Structures,solvation forces and shear of molecular films in a rough nano-confinement

Investigations of surface roughness effects on the structure, dynamics and rheology of a molecular fluid (hexadecane) confined between solid (gold) surfaces, through the use of large-scale molecular dynamics simulations, reveal a remarkable sensitivity to the confining surface morphology. A most significant reduction of the ordering propensity is found in films confined by stationary rough surfaces with a consequent strong suppression of solvation forces and the development of liquid-like dynamic and response characteristics. When the rough-surface boundaries are set in motion at a high shear rate, the interfacial layers of the film stick to the adjacent solid boundaries, resulting in partial slip inside the film with the development of shear stress in the viscous molecular fluid, unlike the case of atomically flat crystalline boundaries where slip of the confined film at the boundaries is accompanied by vanishingly small shear stress in the film. These results are discussed in the context of the effect of roughness on the boundary conditions used in modeling fluid flow past surfaces, and they suggest that morphological patterning of surfaces could provide ways for controlled modifications of frictional processes in thin-film lubricated nanotribological systems.     

A Molecular Dynamics Study of the Transition from Ultra-Thin Film Lubrication Toward Local Film Breakdown

The transition from ultra-thin lubrication to dry friction under high pressure and shear is studied using molecular dynamics: the quantity of lubricant in the confined film is progressively reduced toward solid-body contact. A quantized layer structure is observed for n-alkanes confined between smooth, wettable walls, featuring an alternation of well-layered, low friction configurations, and disordered ones, characterized by high friction, and heat generation. The molecular structure influences the ordering of the fluid and the resulting shear stress. In fact, Lennard-Jones fluids are characterized by low friction due to the absence of interlayer bridges, opposed to the always entangled states and high shear stresses for branched molecules. Surface geometry and wettability also affect the behavior of the confined lubricant. The presence of nanometer-scale roughness frustrates the ordering of the fluid molecules, leading to high friction states. Furthermore, local film breakdown can be observed when the asperities come into contact, with strong wall–wall interactions causing the maximum in shear stress. Finally, friction is limited to a small, constant value by the presence of smooth, non-wettable surfaces in the system due to the occurrence of wall slip.     

Thin lubricant films confined between crystalline surfaces: Gold versus mica

In this paper we report molecular dynamics simulation results of lubricant films only a few nanometres thick confined between atomically smooth gold or model mica surfaces. We have studied dodecane (C₁₂H₂₆) of various film thicknesses. We show below a critical film thickness structural transitions take place with the formation of crystal bridges. We demonstrate this critical film thickness is larger with Au(1 0 0) surface. Below this critical film thickness we observe a large enhancement of apparent shear viscosity. This enhancement, however, is much lower for films confined by gold. We find the extrapolated zero shear viscosity of a ∼2.4-nm-thick film confined by gold is almost two orders of magnitude lower than that of a film, of the same thickness, confined by model mica. We find the source of this difference is the weaker pinning of the layers next to the surface of the gold. This leads to stronger slip, at the lubricant-gold interface, which persists at solid-like and liquid-like states. For mica a larger part of shearing takes place inside the film. The solid-like structure of the films confined by gold shows stronger resilience to shear and melt at considerably larger shear rates than those confined by model mica surfaces. These differences could be important in developing low friction devices at microscale.     

Structure in confined fluids: phase separation of binary simple liquid mixtures

One- to five-layer cyclohexane and octamethyltetracyclosiloxane(OMCTS) films confined between mica-like surfaces are studied to elucidate changes in the lattice type and composition of the films. Grand canonical ensemble Monte Carlo computer simulations are used to study the laterally confined film. In contrast to previous studies, solid-like order is induced primarily by the strong fluid-solid interaction and is largely a function of pore width. Solid-like order within the layers causes the composition of the pore fluid to shift from the bulk composition, favoring either cyclohexane or OMCTS, depending on the pore width. A shift in the relative alignment of the surfaces perturbs the solid-like fluid structure but does not cause the sudden shear melting transition associated with epitaxial alignment of the fluid atoms with the surface. This revised version was published online in June 2006 with corrections to the Cover Date.     

