Molecular simulation study of the adhesion work for water droplets on water monolayer at room temperature

The wetting phenomenon of water droplets coexisting with the ordered water monolayer termed an unexpected phenomenon of "water that does not wet a water monolayer" at room temperature has been found on several solid surfaces.Although the hydrogen bond saturation inside the monolayer can qualitatively describe this phenomenon, whether the Young–Dupré equation still holds under this unconventional wetting framework is still not answered. In this work, we have calculated the contact angle values of the droplets as well as the work of adhesion between the droplets and the monolayer based on an extended phantom-wall method. The results show that similar to the conventional solid–liquid interface,classical Young–Dupré equation is also applicable for the interface of liquid water and ordered water monolayer.     

Effect of temperature on the morphology of nanobubbles at mica/water interface

The great implication of nanobubbles at a solid/water interface has drawn wide attention of the scientific community and industries. However, the fundamental properties of nanobubbles remain unknown as yet. In this paper, the temperature effects on the morphology of nanobubbles at the mica/water interface are explored through the combination of AFM direct image with the temperature control. The results demonstrate that the apparent height of nanobubbles in AFM images is kept almost constant with the increase of temperature, whilst the lateral size of nanobubbles changes significantly. As the temperature increases from 28℃ to 42℃, the lateral size of nanobubbles increases, reaching a maximum at about 37℃, and then decreases at a higher temperature. The possible explanation for the size change of nanobubbles with temperature is suggested.     

Suspended penetration wetting state of droplets on microstructured surfaces

When a water droplet on a micropillar-structured hydrophobic surface is submitted to gradually increased pressure, the CassieBaxter wetting state transforms into the Wenzel wetting state once the pressure exceeds a critical value. It has been assumed that the reverse transition(Wenzel-to-Cassie-Baxter wetting state) cannot happen spontaneously after the pressure has been removed.In this paper, we report a new wetting-state transition. When external pressure is exerted on a droplet in the Cassie-Baxter wetting state on textured surfaces with high micropillars to trigger the breakdown of this wetting state, the droplet penetrates the micropillars but does not touch the base of the surface to trigger the occurrence of the Wenzel wetting state. We have named this state the suspended penetration wetting state. Spontaneous recovery from the suspended penetration wetting state to the initial Cassie-Baxter wetting state is achieved when the pressure is removed. Based on the experimental results, we built models to establish the penetration depth that the suspended penetration wetting state could achieve and to understand the energy barrier that influences the equilibrium position of the liquid surface. These results deepen our understanding of wetting states on rough surfaces subjected to external disturbances and shed new light on the design of superhydrophobic materials with a robust wetting stability.     

Stable water droplets on composite structures formed by embedded water into fully hydroxylated β-cristobalite silica

Using molecular dynamics simulations, we have revealed a novel wetting phenomenon with a droplet on composite structures formed by embedded water into(111) surface of β-cristobalite hydroxylated silica. This can be attributed to the formation of a composite structure composed of embedded water molecules and the surface hydroxyl(–OH) groups,which reduces the number of hydrogen bonds between the composite structure and the water droplet above the composite structure. Interestingly, a small uniform strain(±3%) applied to the crystal lattice of the hydroxylated silica surface can result in a notable change of the contact angles(> 40°) on the surface. The finding provides new insights into the correlation between the molecular-scale interfacial water structures and the macroscopic wettability of the hydroxylated silica surface.     

Modulation Spectroscopy of GaAs Covered by InAs Quantum Dots

Contactless electroreflectance has been employed at room temperature to study the Fermi level pinning at undoped-n^ GaAs surfaces covered by 1.6 and 1.8 monolayer (ML) InAs quantum dots (QDs).It is shown that the 1.8 ML InAs QD moves the Fermi level at GaAs surface to the valence band maximum by about 70 meV compared to bare GaAs,whereas 1.6 ML InAs on GaAs does not modify the Fermi level.It is confirmed that the modification of the 1.8 ML InAs deposition on the Fermi Level at GaAs surface is due to the QDs,which are surrounded by some oxidized InAs facets,rather than the wetting layer.     

