Microscopic spreading of nonreactive perfluoropolyalkylether film on amorphous carbon surfaces

A spreading mechanism of nonfunctional perfluoropolyalkylehter (PFPE) on carbon surfaces is proposed. For the thin thin-film regime, adsorption-desorption is a main driving force for spreading, and the surface diffusion coefficients increase as the film thickness increases. A two-dimensional virial equation is employed to explain the dependency of surface diffusion coefficient on the film thickness. For the thick thin-film regime, the spreading characteristic is determined by the disjoining pressure gradient. We adopt a slip boundary condition to analyze the thick thin-film regime. This modification of the boundary condition reasonably explains the dependence of surface diffusion coefficients on film thickness.     

Synthesis and characterisation of surface gradient thin conversion films on zinc coated steel

Surface gradient layers on hot-dip galvanised steel were synthesised in order to determine the barrier properties and corrosion resistance of thin amorphous conversion coatings as a function of layer thickness and processing time. For this purpose, a dip coating procedure was established that yields well-defined gradient layers. As a model system for conversion film formation on zinc coated steel, a zirconium based bath chemistry was used. The synthesised zirconium oxyhydroxide gradient films were characterised by localised electrochemical techniques, such as Scanning Kelvin Probe (SKP) and electrochemical impedance spectroscopy using an electrochemical capillary cell. Microscopic infrared reflection absorption spectroscopy (μ-FT-IRRAS) measurements and small-spot X-ray photo electron spectroscopy (XPS) were used as complementary surface analytical techniques. The applied analysing techniques provide a spatial resolution of 100-1600 μm. Thereby, a complete variation of thin film properties, such as thickness, barrier properties, corrosion resistance and chemical composition can be measured as function of the time of film growth on a sample with a length of a few centimetres. This approach allows a precise and accurate determination of structure-to-property relationships of thin conversion films. Moreover, it could be shown that a surface gradient film analysis significantly rationalises experimental time and increases the reliability of the experimental results.     

温度和活性剂浓度梯度协同驱动的液滴铺展特性

引言液滴在固体表面上的铺展(spreading)过程是一个学科交叉的研究热点,在镀膜、印刷、采油、矿物浮选、胶片制造、磁流体材料制备以及医疗等领域有着广泛的应用,近些年来开展了大量理论和实验研究工作。     

Finite element modeling of resistive surface layers by micro-contact impedance spectroscopy

Micro-contact impedance spectroscopy (MCIS) is potentially a powerful tool for the exploration of resistive surface layers on top of a conductive bulk or substrate material. MCIS employs micro-contacts in contrast to conventional IS where macroscopic electrodes are used. To extract the conductivity of each region accurately using MCIS requires the data to be corrected for geometry. Using finite element modeling on a system where the resistivity of the surface layer is at least a factor of ten greater than the bulk/substrate, we show how current flows through the two layers using two typical micro-contact configurations. This allows us to establish if and what is the most accurate and reliable method for extracting conductivity values for both regions. For a top circular micro-contact and a full bottom counter electrode, the surface layer conductivity (σ_s) can be accurately extracted using a spreading resistance equation if the thickness is ~10 times the micro-contact radius; however, bulk conductivity (σ_b) values can not be accurately determined. If the contact radius is 10 times the thickness of the resistive surface, a geometrical factor using the micro-contact area provides accurate σ_s values. In this case, a spreading resistance equation also provides a good approximation for σ_b. For two top circular micro-contacts on thin resistive surface layers, the MCIS response from the surface layer is independent of the contact separation; however, the bulk response is dependent on the contact separation and at small separations contact interference occurs. As a consequence, there is not a single ideal experimental setup that works; to obtain accurate σ_s and σ_b values the micro-contact radius, surface layer thickness and the contact separation must all be considered together. Here we provide scenarios where accurate σ_s and σ_b values can be obtained that highlight the importance of experimental design and where appropriate equations can be employed for thin and thick resistive surface layers.     

Development of a model for thin-film stability and spreading in solid–liquid–liquid systems

The presence of thin aqueous films and their stability has a profound effect on reservoir rock–fluids interactions involved in spreading and adhesion. The stability of thin wetting aqueous films on rock surfaces is governed by several variables including pH, brine and crude oil compositions, and capillary pressure. These variables govern the wetting states in the solid–liquid–liquid systems. The wetting states influence the residual oil saturation and the oil-water relative permeabilities and, consequently, the oil recovery. The objective of this study was to deduce a functional dependence of thin-film stability on the above parameters by considering intermolecular and surface interactions in rock–crude oil–brine systems. The surface forces are manifested as disjoining pressure in thin films. The disjoining pressure isotherms for the selected solid–liquid–liquid systems have been computed in terms of the bulk properties of the media. The equilibrium contact angles have also been computed from the integration of the Young–Laplace equation, which relates contact angle to the capillary pressure and disjoining pressure isotherm of the system. The contact-angle data obtained from sessile-drop experiments have been compared with the calculated results, as well as with other published results. Adhesion maps, which relate the film stability to brine pH and molarity, have been developed. The rock–fluids systems considered for this study consisted of smooth glass, quartz and Yates reservoir fluids. The DLVO theory has been used to model the intermolecular forces. The structural forces are incorporated to overcome the limitations of the DLVO theory. A charge regulation model has been used to analyze the crude oil–brine and glass–brine interfaces. The effects of multivalent ions have been incorporated using an equivalent molarity concept. The overall computational model developed in this study is aimed at providing a priori prediction capability of rock-fluids interactions in petroleum reservoirs for inclusion in reservoir simulators.     