非牛顿介质中图形化表面对减阻效果的影响

为研究图形化表面在非牛顿介质中对减阻效果的影响,在微摩擦试验机(UMT)上进行了销盘润滑实验。实验中在硅片的表面刻蚀了纵向和横向2种沟槽以形成不同的表面图形。实验表明,与光滑表面相比,当剪切速率处于牛顿流动区时,沟槽具有一定的减阻效果;而当剪切速率处于剪切稀化区时,沟槽表面的阻力上升;并且横向沟槽表面的阻力总是大于纵向沟槽表面的阻力。流体数值计算的结果表明,横向沟槽侧壁的压差阻力和纵向沟槽侧壁的粘性阻力无法完全补偿沟槽底面上的阻力损失,因而产生了减阻效果。     

A Molecular Dynamics Study of Lubrication Rheology of Polymer Fluids

This study investigated the characteristics of lubrication rheology for polymer fluids using the molecular dynamics method. The confining potential and FENE potential were used to describe the bonding force in a polymer chain. Both Poiseuille and Couette flows were simulated and the results are presented for the density distribution, velocity profiles, slip ratio, viscosity, and relation between shear stress and shear rate. The effects of the number of molecular layers and film thickness on the rheology characteristics were also investigated.     

Hilbert diagnostics of Rayleigh-Benard convection in fluids

An combined optical and thermal imaging experimental system was designed to investigate Rayleigh-Benard convection of a fluid in a layer with two rigid isothermal boundaries and a free upper boundary under steady and unsteady thermal boundary conditions. The fluid surface structure was visualized using methods of reflected-light Hilbert optics. Noncontact control of the fluid layer thickness was performed using a specially designed remote meter based on an MBR-1 microscope with a smooth focusing unit based on the Meyer mechanism. The evolution of the dynamic structure of the surface and temperature field of the fluid being analyzed were studied experimentally, and the existence of flows in the form of two-dimensional rolls with axes of rotation parallel to the lateral boundaries (the walls of the cavity) was confirmed. It is shown that the highly viscous fluid flow has a thermal gravitational nature. Correspondence is found between the evolution of the thermograms and Hilbert schlieren patterns of surface structures in different modes of Rayleigh-Benard convection.     

The role of viscous deformation in the machining of polymers

This study aims to evaluate the machinability of typical thermoplastic and thermosetting polymers and understand the effect of their viscous properties on surface integrity, chip formation and machining forces. The interaction between the strain rate and temperature during machining was particularly addressed. It was found that the viscous deformation of a polymer plays a decisive role in determining the quality of a machined surface. To minimize the surface roughness, for instance, the machining conditions must be selected in such a way that the material removal deformation falls in the regime without visco-plastic scaling/tearing and brittle cracking. The optimal machining condition must be based on the polymer properties, such as the glass transition temperature, fracture toughness and molecular mobility. The shear stress in the shear plane of chipping is a good measure of the coupled effect of strain rate and temperature rise. In addition, the study discovered two new types of chips whose deformation and curling were in close relation to the surface integrity of the machined components.     

Hydrodynamic lubrication of porous journal bearings using a modified Brinkman-extended Darcy model

A numerical solution for the hydrodynamic lubrication of finite porous journal bearings considering the flexibility of the liner is introduced. The Brinkman-extended Darcy equations and the Stokes' equations are utilized to model the flow in the porous region and fluid film region, respectively. A stress jump boundary condition at the porous media/fluid film interface and effects of viscous shear are included into the lubrication analysis. Elrod's cavitation algorithm, which automatically predicts film rupture and reformation in the bearing, is implemented in the solution scheme. The present analysis predictions for pressure distributions, load carrying capacity, and friction factor are in good agreement with three different sets of experimental results available in the literature. Furthermore, the effects of dimensionless permeability parameter, and stress jump parameter on performance parameters such as load carrying capacity, side leakage, friction factor, and attitude angle, are presented and discussed.     