Statistical model for combustion of high-metal magnesium-based hydro-reactive fuel

<正>We investigate experimentally and analytically the combustion behavior of a high-metal magnesium-based hydroreactive fuel under high temperature gaseous atmosphere.The fuel studied in this paper contains 73%magnesium powders.An experimental system is designed and experiments are carried out in both argon and water vapor atmospheres. It is found that the burning surface temperature of the fuel is higher in water vapor than that in argon and both of them are higher than the melting point of magnesium,which indicates the molten state of magnesium particles in the burning surface of the fuel.Based on physical considerations and experimental results,a mathematical one-dimensional model is formulated to describe the combustion behavior of the high-metal magnesium-based hydro-reactive fuel.The model enables the evaluation of the burning surface temperature,the burning rate and the flame standoff distance each as a function of chamber pressure and water vapor concentration.The results predicted by the model show that the burning rate and the surface temperature increase when the chamber pressure and the water vapor concentration increase,which are in agreement with the observed experimental trends.     

Sessile droplet freezing and ice adhesion on aluminum with different surface wettability and surface temperature

This paper focused on the sessile droplet freezing and ice adhesion on aluminum with different wettability(hydrophilic, common hydrophobic, and superhydrophobic surfaces, coded as HIS, CHS, SHS, respectively) over a surface temperature range of ?9°C to ?19°C. It was found that SHS could retard the sessile droplet freezing and lower the ice adhesion probably due to the interfacial air pockets(IAPs) on water/SHS interface. However, as surface temperature decreasing, some IAPs were squeezed out and such freezing retarding and adhesion lowering effect for SHS was reduced greatly. For a surface temperature of ?19°C, ice adhesion on SHS was even greater than that on CHS. To discover the reason for the squeezing out of IAPs, forces applied to the suspended water on IAPs were analyzed and it was found that the stability of IAPs was associated with surface micro-structures and surface temperature. These findings might be helpful to designing of SHS with good anti-icing properties.     

Two-Step Oxidation of Pb（111） Surfaces

We report on a two-step method for oxidation of Pb（111） surfaces, which consists of low temperature （90K） adsorption of 02 and subsequent annealing to room temperature. In situ scanning tunnelling microscopy observation reveals that oxidation of Pb（111） can occur effectively by this method, while direct room temperature adsorption results in no oxidation. Temperature-dependent adsorption behaviour suggests the existence of a precursor state for 02 adsorption on Pb（111） surfaces and can explain the oxidation-resistance of clean Pb（111） surface at room temperature.     

Structure and dynamics of ordered water in a thick nanofilm on ionic surfaces

The structure and dynamics of water in a thick film on an ionic surface are studied by molecular dynamic simulations. We find that there is a dense monolayer of water molecules in the vicinity of the surface. Water molecules within this layer not only show an upright hydrogen-down orientation, but also an upright hydrogen-up orientation. Thus, water molecules in this layer can form hydrogen bonds with water molecules in the next layer. Therefore, the two-dimensional hydrogen bond network of the first layer is disrupted, mainly due to the O atoms in this layer, which are affected by the next layer and are unstable. Moreover, these water molecules exhibit delayed dynamic behavior with relatively long residence time compared with those bulk-like molecules in the other layers. Our study should be helpful to further understand the influence of water film thickness on the interfacial water at the solid-liquid interface.     