分离压/结合压作用下的活性剂液滴演化特征

当含活性剂液膜厚度小于100 nm时,分子间力表现出的分离压或结合压效应将对其演化特征及去润湿特性产生重要影响。针对可溶性活性剂液滴的铺展历程,采用润滑理论建立了液膜厚度、活性剂表面浓度和内部浓度的演化模型,模拟了正负体系中受分离压或结合压影响的液滴演化过程。结果表明,分离压可促进正体系下的液滴稳定铺展,并抑制去润湿现象的发生;而负体系下,分离压则加剧不稳定性,并改变Marangoni负效应对液滴演化的影响。正体系下液滴受结合压作用呈现不稳定特征,在极短时间内发生破断;结合压对负体系下的液滴演化影响与分离压作用相似,具有促进液滴不稳定演化的作用。     

Gas transmission through polymers

Little data is available on gas transmission through thick polymer samples. There is, therefore, a temptation to assume an inverse relationship between gas transmission rate and thickness and to calculate flow through thick samples using permeability data obtained on thin films. This is incorrect, as the calculation assumes steady flow, and with thick samples this state may not be reached for months or even years. Thus both diffusivity and permeability are needed to calculate the gas flow and the calculations are more complex than simple permeability calculations. Because of the long time scale, the effects of varying the thickness are difficult to demonstrate experimentally. We describe a computer simulation of the manometric method which compares the behavior of two materials with similar permeabilities but different diffusivities. Although thin samples behave identically, they differ increasingly with thickness. The pitfalls in using the gradient and time lag of the pressure curve to calculate permeability and diffusivity are also discussed.     

DLVO AND NON-DLVO FORCES IN THIN LIQUID FILMS FROM RHAMNOLIPIDS

Microscopic foam films (r?=?100?μm) stabilized with a single rhamnolipid with a well-known structure (α-L-rhamnopyranosyl-β-hydroxydecanoyl-β-hydroxydecanoate (R1)) are investigated, and the obtained results are compared with results obtained from studies of foam films formed from solutions of rhamnolipid mixtures. The studies are carried out employing the Scheludko-Exerowa microinterferometric method. The dependence of foam film thickness on the electrolyte concentration (C _el ) of the solution is monitored, and formation of common films (CF), common black films (CBF) and Newton black films (NBF) is found. The continuous CBF-to-NBF transition is considered as evidence of the action of repulsive forces that are not described by the classic Derjaguin–Landau–Verwey–Overbeek (DLVO) theory of colloid stability. These non-DLVO repulsive forces lead to an additional positive component of the disjoining pressure. To understand better the surface forces operating in the rhamnolipid foam films, direct measurements of the disjoining pressure/film thickness (Π(h)) isotherms are carried out employing the thin liquid film–pressure balance technique. The comparison of the obtained experimental Π(h) isotherm for CF (C _el ?=?10^?3 mol dm^?3 NaCl) to the DLVO theoretical predictions yields a diffuse electric layer potential of?～?5?mV and surface charge density of?～?50?mC?m^?2 at the film solution–air interfaces. The deviation of the experimental curve from the theoretical one found for films thinner than about 40?nm evidences the action of non-DLVO surface forces. The experimental steplike Π(h) isotherms obtained for the CBF (C _el ?=?0.15?mol?dm^?3 NaCl) are considered to result from an aggregation process, leading to the formation of lamellar structures in the foam film. The obtained results show that the surface forces operative in rhamnolipid foam films are determined not only by the structure and organization of the adsorbed layers but also by the molecular state of the bulk solution.     

MICROSCOPIC SPREADING CHARACTERISTICS OF NONPOLAR PERFLUOROPOLYETHER FILMS

The surface diffusion characteristics of nonpolar perfluoropolyether (PFPE) Z on carbon surfaces are investigated in two regimes, submonolayer and multilayer, for nano-thin films. For the submonolayer regime, the two-dimensional, cubic van der Waals equation of state is applied to determine the dependence of the surface diffusion coefficient on the film thickness, as experimental surface diffusion coefficients increase with increasing film thickness. For the multilayer regime, a conventional fluid mechanics analysis with position dependent viscosity and a van der Waals disjoining pressure gradient is applied to investigate the surface diffusion characteristics. The present theoretical analysis qualitatively agrees with the experimental results.     

