, which governs the uptake and release of trace gases by the oceans and cloud droplets. In this article we review some of the recent experimental and theoretical advances in our knowledge of the properties of aqueous interfaces and discuss open questions and gaps in our understanding.

Surface Structure Dependence in Desorption and Crystallization of Thin Interfacial Water Films on Platinum.

We study the influence of substrate structure on desorption and crystallization of water deposited at 100 K on platinum. We use ultrathin water films adsorbed to well-defined but highly corrugated Pt(211) and Pt(221) surfaces. Desorption spectra reveal variations in the wetting and subsequent layers that critically depend on step type. Crystallization is induced at much lower substrate temperatures as compared to Pt(111). The crystalline ice (CI)-like layer is also significantly more stable on stepped surfaces as evidenced by a higher desorption energy. Crystallinity of the CI-like layers is maintained over a thickness that varies strongly with step type.

Erratum to: Effect of wetted graphene on the performance of Pt/PPy-graphene electrocatalyst for methanol electrooxidation in acid medium

Polypyrrole (PPy)-modified graphene can be used as a support of electrocatalyst in methanol oxidation reaction. In this paper, the Pt/PPy-graphene electrocatalyst is prepared through in situ synthetic method. The structure and electrochemical property of the electrocatalyst were investigated. The results show that the ultrathin water film between graphene and PPy was favorable as the PPy was distributed uniformly on the upper surface of graphene. The PPy-modified graphene enables the formation of Pt nanoparticles having a very narrow size profile centered around 8 nm and a very even spatial distribution on the graphene surface. The Pt/PPy-graphene catalyst exhibits a noticeably higher electrochemical activity in methanol oxidation reaction and performance durability than the Pt/graphene catalyst, and thus, PPy-graphene is a promising alternative catalyst support in direct methanol fuel cells.

Molecular Dynamics Simulation on Rapid Boiling of Thin Water Films on Cone-Shaped Nanostructure Surfaces

Molecular dynamics simulations (MDS) are employed to investigate the effects of the size of a nanocone on rapid boiling of an ultrathin liquid water film that is suddenly heated by a hot aluminum plate. A physically sound thermostat is applied to control the temperature of the aluminum plate and then to heat the water molecules that are placed on the solid surface. The results show that the cone nanostructures drastically enhance heat transfer from the solid aluminum plate to liquid water and the phase change process from liquid water to vapor. They also have significant effects on temperature histories and the density distributions in the system. In all cases studied, the water molecules above the solid surface rapidly boil after contact with an extremely hot aluminum plate and consequently a cluster of liquid water is observed to move upward during the phase change. It is also observed that the separation temperature associated with separation of liquid water film from the solid surface and its final temperature when the system is at equilibrium strongly depend on the height of the nanocone. Furthermore, in all cases, at a specific time after beginning of boiling, a nonvaporized water molecular layer is formed above the surface of the aluminum plate.

Optical investigation of diffusion of single Ag markers in confined water films

Silver (Ag) clusters with sub-nanometer diameter show as a kind of marker fluorescence emission in the visible range and allow besides optical single-cluster tracking for spectroscopy of single clusters revealing a vibronic coupling of the Ag clusters to the interface. Fluorescence intensity intermittence (blinking) and laser power-dependent photobleaching are observed. The spatial diffusion of monodisperse silver nanoclusters explores ultrathin nanostructured water films on silicon dioxide (SiO2). These heterogeneous diffusion dynamics depend strongly on the thickness of wetting with ultrathin water films and the chemical composition of the SiO2 interface. Thickness-dependent structural changes of the water film have significant influence on diffusion dynamics. We present a numerical model which explains the heterogeneous diffusion across the layers of water with varying viscosity. Our results do not only demonstrate the feasibility to use Ag nano markers to explore nanostructures but also reveal the complexity of nanostructured liquids at solid interfaces.

Molecular dynamics simulations of disjoining pressure effect in ultra-thin water film on a metal surface

Molecular dynamics (MD) simulations are used to examine the disjoining pressure effect of a water thin film adsorbed on a metal surface. The model was validated against experiments and verified against previous MD simulations. The variation of vapor pressure with film thickness was examined for a water thin film adsorbed on a gold surface. The results agree well with the classic disjoining pressure theory without surface charges and show that liquid layering does not affect disjoining pressure. However, surface charges of the gold substrate enhance the disjoining pressure of the water thin film, consistent with experimental evidences for polar liquids.

Viscosity of Ultrathin Water Films Confined between Aluminol Surfaces of Kaolinite: Ab Initio Simulations

Ab initio molecular dynamics simulations of water confined between kaolinite walls were conducted in an effort to make contact with experiments implying dramatic viscosity enhancement for water in nanometer-scale, hydrophilic channels. An earlier ground-state structural optimization of a single water layer on a flat kaolinite(001) surface had yielded a molecular arrangement, which, by appearing hydrophobic to subsequent layers, suggested the possibility of very low flow resistance. Well above the freezing point, however, and under shear, the surface became hydrophilic, as a percentage of first-layer water molecules flipped to expose dangling hydroxyls. This led to simulated steady-state velocity fields consistent with a “no-slip” boundary condition and viscous flow. The magnitude of the viscosity derived from the simulations, only a few times that of bulk water, does not lend theoretical weight to the notion of dramatic enhancement under nanoconfinement.