STM study of selenium adsorption on Au(111) surface

Adsorption of chalcogen atoms on metal surfaces has attracted increasing interest for both the fundamental research and industrial applications. Here, we report a systematic study of selenium(Se) adsorption on Au(111) at varies substrate temperatures by scanning tunneling microscopy. At room temperature, small Se clusters are randomly dispersed on the surface. Increasing the temperature up to 200℃, a well-ordered lattice of Se molecules consisting of 8 Se atoms in ringlike structure is formed. Further increasing the temperature to 250℃ gives rise to the formation of Se monolayer with Au(111)-3~(1/2) ×3~(1/2) lattices superimposed with a quasi-hexagonal lattice. Desorption of Se atoms rather than the reaction between the Se atoms and the Au substrate occurs if further increasing the temperature. The ordered structures of selenium monolayers could serve as templates for self-assemblies and our findings in this work might provide insightful guild for the epitaxial growth of the two-dimensional transition metal dichalcogenides.     

A Thermodynamic Cavitation Model for Cavitating Flow Simulation in a Wide Range of Water Temperatures

A thermodynamic cavitation model is developed to simulate the cavitating water flow in a wide temperature range. The thermal effect on bubble growth during cavitation is introduced in the developed model by considering both pressure difference and heat transfer between the vapor and liquid phase. The cavitating turbulent flow over a NACA0015 hydrofoil has been simulated at various temperatures from room temperature to 150°C by using the present cavitation model, which has been validated by the experimental data. It is seen that the thermodynamic effects of cavitation, vapor depression and temperature depression are much more predominant in high temperature water compared with those in room temperature water. These results indicate that the proposed thermodynamic cavitation model is reasonably applicable to the cavitating water flow in a wide temperature range.     

Performance study of the PTFE-THGEM at room and low temperature

GPM based on THGEM has shown its competitive strength compared to the conventional PMT,especially in the low background research such as dark matter detection. A kind of THGEM made from PTFE,named PTFE-THGEM,is developed for the GPM to be used in CDEX. The PTFE has a lot of advantages especially its low level radioactivity. The PTFE-THGEM was tested at room and cryogenic temperature. It has a high gain in different gases and shows good stability at room temperature. The gain of a single PTFE-THGEM reached 112 at 117 K. The penning effect is also discussed in this paper to explain the “abnormal” phenomena of the gain in different gases.     

Performance study of the PTFE-THGEM at room and low temperature

GPM based on THGEM has shown its competitive strength compared to the conventional PMT, espe- cially in the low background research such as dark matter detection. A kind of THGEM made from PTFE, named PTFE-THGEM, is developed for the GPM to be used in CDEX. The PTFE has a lot of advantages especially its low level radioactivity. The PTFE-THGEM was tested at room and cryogenic temperature. It has a high gain in different gases and shows good stability at room temperature. The gain of a single PTFE-THGEM reached 112 at 117 K. The penning effect is also discussed in this paper to explain the ＂abnormal＂ phenomena of the gain in different gases.     

Droplet impact on regular micro-grooved surfaces

We have investigated experimentally the process of a droplet impact on a regular micro-grooved surface. The target surfaces are patterned such that micro-scale spokes radiate from the center, concentric circles, and parallel lines on the polishing copper plate, using Quasi-LIGA molding technology. The dynamic behavior of water droplets impacting on these structured surfaces is examined using a high-speed camera, including the drop impact processes, the maximum spreading diameters, and the lengths and numbers of fingers at different values of Weber number. Experimental results validate that the spreading processes are arrested on all target surfaces at low velocity. Also, the experimental results at higher impact velocity demonstrate that the spreading process is conducted on the surface parallel to the micro-grooves, but is arrested in the direction perpendicular to the micro-grooves. Besides, the lengths of fingers increase observably, even when they are ejected out as tiny droplets along the groove direction, at the same time the drop recoil velocity is reduced by micro-grooves which are parallel to the spreading direction, but not by micro-grooves which are vertical to the spreading direction.     

Non-monotonic dependence of current upon i-width in silicon p–i–n diodes

Silicon p–i–n diodes with different i-region widths are fabricated and tested. It is found that the current shows the non-monotonic behavior as a function of i-region width at a bias voltage of 1.0 V. In this paper, an analytical model is presented to explain the non-monotonic behavior, which mainly takes into account the diffusion current and recombination current contributing to the total current. The calculation results indicate that the concentration ratio of p-region to n-region plays a crucial role in the non-monotonic behavior, and the carrier lifetime also has a great influence on this abnormal phenomenon.     