Dynamics of Partial Wetting

The kinetics of partial wetting was investigated for the case of spreading of cylindrical and axisymmetric drops over a horizontal solid surface under the action of capillary forces. The Brochard-Wyart and de Gennes (1992) relation, having a cut off of molecular size below which the continuum theory breaks down, is used to provide a dynamic contact angle boundary condition. The dynamic contact angle relation and the viscous dissipation are shown to be valid even in the case of axisymmetric spreading when using the approximation of a wedge flow pattern near the leading edge. To obtain an analytical solution for the dynamics of spreading, the departure of the outer region solution from the spherical cap profile, near the inflection point, was neglected, and the accuracy of the approximation was examined. The analytical solution obtained is a powerseries expression. Simple short-time and long-time asymptotic solutions are also provided. The model for the dynamics of partial wetting does not require any fitting parameter. The results are found to agree favorably with the available published experimental data even at dynamic contact angles as large as about 90°, which is beyond the range of applicability of the lubrication theory. The model matches very closely the dynamics of spreading obtained with a previous model by Chebbi (2010), in which a numerically similarity solution was used to account for the deviation from the spherical cap profile near the inflection point.     

A CONTINUUM ANALYSIS OF SURFACE TENSION IN NONEQUILIBRIUM SYSTEMS

The intermolecular forces that cause surface tension in multiphase systems at equilibrium give rise to pressure gradients in nonequilibrium systems. The present paper treats such systems within a framework of thermodynamics and continuum mechanics and uses a generalization of the conceptual experiment of Rowlinson and Widom which applies to systems with curved interfaces and to nonequilibrium situations where the phase interfaces are not fully developed. The pressure tensor has a component p_n acting in the direction normal to the concentration gradient and a component p_t acting in the plane tangent to the surface of constant concentration where

P_n —P_t = k(∇c)² The concentration gradient causes an additional volumetric force not present in homogeneous fluids

Here, k.- is the gradient energy parameter appearing in the Landau-Ginzburg functional, c is the concentration of the key component, ₁, and R₂ are the principal radii of curvature for the surface of constant constration, n is a unit vector normal to that surface, and ∇, is the gradient along the surface. The volumetric force generates pressure gradients in systems with curved interfaces at equilibrium or can drive flow in nonequilibrium situations.     

SURFACE TENSION RETARDATION OF MARANGONI WAVES

When a chemical reaction creates, in a system with a fluid interface, a concentration pattern of a surface active species, a hydrodynamic structure can develop as a result of Marangoni tractions derived from the concentration waveform. This concept was demonstrated by Dagan and Pismen (1984) who showed that a bistable reaction which produces a solitary, propagating chemodiffusive wave can induce a corresponding hydrodynamic wave on the surface of a thin film. The object of this paper is to study the influence of surface tension on the propagating wave, an effect which was not considered by Dagan and Pismen. Results indicate that the surface deflection which accompanies the hydrodynamic wave is diminished as the surface tension is increased. In addition, it is shown that surface tension acts to slow the propagating wave down. This behavior is attributed to an adverse pressure gradient which develops in a direction opposite to the Marangoni flow.     

注塑充填过程变厚度截面速度压力场数值模拟

采用有限元法,对注塑充填过程中“凸”型和“刀”型塑件截面的速度场和压力场进行数值模拟。通过结果分析得出：广义Hele-Shaw流动假设在熔体前沿部分不合理,在塑件截面突变处Hele-Shaw假设也失效,尤其在突变的较宽截面处误差更大。     

Probing of Thin Slipping Films by Persistent External Disturbances

This paper investigates the propagation of thickness disturbances on the free surface of a thin viscous liquid film on a solid substrate. On the free surface of the film the disturbances are induced by moving local external pressure perturbations acting on the surface. The analysis is performed by the Fourier‐Laplace transform applied to the linearized perturbation equations for small amplitudes. The amplitude of the interface deflection caused by the disturbance, is reconstructed by the inverse Fourier‐Laplace transform and numerically evaluated in the long time limit in long wave approximation. The proposed technique appears promising for probing the slip length of a thin film by recording its free surface response to a moving perturbation.     

One-step Sinter-HIP method for preparation of functionally graded cemented carbide with ultrafine grains

Ultrafine crystalline functionally graded cemented carbides (FGCCs) with a surface zone enriched in binder phase were prepared by a one-step Sinter-HIP method. The influence of sintering pressure and cubic carbide composition on the formation of gradient layer was examined. The results show that the ultrafine FGCC with surface zone enriched in binder phase can be formed by the one-step Sinter-HIP method. The process of the gradient layer formation is accelerated under higher sintering pressure; the gradient layer thickness increases with the sintering pressure increasing. The gradient layer thickness is controlled by diffusion distance of cubic carbide formers, such as Ti, Ta and Nb. The addition of (Ta,Nb)C leads to decrease the thickness of gradient layer.     

