受限空间内油水界面动态特性及粘附行为的荧光原位观测

水污染作为润滑油污染的常见形式,对润滑油本身以及机械系统都有巨大的危害。为了模拟实际非均匀多相系统中的界面行为,本文搭建了高精度点接触实验台来研究传统的不溶相替换问题。将目前静态平行受限空间内油水界面行为的研究推广到动态点接触楔形受限空间内,探究了游离水滴穿过点接触狭缝间毛细油池过程中的界面特性。重点关注固壁润湿性以及固壁的分离运动对整个侵入过程中液滴动态行为的影响。实验发现了铺展系数是决定油水界面融合和分离特性的关键因素,揭示了固壁润湿性和球盘间的相对分离运动会影响游离水滴穿过毛细油池之后的粘附行为。表面张力和液体与壁面之间的粘附功能够解释观测的实验现象。     

Structure of classical fluid of charged hard spheres and point lons in the perturbation of a small non-responsive,uniform background density

Abstract

In liquid Te, the model of Cabane and Friedel is based on the coexistence of a network of strong covalent bonds with a metallic-like electron gas. We have attempted to simulate the gross features of this situation by studying a classical fluid of charged hard spheres, plus point charges (classical electrons!), all moving in a small neutralizing negative background density.

An exact solution of even this model will require computer simulation. We have therefore worked in the approximation of the mean spherical model. Also, we treat the neutralizing background only to first order in the background density.

The main effect of the background is to increase the height of the contact value of g(r) for the hard sphere pair correlations. This appears to correspond qualitatively with the high temperature behaviour of liquid Te.     

On the equilibrium calculation of the friction coefficient for liquid slip against a wall

We demonstrate that the linear response theory of interface friction presented by Bocquet and Barrat [Phys. Rev. E 49, 3079 (1994)] results in a friction coefficient that is not an intrinsic property of the interface and thus does not correspond to the actual interfacial friction coefficient. We point out that this previous derivation includes an unsubstantiated identification of the velocity field in the nonuniform system with the perturbation applied to the equations of the motion. We present an alternative equilibrium theory of the friction associated with the confined fluid and show how this friction is related to the intrinsic interfacial friction.     

烷基苯磺酸盐在油水界面行为的介观模拟

采用耗散颗粒动力学(DPD)方法在介观层次上模拟了表面活性剂烷基苯磺酸盐在油/水界面的排布行为, 考察了分子结构、浓度、盐度、油相等因素对表面活性剂界面密度和界面效率的影响, 并探讨了利用表面活性剂复配协同效应提高界面活性的理论机制. 分子模拟给出的分子水平的微观信息为强化采油技术中配方筛选和表面活性剂的有效应用提供指导.     

Structure of a dipolar fluid in contact with a solid: Adsorption and surface potential

The structure of a dipolar fluid in contact with a solid is investigated using the optimized cluster theory. The solid-liquid interaction is described by an effective one-dimensional potential. The model of a perfect impenetrable wall is dropped. An adsorption potential similar to the non-electrostatic potential appearing in the classical models of the double layer is introduced. We show that the effect of a reasonable adsorption potential is localized in the first monolayer in contact with the solid although the total effect of the solid is non localized in this monolayer. No states of orientation predominate in such a way that we can justify a two- or three-state model. The potential drop g^s(dip) across the interface and the change in the surface Gibbs energy due to the adsorption potential are calculated. An adsorption potential of magnitude 1kT gives g^s(dip) - ?0.6 V. In order to obtain the values of g^s(dip) generally accepted in the literature, no dielectric constant or clusters should be introduced. Because of the competition between the dipolar interaction and the adsorption potential, an increase of the dipole moment does not necessarily increase g^s(dip).     

