Determination of the surface properties and adsorption states of nanoporous materials using the zeta adsorption isotherm.

The adsorption process of porous materials has always been a popular field of research in interfacial physics, and the surface physical parameters of materials can be obtained from their adsorption characteristics, which has a great influence on the performance of materials. Based on the zeta adsorption isotherm, we propose a method based on the zeta adsorption isotherm to predict the entire adsorption process of porous materials and determine material surface properties from the measured isotherm data in the heterogeneity-free range. We applied the zeta constants of the silica adsorption system to the corresponding adsorption isotherm of the porous material. The results showed that the predicted adsorption isotherms are in good agreement with the experimental measurements before pore filling and can effectively identify the pressure ratios at the beginning and end of pore filling. In the region of high-pressure ratios, the Kelvin equation was utilized to calculate the pressure ratio at a contact angle of 0°. The surface parameters of the materials were determined by geometrically calculating the variation of the adsorption amount and the desorption isotherms in the high-pressure ratio range were calculated from these surface parameters. The predicted desorption isotherms can well reflect the adsorption process of silica porous materials in the region of a high-pressure ratio. In addition, for the surface parameters of the materials, the specific surface area calculated from the adsorption and desorption isotherms, respectively, differed by less than 7.9%, and the reliability of the method was verified by comparing the results with those of the argon adsorption systems.

Atomistic investigation on the kinetic behavior of vapour adsorption and cluster evolution using a statistical rate theory approach.

The kinetic behavior of vapor adsorption on a solid surface in an isobaric-isothermal system is investigated by means of molecular dynamics simulations combined with theoretical studies through a statistical rate theory approach. The molecular insights into the formation and evolution of clusters in the adsorbate are presented. Results show that the argon vapor is adsorbed on the silicon surface as different types of clusters. In the initial stage of adsorption, the empty adsorption sites on the surface decrease, and the adsorbed single-molecule-cluster grows rapidly and dominates the interface. The increasing rate of the adsorbed cluster and the declining rate of the empty adsorption site are dependent on the pressure ratio. For a large pressure ratio, the single-molecule-clusters are aggregated to incubate large clusters, and the fraction of a single-molecule-cluster is decreased with time. When the adsorption isotherm is determined, the chemical potential of the adsorbed cluster is expressed from the zeta isotherm model. Then the adsorption kinetics are analyzed through the statistical rate theory. The molecular exchange rate and the instantaneous driving force are calculated. The higher pressure ratio induces the larger chemical potential difference and accelerates the net adsorption rate. The adsorption kinetics derived from MD simulations are in close agreement with the theoretical analysis of the statistical rate theory.

Study on Evaporation Characteristics of Water in Annular Liquid Pool at Low Pressures.

In order to investigate the energy transfer mechanism and the nonequilibrium effect during water evaporation in its own pure vapor at low pressures, a series of precise measurements are conducted to obtain the temperature profile near the liquid-vapor interface and the evaporation rates in an annular pool in a closed chamber. The results show that the interface temperature of the vapor side is higher than that of the liquid side when water evaporates in its own pure vapor at low pressures (ranging from 394 to 1467 Pa), the temperature discontinuity across the interface exists in all experimental conditions. The magnitude of the temperature discontinuity is strongly affected by the vapor pressure. A uniform temperature layer with a thickness of about 2 mm is found below the evaporating interface because of the coupling effect of evaporation cooling and thermocapillary convection. The energy required for evaporation is mainly transferred by thermocapillary convection in the uniform temperature layer. Furthermore, the numerical simulation results confirm that the evaporation flux near the cylinders is much larger than that at the middle region, which implies that most of the latent heat required for evaporation is transferred to the interface near the cylinders.

Molecular insight into the formation of adsorption clusters based on the zeta isotherm.

This work presents a series of molecular dynamics simulations of argon adsorption on a silicon substrate with different lattice orientations. From the simulation results, the density profiles are discussed and the amount of adsorbed particles is obtained at different pressures. It is found that the solid surface orientation has a great influence on the density distributions and atomic arrangements near the surface. With the collected data, the thermal constants derived from the expression of zeta adsorption isotherms are determined. The calculated isotherms agree well with the simulation results. Also, from a microscopic point of view, the molecular insights show that the structures of the adsorbates are present as clusters with different numbers of particles. The size of the clusters changes with pressure. At a relatively small pressure ratio, most of the clusters consist of a single molecule. As the pressure ratio increases, larger sized clusters appear, forming various cluster-types. The molecular cluster distributions are closely consistent with the basic approximation of the zeta adsorption isotherm. Furthermore, the surface adsorption sites determined from molecular dynamics simulation show good agreement with that predicted by the zeta isotherm model, which reaffirms the effectiveness of the theoretical model. When the isotherm is extended to a pressure ratio greater than unity, a finite amount of adsorption is predicted and the wetting conditions are obtained. Affected by the solid surface orientations, the pressure ratio at wetting for the silicon substrate with the (111) surface plane is larger than those of the (100) and (110) surfaces, indicating that a higher subcooling is required for the wetting transition.

Molecular Dynamics Simulation of Heat Transport through Solid-Liquid Interface during Argon Droplet Evaporation on Heated Substrates.

This paper presents a series of molecular dynamics simulations of the evaporating process of an argon droplet on heated substrates and the energy transport mechanism through the solid-liquid interface. Results indicate that the mass density through the liquid-vapor interface decreases sharply when the evaporation is in the steady state. Meanwhile, there is an adsorption layer in the form of clusters at the solid-liquid interface, which has a higher mass density than the droplet inside. Furthermore, the wetting property of the solid substrate is related to the system's initial temperature and the solid-liquid potential energy parameter. The contact angle decreases with the increase of initial temperature and solid-liquid potential energy parameter. During the accelerated evaporation process, small part of energy transports into the liquid in the perpendicular direction to the solid-liquid interface and most of the energy transports along the parallel direction to the solid-liquid interface in the adsorption layer to the three-phase contact line. The heat-transfer process from the solid substrate to the droplet inside is hindered by the Kapitza resistance at the solid-liquid interface, no matter the solid substrate is hydrophilic or hydrophobic. Meanwhile, the Kapitza resistance gradually increases with the increase of the initial temperature and decreases with the increase of the solid-liquid energy parameter.

Pattern formation of Rayleigh-Bénard convection of cold water near its density maximum in a vertical cylindrical container.

In order to understand the onset of convective instability and multiple stable convection patterns of buoyancy-driven convection of cold water near its density maximum in a vertical cylindrical container heated from below, a series of three-dimensional numerical simulations were performed. The aspect ratio of the container was 2 and Prandtl number of cold water was 11.57. The sidewall was considered to be perfectly adiabatic, and the density inversion parameter was fixed at 0.3. The result shows that the density inversion phenomenon in cold water has an important effect on the critical Rayleigh number for the onset of convection and the pattern formation at higher Rayleigh numbers. When the Rayleigh number varies from 3×10(3) to 1.2×10(5), eight stable, steady convection patterns are obtained under different initial conditions. The coexistence of multiple stable steady flow patterns is also observed within some specific ranges of the Rayleigh number.