Modeling and analysis of thickness gradient and variations in vacuum‐assisted resin transfer molding process

As vacuum‐assisted resin transfer molding (VARTM) is being increasingly used in aerospace applications, the thickness gradient and variation issues are gaining more attention. Typically, thickness gradient and variations result from the infusion pressure gradient during the process and material variations. Pressure gradient is the driving force for resin flow and the main source of thickness variation. After infusion, an amount of pressure gradient is frozen into the preform, which primarily contributes to the thickness variation. This study investigates the mechanism of the thickness variation dynamic change during the infusion and relaxing/curing processes. A numerical model was developed to track the thickness change of the bagging film free surface. A time‐dependent permeability model as a function of compaction pressure was incorporated into an existing resin transfer molding (RTM) code for obtaining the initial conditions for relaxing/curing process. Control volume (CV) and volume of fluid (VOF) methods were combined to solve the free surface problem. Experiments were conducted to verify the simulation results. The proposed model was illustrated with a relatively complex part. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Observation of Cell Structures Developed by Marangoni Convection in Glass Thick Films

Cell structures developed by convection in glass thick films have been studied with special emphasis on the effects of material parameters and heating conditions on the cell structure. Borosilicate glass films from 10 to 30 μm thick were prepared on alumina substrates by printing and firing at temperatures between 750° and 950°C up to 2 h, and surface flow patterns were observed with an optical microscope under dark-field illumination. In most cases a flow pattern developed and changed with time, finally reaching a steady state. Cell structures with regular polygons of four to seven sides from 15 to 150 μm across developed, depending on the glass composition. The effects of heating time, temperature, film thickness, and glass chemistry on the cell structure have been examined. From observations of cell structures, the driving force for the convection in glass thick films has been identified to be the gradient in surface tension due to small temperature fluctuations on the surface. Hence, it is concluded that the convection in glass thick films is of Marangoni type.     

PREDICTION OF MEMBRANE SEPARATION CHARACTERISTICS BY PORE DISTRIBUTION MEASUREMENTS AND SURFACE FORCE-PORE FLOW MODEL

Pore size distribution of polyamide thin film composite reverse osmosis membranes and a cellulosic ultrafiltration membrane were obtained from vapor adsorption data of CO₂ and N₂ gases. The surface: force-pore flow model previously reported in the literature was utilized in this work. The pore distribution data were further used with solutes separation data at a particular pressure to obtain the values of solute-solvent-membrane wall forces involved. The interaction parameters were obtained by the simultaneous solution of the ordinary differential equations describing the model using the software package COLSYS. From the knowledge of the pore distribution data and the interactions forces, the solutes separation data (for nonionized organics and sodium chloride) were predicted over a wide range of pressures and showed excellent agreement with experimental separation data.     

HYDRODYNAMIC MODEL AND EXPERIMENTS FOR CROSSFLOW MICROFILTRATION

Crossflow microfiltration, in which a suspension is passed through a pressurized open-ended tube or channel having microporous membrane walls, is an effective means of filtering fine particles from a liquid and is finding increasing application in separations involving microbial suspensions and products. The particles, which are carried toward the walls with the filtrate cross-flow, form a thin cake layer on the membrane surface which does not accumulate substantially but is rather swept along the channel by the tangential flow of suspension. This paper presents a stratified-flow model of this phenomenon which predicts the steady-state permeation flux, and the velocity, pressure, and concentrated particle layer thickness profiles, as functions of the system parameters. In addition, the results of laboratory experiments which used a crossflow microfiltration channel with glass sides are reported. The measured steady-state thickness of the cake layer as a function of distance from the channel entrance shows good agreement with the theory, except for the case of a relatively thick layer when it is believed that a stagnant sublayer had formed beneath the flowing cake layer.     

Nonisothermal analysis of a couple stress fluid in blade coating process

In this paper, a nonisothermal analysis for the process of blade coating of an incompressible couple stress fluid is presented using both plane and exponential coaters. The governing system is simplified using lubrication approximation theory (LAT). The interesting quantities from engineering point of view like normalized pressure, maximum pressure, pressure gradient, velocity, and effects of involved parameters on temperature distribution, which influence the coating process are evaluated. It is observed that pressure is at maximum near the edge of the blade whereas fluid velocity and temperature are at maximum near the substrate. An increasing couple stress parameter increases the load and decreases the coating thickness. It is worth mentioning that load and pressure are the most significant outcomes of the present exertion as these two physical quantities ensure thickness and the quality of coating.