Contact line motion in confined liquid-gas systems: Slip versus phase transition

In two-phase flows, the interface intervening between the two fluid phases intersects the solid wall at the contact line. A classical problem in continuum fluid mechanics is the incompatibility between the moving contact line and the no-slip boundary condition, as the latter leads to a nonintegrable stress singularity. Recently, various diffuse-interface models have been proposed to explain the contact line motion using mechanisms missing from the sharp-interface treatments in fluid mechanics. In one-component two-phase (liquid-gas) systems, the contact line can move through the mass transport across the interface while in two-component (binary) fluids, the contact line can move through diffusive transport across the interface. While these mechanisms alone suffice to remove the stress singularity, the role of fluid slip at solid surface needs to be taken into account as well. In this paper, we apply the diffuse-interface modeling to the study of contact line motion in one-component liquid-gas systems, with the fluid slip fully taken into account. The dynamic van der Waals theory has been presented for one-component fluids, capable of describing the two-phase hydrodynamics involving the liquid-gas transition [A. Onuki, Phys. Rev. E 75, 036304 (2007)]. This theory assumes the local equilibrium condition at the solid surface for density and also the no-slip boundary condition for velocity. We use its hydrodynamic equations to describe the continuum hydrodynamics in the bulk region and derive the more general boundary conditions by introducing additional dissipative processes at the fluid-solid interface. The positive definiteness of entropy production rate is the guiding principle of our derivation. Numerical simulations based on a finite-difference algorithm have been carried out to investigate the dynamic effects of the newly derived boundary conditions, showing that the contact line can move through both phase transition and slip, with their relative contributions determined by a competition between the two coexisting mechanisms in terms of entropy production. At temperatures very close to the critical temperature, the phase transition is the dominant mechanism, for the liquid-gas interface is wide and the density ratio is close to 1. At low temperatures, the slip effect shows up as the slip length is gradually increased. The observed competition can be interpreted by the Onsager principle of minimum entropy production.     

On the Degree to which the Contact Angle is Affected by the Adsorption onto a Solid Surface of Vapor Molecules Originating from the Liquid Drop

ABSTRACT

From surface tensions of liquids and Lifshitz-van der Waals (LW) and Lewis acid-base (AB) surface tension components and the AB electron-acceptor γ⁺ and electron-donor γ^˙ parameters determined by contact angle (θ) measurements (using the Young-Dupré equation for polar systems), the interfacial work of salvation (W_st) between various contact angle liquids (L) and a moderately polar solid (S), such as polymethylmethacrylate (PMMA) could be determined. From these W_SL -values the maximum values of the equilibrium association constant, K_a, are obtained for the adsorption of molecules of liquids, L, onto a solid substratum, S. From the K_a-values and the vapor pressures of the various liquids, the maximum number of liquid molecules adsorbed from the gaseous phase onto the solid surface can be determined, at 20^°C and 76cm Hg ambient atmospheric pressure. This yields the maximum value for the fraction, ?, of the surface area of the solid that will be covered by molecules of the liquid, L, emanating from the liquid drop, via the gaseous state. From these ?-values, using Cassie's approach, the maximum amount, Δθ, can be determined by which the observed contact angle is lower than the ideal contact angle, as a consequence of the coverage of the solid substratum by adsorbed molecules originating from the contact angle liquid.

For most of the contact angle liquids used, the maximum deviation, Δθ, is well under 1^°; for water on PMMA it is about 1½^°.     

The surface tension of a solid at the solid-vacuum interface, an evaluation from adsorption and wall potential calculations

A method for the evaluation of quantities that are experimentally inaccessible such as the surface tension at the solid-vacuum interface and the superficial tension of the fluid in contact with the solid is presented. The approach is based on consideration of an equilibrium of a fluid in solid capillary wherein a balance between surface and capillary forces has been replaced by conceptual alternative interfacial and centrifugal forces. This approach involves the simultaneous numerical solution one the special forms of the Gibbs equation for solid-fluid interface and a generalized Kelvin equation derived earlier. The latter equation takes into account interactions between the solid thick cylindrical wall and confined fluid, this body-body interaction potential has been primarily calculated using the Lennard-Jones (6-12) expression for the atom-atom pair potentials and expressed by hypergeometrical functions having good convergences. All numerical calculations shown here have been performed for the model graphite-argon system at 90 K. Finally, an analysis of the accuracy of the proposed method is considered.     

Contact Angle Measurement for Dispersed Microspheres Using Scanning Confocal Microscopy

Abstract

Scanning confocal microscopy was used for contact angle measurement of individual microspheres. The measurements were carried out by using different laser‐scanned layers of the particle floating on the air–water interface. The ratio of the diameter for the cross‐section of the protruded area of the particle at the air–water interface to the actual diameter of the particle is used for contact angle measurements. Two systems, i.e., glass and polystyrene microspheres with diameters of 3–10 and 6 µm, respectively, with water were used for this investigation (this size range of particles are most relevant to inhalation applications). Using the developed methodology, contact angles of 27° and 41° were measured (with water) for glass and polystyrene particles, respectively. The theoretical error in contact angle measurement for the developed methodology is determined to be generally about 1° with a maximum of 3° for contact angle of particles ranging from 2 to 24 µm in size; the experimental error was 4–6°. The contact angles of glass and polystyrene particles were compared to those obtained from pendant drop method and confirmed.     

