Nanofluidics: viscous dissipation in layered liquid films

We studied the layer-by-layer collapse of molecularly thin films of a model lubricant confined between two atomically smooth substrates. The dynamics of the consecutive expulsion of four molecular layers were found to slow down with decreasing film thickness but showed no evidence for confinement-induced solidification. Using a hydrodynamic model, we show that the sliding friction of liquid layers on top of the solid substrates is approximately 18 times higher than the mutual friction between adjacent liquid layers. The latter was independent of film thickness and in close agreement with the bulk viscosity.     

Dynamics of layering transitions in confined liquids

Multiple beam interferometry and video microscopy were used to investigate the layering transition of thin liquid films of 1-undecanol confined between atomically smooth mica surfaces. The expulsion of a molecularly thin lubricant layer was followed directly in two dimensions. Overall, the dynamics of the transition follows theoretical predictions based on two-dimensional hydrodynamics. Frequently, pockets of liquid remain trapped inside the contact area at the end of the transition. The trapped pockets undergo shape transformations to minimize elastic and interfacial energy.     

Effect of solid surface on the formation of thin confined lubricating film of water with micro-content of oil

The formation of confined film between two contacting surfaces is significant for evaluating the lubricating ability of liquid. A micro-content of oil in water was experimentally demonstrated to be significantly effective to the film formation of water, which was much thicker than predicted by elastohydrodynamic lubrication theory. The effect of solid surface characteristics on the liquid film confined in a nanogap has been investigated. The film forming performances of such films were presented. The work of adhesion between two different phases was calculated, and the competitive wetting behaviours of water and oil on different solid surfaces were employed to understand the film formation mechanism.     

Thin liquid layers in shear: non-Newtonian effects

When shear flow is generated in molecularly thin liquid films of simple liquids confined between two parallel plates, the effective viscosity of the liquid increases by many orders of magnitude compared to its bulk value. Non-Newtonian effects such as shear thinning with a universal power law exponent of are observed in experiments and computer simulations. We present a simple model of these phenomena based on shear melting of solid-like layers induced by the strong coupling with the crystalline walls.     

Lubrication by molecularly thin water films confined between nanostructured membranes

We use molecular dynamics simulations to study thermal sliding of two nanostructured surfaces separated by nanoscale water films. We find that friction at molecular separations is determined primarily by the effective free energy landscape for motion in the plane of sliding, which depends sensitively on the surface character and the molecular structure of the confined water. Small changes in the surface nanostructure can have dramatic effects on the apparent rheology. Whereas porous and molecularly rough interfaces of open carbon nanotube membranes are found to glide with little friction, a comparably smooth interface of end-capped nanotubes is effectively stuck. The addition of salt to the water layer is found to reduce the sliding friction. Surprisingly, the intervening layers of water remain fluid in all cases, even in the case of high apparent friction between the two membranes.     

Transverse shear oscillator investigation of boundary lubrication in weakly adhered films

We investigate the boundary lubrication in weakly adhered molecularly thin films deposited between a sphere and a plane, below the sliding threshold. The shear contact stiffness and interfacial dissipation at the micrometer scale are determined with a high-frequency quartz oscillator. Two distinct behaviors are found as a function of the shear oscillation: a linear viscoelastic response at low amplitude and a nonlinear frictional microslip at high amplitude. A friction model is proposed to analyze the data, which allows evaluating the shear strength, the friction coefficient, and the interfacial viscosity at different solid interfaces under low load.     

Frictional dissipation in stick-slip sliding

The time variation of the frictional force between two surfaces, undergoing stick-slip sliding across a molecularly thin film of a confined model liquid, was examined at high time and force resolution, showing clearly that dissipation of energy occurs both during the slip, and at the instant of stick (via transfer of residual momentum). Detailed analysis indicates that, in marked contrast to earlier suggestions, of order 90% or more of the dissipation occurs by viscous heating of the confined shear-melted film during the slip, and only a small fraction of the energy is dissipated at the instant of stick.     

Spreading and two-dimensional mobility of long-chain alkanes at solid/gas interfaces

Long-chain n alkanes on solid surfaces can form partially wetting liquid alkane droplets coexisting with solid multilayer terraces. We propose a diffusivelike alkane flow between terrace edge and droplet perimeter through a molecularly thin "precursorlike" film. Depending on the (uniform!) sample temperature, either droplet or terrace edge are not in thermodynamic equilibrium. This leads to a chemical potential gradient, which drives the reversible alkane flow. The gradient can be adjusted and calculated independently from the phenomenological diffusion coefficient.     