Molecular dynamics simulation of surface melting behaviours of the V（110） plane

The modified analytic embedded-atom method and molecular dynamics simulations are applied to the investigation of the surface premelting and melting behaviours of the V（110） plane by calculating the interlayer relaxation, the layer structure factor and atomic snapshots in this paper. The results obtained indicate that the premelting phenomenon occurs on the V（110） surface at about 1800K and then a liquid-like layer, which approximately keeps the same thickness up to 2020K, emerges on it. We discover that the temperature 2020K the V（110） surface starts to melt and is in a completely disordered state at the temperature of 2140K under the melting point for the bulk vanadium.     

Wetting failure condition on rough surfaces

Wetting states and processes attract plenty of interest of scientific and industrial societies. Air entrainment, i.e.,wetting failure, on smooth plate in wetting process has been investigated carefully before. Liquid bath entries of "rough"silicon wafers are studied experimentally in the present work, and the air entrainment condition is analyzed specially with the lubrication theory. The roughness effects on the moving contact lines are therefore explored. The contact line pinning is found to be the main reason for the dynamically enhanced hydrophobicity of rough surface, which implies an effective microscopic contact angle of θ_e = θ_Y + 90° where θY is the Young's contact angle of the material. Our results suggest that the solid surfaces can be considered as hydrophobic ones for a wide range of dynamic process, since they are normally rough. The work can also be considered as a starting point for investigating the high-speed advancing of moving contact line on rough surfaces.     

Quartic coupling and its effect on wetting behaviors in nematic liquid crystals

Based on the fact that rubbed groove patterns also affect the anchoring of liquid crystals at substrates,a quartic coupling is included in constructing the surface energy for a liquid crystal cell.The phase diagram and the wetting behaviors of the liquid crystal cell,bounded by surfactant-laden interfaces in a magnetic field perpendicular to the substrate are discussed by taking the quartic coupling into account.The nematic order increases at the surface while it decreases in the bulk as a result of the introduction of quartic substrate-liquid crystal coupling,indicating that the groove anchoring makes the liquid crystal molecules align more orderly near the substrate than away from it.This causes a different wetting behavior:complete wetting.     

Water Gas Shift Reaction： A Monte Carlo Simulation

The water gas shift （WGS） reaction is reacts with water on a catalytic surface a process of industrial importance to form CO2 and H2. We study this In this reaction carbon monoxide reaction with thermal （Langmuir- Hinshelwood） and non-thermal （precursor and Eley-Rideal） reaction mechanisms using the techniques of Monte Carlo computer simulation. The details of surface coverages and production rates are given as a function of CO partial pressure. The diffusion of species on the surface as well as their desorption from the surface is also introduced to include temperature effects. The phase diagrams of the system have been drawn to observe the behaviour of reacting species on the surface. The study reveals that the production rates are higher for non-thermal precursor mechanism and are in agreement with the experimental finding.     

Residual stress induced wetting variation on electric brush-plated Cu film

Nanocrystalline Cu film with a mirror surface finishing is prepared by the electric brush-plating technique. The as- prepared Cu film exhibits a superhydrophilic behavior with an apparent water contact angle smaller than 10°. A subsequent increase in the water contact angle and a final wetting transition from inherent hydrophilicity with water contact angle smaller than 90° to apparent hydrophobicity with water contact angle larger than 90° are observed when the Cu film is subjected to natural aging. Analysis based on the measurement of hardness with nanoindentation and the theory of the bond-order-length-strength correlation reveals that this wetting variation on the Cu film is attributed to the relaxation of residual stress generated during brush-plating deposition and a surface hydrophobization role associated with the broken bond polarization induced by surface nanostructure.