Electroosmotic flow in a water column surrounded by an immiscible liquid

In this paper, we conducted numerical simulation of the electroosmotic flow in a column of an aqueous solution surrounded by an immiscible liquid. While governing equations in this case are the same as that in the electroosmotic flow through a microchannel with solid walls, the main difference is the types of interfacial boundary conditions. The effects of electric double layer (EDL) and surface charge (SC) are considered to apply the most realistic model for the velocity boundary condition at the interface of the two fluids. Effects on the flow field of ?-potential and viscosity ratio of the two fluids were investigated. Similar to the electroosmotic flow in microchannels, an approximately flat velocity profile exists in the aqueous solution. In the immiscible fluid phase, the velocity decreases to zero from the interface toward the immiscible fluid phase. The velocity in both phases increases with ?-potential at the interface of the two fluids. The higher values of ?-potential also increase the slip velocity at the interface of the two fluids. For the same applied electric field and the same ?-potential at the interface of the two fluids, the more viscous immiscible fluid, the slower the system moves. The viscosity of the immiscible fluid phase also affects the flatness of the velocity profile in the aqueous solution.     

Graphitic Carbon Nitride (g‐C3N4): An Interface Enabler for Solid‐State Lithium Metal Batteries

Solid‐state Li metal batteries (SSLMBs) have attracted considerable interests due to their promising energy density as well as high safety. However, the realization of a well‐matched Li metal/solid‐state electrolyte (SSE) interface remains challenging. Herein, we report g‐C₃N₄ as a new interface enabler. We discover that introducing g‐C₃N₄ into Li metal can not only convert the Li metal/garnet‐type SSE interface from point contact to intimate contact but also greatly enhance the capability to suppress the dendritic Li formation because of the greatly enhanced viscosity, decreased surface tension of molten Li, and the in situ formation of Li₃N at the interface. Thus, the resulting Li‐C₃N₄|SSE|Li‐C₃N₄ symmetric cell gives a significantly low interfacial resistance of 11 Ω cm² and a high critical current density (CCD) of 1500 μA cm^?2. In contrast, the same symmetric cell configuration with pristine Li metal electrodes has a much larger interfacial resistance (428 Ω cm²) and a much lower CCD (50 μA cm^?2).     

Thermodynamic Evaluation of Solid-State Reactions in which a Volatile Product is Formed

Summary. The use of equilibrium thermodynamics in describing interfacial reactions between non-ionic inorganic solids is demonstrated using examples of high-temperature interactions in the Ti–Si–N and Mo–Si–N systems. In the case of a diffusion-controlled process, solid-state reactions can be interpreted with chemical potential (activity) diagrams. The role of volatile reaction products formed during interaction in developing the diffusion zone morphology is analysed. The interfacial phenomena in systems based on dense Si₃N₄ and non-nitride forming metals can be explained by assuming a nitrogen pressure build-up at the contact surface. This pressure determines the chemical potential of Si at the interface and, hence, the reaction products in the diffusion zone.     

Thermodynamic Evaluation of Solid-State Reactions in which a Volatile Product is Formed

The use of equilibrium thermodynamics in describing interfacial reactions between non-ionic inorganic solids is demonstrated using examples of high-temperature interactions in the Ti–Si–N and Mo–Si–N systems. In the case of a diffusion-controlled process, solid-state reactions can be interpreted with chemical potential (activity) diagrams. The role of volatile reaction products formed during interaction in developing the diffusion zone morphology is analysed. The interfacial phenomena in systems based on dense Si₃N₄ and non-nitride forming metals can be explained by assuming a nitrogen pressure build-up at the contact surface. This pressure determines the chemical potential of Si at the interface and, hence, the reaction products in the diffusion zone.     

Interfacial slip friction at a fluid-solid cylindrical boundary

Recently we proposed a method to calculate the interfacial friction coefficient between fluid and solid at a planar interface. In this work we extend the method to cylindrical systems where the friction coefficient is curvature dependent. We apply the method to methane flow in carbon nanotubes, and find good agreement with non-equilibrium molecular dynamics simulations. The proposed method is robust, general, and can be used to predict the slip for cylindrical nanofluidic systems.     