Slippage and nanorheology of thin liquid polymer films

Thin liquid films on surfaces are part of our everyday life; they serve, e.g.,?as coatings or lubricants. The stability of a thin layer is governed by interfacial forces, described by the effective interface potential, and has been subject of many studies in recent decades. In recent years, the dynamics of thin liquid films has come into focus since results on the reduction of the glass transition temperature raised new questions on the behavior of especially polymeric liquids in confined geometries. The new focus was fired by theoretical models that proposed significant implication of the boundary condition at the solid/liquid interface on the dynamics of dewetting and the form of a liquid front. Our study reflects these recent developments and adds new experimental data to corroborate the theoretical models. To probe the solid/liquid boundary condition experimentally, different methods are possible, each bearing advantages and disadvantages, which will be discussed. Studying liquid flow on a variety of different substrates entails a view on the direct implications of the substrate. The experimental focus of this study is the variation of the polymer chain length; the results demonstrate that inter-chain entanglements and in particular their density close to the interface, originating from non-bulk conformations, govern the liquid slip of a polymer.     

Thickness transitions in thin films: The equilibrium

The thermodynamic and thickness equilibria of planar solid, liquid or gaseous thin films are described in terms of the film excess chemical potential μ_ex. A general formula is given which relates μ_ex to the short- and/or long-range surface forces associated with the two film surfaces and the dependence of the film specific surface free energy σ on the film thickness h is expressed through μ_ex. The conditions under which the thin films are in stable or metastable thermodynamic equilibrium are analyzed and it is shown that the metastable thin films are subject to thickness transitions involving abrupt changes of h. The thin film thickness equilibrium, i.e. the coexistence between two films of different thickness, is also considered and the film equilibrium chemical potential characterizing this equilibrium is determined by means of μ_ex and σ.     

微间隙受限液体行为与昆虫爪垫在光滑壁面的粘着机理

自然界中许多昆虫通过分泌一层油性液体薄膜实现其爪垫表皮和光滑壁面之间粘附和解粘附,从而实现在光滑壁布的快速爬行.为了提示昆虫爪垫与光滑壁面间微量液体薄膜对生物粘着的意义,基于自行研制的粘着接触实验仪,采用微量的[emim[Tf2N]离子液体和聚α烯烃油,观测其受限在纳米级光滑钢球表面与玻璃表面之间的接触行为以及法向粘着力.实验发现,临界体积(10^-12—10^-9L)范围内的受限液滴达到临界厚度（小于2μm）后会出现自动铺展和瞬时收缩行为，并同时提供幅值稳定且数值 关键词： 受限液体 粘着力 昆虫爪垫 类固化     

Foams in contact with solid boundaries: Equilibrium conditions and conformal invariance

A liquid foam in contact with a solid surface forms a two-dimensional foam on the surface. We derive the equilibrium equations for this 2D foam when the solid surface is curved and smooth, generalising the standard case of flat Hele-Shaw cells. The equilibrium conditions at the vertices in 2D, at the edges in 3D, are invariant by conformal transformations. Regarding the films, conformal invariance only holds with restrictions, which we explicit for 3D and flat 2D foams. Considering foams confined in thin interstices between two non-parallel plates, normal incidence and Laplace’s law lead to an approximate equation relating the plate profile to the conformal map. Solutions are given for the logarithm and power laws in the case of constant pressure. The paper concludes on a comparison with available experimental data.     

Effective viscosity of confined hydrocarbons

We present molecular dynamics friction calculations for confined hydrocarbon films with molecular lengths from 20 to 1400 carbon atoms. We find that the logarithm of the effective viscosity η(eff) for nanometer-thin films depends linearly on the logarithm of the shear rate: log η(eff)=C-nlog ?γ, where n varies from 1 (solidlike friction) at very low temperatures to 0 (Newtonian liquid) at very high temperatures, following an inverse sigmoidal curve. Only the shortest chain molecules melt, whereas the longer ones only show a softening in the studied temperature interval 0相似文献   

Tribological behaviors of lanthanum-based phosphonate 3-aminopropyltriethoxysilane self-assembled films

Lanthanum-based thin films deposited on the phosphonate 3-aminopropyltriethoxysilane (APTES) self-assembled monolayer (SAM) were prepared on the hydroxylated glass substrate by a self-assembling process from specially formulated solution. Chemical compositions of the films and chemical state of the elements were detected by X-ray photoelectron spectrometry (XPS). The thickness of the films was determined with an ellipsometer, while the morphologies of the original and worn surfaces of the samples were analyzed by means of atomic force microscopy (AFM) and scanning electron microscopy (SEM), respectively. The tribological properties of the films sliding against GCr15 steel ball were evaluated on a UMT-2MT reciprocating friction and wear tester. As the results, the target film was obtained and reaction may have taken place between the film and the glass substrate. The tribological results show that lanthanum-based thin films are superior in reducing friction and resisting wear compared with APTES-SAM and phosphorylated APTES-SAM. SEM observation of the morphologies of worn surfaces indicates that the wear of APTES-SAM and the phosphorylated APTES-SAM is characteristic of brittle fracture and severe abrasion. Differently, slight abrasion and micro-crack dominate the wear of lanthanum-based thin films. The superior friction reduction and wear resistance of lanthanum-based thin films are attributed to the enhanced load-carrying capacity of the inorganic lanthanum particles in the lanthanum-based thin films as well as good adhesion of the films to the substrate.     

Superlubricity: a paradox about confined fluids resolved

Using the method of Frantz and Salmeron to cleave mica [Tribol. Lett. 5, 151 (1998)]] we investigate alkane fluids in a surface forces apparatus and confirm several predictions of molecular dynamics (MD) simulation. An oscillatory force-distance profile is observed for the methyl-branched alkane, squalane. Boundary slip is inferred from the frictional sliding of molecularly thin fluids and also from the hydrodynamic flow of thicker films. These findings resolve the paradox that prior experiments disagreed with these aspects of MD predictions, and demonstrate that exceptionally low energy dissipation is possible when fluids move past solid surfaces that are sufficiently smooth.     

The mica slit-pore as a tool to control the orientation and distortion of simple liquid monolayers

Grand canonical ensemble Monte Carlo computer simulations have been used to study mono-layer octamethylcyclotetrasiloxane (OMCTS) and cyclohexane films confined between mica-like surfaces to determine the effect of the mica surfaces on the orientation and distortion of the films at different surface alignments. The film molecules are packed as a highly ordered lattice. The orientation of the lattice is fixed relative to the mica surfaces and depends on the size of the film molecule. Registry shifts distort the film lattice by effectively stretching it along a particular direction that depends on the size of the film molecule. For a particular registry, OMCTS and cyclohexane monolayers are stretched in perpendicular directions. Coupling between the monolayers and the mica surfaces generates a nonzero shear stress when the surfaces are out of alignment, but the film does not become disordered or melt. It is possible that precisely controlled solid surfaces could be used to create packed arrays of film molecules with desired orientation and degree of distortion that may be useful in nanotechnological applications.     

The origins of fast segmental dynamics in 2 nm thin confined polymer films

Molecular-Dynamics computer simulations were used to study 2 nm wide polystyrene films confined in slit pores, defined by inorganic crystalline surfaces. The simulated systems mimic experimentally studied hybrid materials, where polystyrene is intercalated between mica-type, atomically smooth, crystalline layers. A comparison between the experimental findings and the simulation results aims at revealing the molecular origins of the macroscopically observed behavior, and thus provide insight about polymers in severe/nanoscopic confinements, as well as polymers in the immediate vicinity of solid surfaces. Pronounced dynamic inhomogeneities are found across the 2 nm thin film, with fast relaxing moieties located in low local density regions throughout the film. The origins of this behavior are traced to the confinement-induced density inhomogeneities, which are stabilized over extended time scales by the solid surfaces. Received 9 August 2001 and Received in final form 7 January 2002     

Fabrication of durable hydrophobic surfaces through surface texturing

Low surface energy polymer thin-films can be applied to surfaces to increase hydrophobicity and reduce friction for a variety of applications. However, wear of these thin films, resulting from repetitive rubbing against another surface, is of great concern. In this study, we show that highly hydrophobic surfaces with persistent abrasion resistance can be fabricated by depositing fluorinated carbon thin films on sandblasted glass surfaces. In our study, fluorinated carbon thin films were deposited on sandblasted and as-received smooth glass using deep reactive ion etching equipment by only activating the passivation step. The surfaces of the samples were then rubbed with FibrMet abrasive papers in a reciprocating motion using an automatic friction abrasion analyzer. During the rubbing, the static and kinetic friction forces were also measured. The surface wetting properties were then characterized using a video-based contact angle measuring system to determine the changes in water contact angle as a result of rubbing. Assessment of the wear properties of the thin films was based on the changes in the water contact angles of the coated surfaces after repetitive rubbing. It was found that, for sandblasted glass coated with fluorinated carbon film, the water contact angle remained constant throughout the entire rubbing process, contrary to the smooth glass coated with fluorinated carbon film which showed a drastic decrease in water contact angle with the increasing number of rubbing cycles. In addition, the static and kinetic friction coefficients of the sandblasted glass were also much lower than those of the smooth glass.     

Dynamics of dewetting at the nanoscale

We use large-scale molecular dynamics simulations to model the dewetting of solid surfaces by partially wetting thin liquid films. As observed experimentally and in previous simulations, the films recede at an initially constant speed, creating a growing rim of liquid with a constant receding dynamic contact angle. Film recession is faster on the more poorly wetted surface to an extent that cannot be explained solely by the increase in the surface tension driving force. Furthermore, the rates of recession of the thinnest films are found to increase with decreasing film thickness. These results suggest not only that the mobility of the liquid molecules adjacent to the solid increases with decreasing solid-liquid interactions, but also that the mobility adjacent to the free surface of the film is higher than in the bulk, so that the average viscosity of the film decreases with thickness. Recent simulations of films with a wide range of solid-liquid interactions lend support to this view.     

Improved liquid－solid－gas interface deposition of nanoparticle thin films

A liquid－solid－gas interface deposition method to prepare nanoparticle thin films is presented in this paper. The nanoparticles in the part of suspension located close to the solid－liquid－gas interface grow on the substrate under the influence of interface force when the partially immersed substrate moves relatively to the suspension. By using statistical theory of the Brownian motion, growth equations for mono-component and multi-component nanoparticle thin films are obtained and some parameters for deposition process are discussed.