On the Stagnation Point Flow of a Special Class of Non-Newtonian Fluids

Abstract

The two-dimensional boundary layer equations for a class of non-Newtonian fluids, for which the apparent viscosity can be expressed as a polynomial in the second scalar invariant of the rate of strain tensor, have been derived. These equations have been employed to analyse the flow near a stagnation point over a stationary impermeable wall. The non-Newtonian effects on the boundary layer velocity profile and the wall skin friction have been studied, and compared with the corresponding Newtonian fluid. The fluid velocity in the boundary layer has been shown to be retarded by the non-Newtonian effect while the skin friction increases proportionate to it.     

Hard and Soft - Core Equations of State for Simple Fluids: VI. General Theory of Termination Temperatures,and a Validity Criterion for the Second Virial Coefficient

Abstract

A general proof is given that the classical second virial coefficient satisfies the requirement for the non-existence of a termination point of any locus of C_v extrema. This validity criterion is applied to some proposed forms for the second virial coefficient. The order of the termination temperatures is verified for a fairly general intermolecular potential. In particular a proof is given that T_F lies between T_C and T_A. Also the hard-core limit of the ratio T_D/T_A(～2) is examined briefly.     

Interfacial stabilization of organic-aqueous two-phase microflows for a miniaturized DNA extraction module

Organic-aqueous liquid (phenol) extraction is one of many standard techniques to efficiently purify DNA directly from cells. The cell components naturally distribute themselves into the two fluid phases in order to minimize interaction energies of the biological components with the surrounding solvents. The membrane components and protein partition to the interface between the organic and aqueous phases while the DNA stays in the aqueous phase. The aqueous phase is then removed with a purified DNA sample. This work studies the first steps towards miniaturizing this liquid extraction technique in a microfluidic device. The first step is to understand how the two liquid phases behave in microchannels. Due to the interfacial tension between the two liquid phases, novel approaches must be examined in order to obtain interfacial stability under flow conditions. The stability of the organic-aqueous interface is improved by reducing the interfacial tension between the two phases by incorporating a surfactant into the aqueous phase. The variation of the interfacial tension as a function of surfactant concentration is also quantified in this work. This has led to the ability to create stable stratified microflows in both a dual inlet and three inlet microfluidic systems. Also, the first step in understanding biological interactions at the organic-aqueous interface is investigated using a fluorescently labeled bovine serum albumin protein.     

Computation of interfacial properties via grand canonical transition matrix Monte Carlo simulation

We examine two free-energy-based methods for studying the wetting properties of a fluid in contact with a solid substrate. Application of the first approach involves examination of the adsorption behavior of a fluid at a single substrate, while the second technique requires investigation of the properties of a system confined between two parallel substrates. Both of the techniques rely upon computation and analysis of the density dependence of a system's surface free energy and provide the contact angle and solid-vapor and solid-liquid interfacial tensions for substrate-fluid combinations within the partial wetting regime. Grand canonical transition matrix Monte Carlo simulation is used to obtain the required free-energy curves. The methods examined within this work are general and are applicable to a wide range of molecular systems. We probe the performance of the methods by computing the interfacial properties for two systems in which an atomistic fluid interacts with a fcc crystal. For both of the systems studied we find good agreement between our results and those obtained via the mechanical definition of the interfacial tension.     

Dilational viscoelasticity of anionic polyelectrolyte/surfactant adsorption films at the water–octane interface

In this article, the interfacial tension and interfacial dilational viscoelasticity of polystyrene sulfonate/surfactant adsorption films at the water–octane interface have been studied by spinning drop method and oscillating barriers method respectively. The experimental results show that different interfacial behaviors can be observed in different type of polyelectrolyte/surfactant systems. Polystyrene sulfonate sodium (PSS)/cationic surfactant hexadecanetrimethyl–ammonium bromide systems show the classical behavior of oppositely charged polyelectrolyte/surfactant systems and can be explained well by electrostatic interaction. In the case of PSS/anionic surfactant sodium dodecyl sulfate (SDS) systems, the coadsorption of PSS at interface through hydrophobic interaction with alkyl chain of SDS leads to the increase of interfacial tension and the decrease of dilational elasticity. For PSS/nonionic surfactant TX100 systems, PSS may form a sub-layer contiguous to the aqueous phase with partly hydrophobic polyoxyethylene chain of TX100, which has little effect on the TX100 adsorption film and interfacial tension.