超薄含水油膜Couette流润滑特性的分子动力学模拟

基于分子动力学方法,建立超薄含水柴油膜的全原子分子模型,进行不同含水率下油膜Couette流的润滑特性研究。在相同剪切速度作用下,分析含水油膜的微观结构、速度分布、整体键取向参数、剪切黏度等性质。发现不含水时油膜形成了类固体层,不具有流动性,且在剪切过程中黏度值下降,即表现出剪切时间稀化现象;而含水工况下,油膜出现分层结构,流速符合Couette流的流动特性;且含水率越高,油膜的分层现象越明显,链烃的有序性越强,致使油水混合薄膜的剪切黏度值也越低,呈现出非牛顿流体性质,此时油膜固有的剪切稀化特性被削弱。研究表明,水分子由于具有较强的分子间作用力,能促使油膜中的有机分子重新排布,从而对油膜的润滑性能产生较大改变。     

The behaviour of hydrostatic pads with grooved lands

A hydrostatic pad is usually made up of a recess surrounded by a land. Viscous fluid is supplied under pressure to the recess. The land, being separated from the bearing surface by a relatively small clearance, will act as the hydraulic impedance needed to separate the required bearing pressure inside the recess from the pressure of the surrounding environment. If the pad is moved relative to the bearing surface, the film of fluid in the clearance, being viscous, will be sheared. This shearing action will initiate viscous shear stresses between the fluid layers and hence viscous drag between the moving pad and the bearing surface. The lands of the pad, having a much smaller clearance from the bearing surface, will be subjected to a much higher drag force than the recess. The power required to overcome such a drag force, and cause the required motion of the pad relative to the bearing surface, will be transformed mainly into heat. Sometimes, especially under high relative speeds and with small clearances, the generated heat can be detrimental to the bearing action, and if excessive, may lead to bearing failure.     

Effect of Base Oil Structure on Elastohydrodynamic Friction

The EHD friction properties of a wide range of base fluids have been measured and compared in mixed sliding–rolling conditions at three temperatures and two pressures. The use of tungsten carbide ball and disc specimens enabled high mean contact pressures of 1.5 and 2.0 GPa to be obtained, comparable to those present in many rolling bearings. The measurements confirm the importance of molecular structure of the base fluid in determining EHD friction. Liquids having linear-shaped molecules with flexible bonds give considerably lower friction than liquids based on molecules with bulky side groups or rings. EHD friction also increases with viscosity for liquids having similar molecular structures. Using pure ester fluids, it is shown that quite small differences in molecular structure can have considerable effects on EHD friction. The importance of temperature rise in reducing EHD friction at slide–roll ratios above about 5% has been shown. By measuring EHD friction at several temperatures and pressures as well as EHD film thickness, approximate corrections to measured EHD friction data have been made to obtain isothermal shear stress and thus EHD friction curves. These show that under the conditions tested most low molecular weight base fluids do not reach a limiting friction coefficient and thus shear stress. However, two high traction base fluids appear to reach limiting values, while three linear polymeric base fluids may also do so. Constants of best fit to a linear/logarithmic isothermal shear stress/strain rate relationship have been provided to enable reconstruction of isothermal EHD friction behaviour for most of the fluids tested.     

纳米粗糙间隙中季戊四醇四酯的薄膜润滑行为

采用分子动力学模拟方法建立光滑和粗糙2种固体壁面结构,研究季戊四醇四酯润滑剂在不同压力、薄膜厚度下,在恒定剪切速度和温度下的薄膜润滑行为。分析壁面间润滑薄膜的密度分布,以及剪切过程中润滑剂的速度分布。输出固体壁面在x向和z向的力学响应,并计算摩擦因数。结果表明：表面纳米结构降低了润滑薄膜的厚度,减弱了润滑薄膜分层现象;当润滑薄膜厚度较大时,V形纳米沟槽有助于减小薄膜润滑系统的摩擦因数;润滑薄膜厚度较小时,V形纳米沟槽表面润滑状态容易从流体润滑转变到边界润滑状态,摩擦因数增大。     

The structural origin of the complex rheology in thin dodecane films: Three routes to low friction

The effective viscosity of confined lubricant films less than 6–7 molecular layers is usually enhanced by many orders of magnitude. For dodecane the high friction film has a strong in-plane order with “mosaic-like” structures that extend across the film and effectively form “crystalline bridges” [Jabbarzadeh A, Harrowell P, Tanner RI. Crystal bridge formation marks the transition to rigidity in a thin lubrication film. Phys Rev Lett 2006; 96: 206102-1/4] resulting in high friction. Using molecular dynamics simulations, we have identified three routes to lower the friction. We show that the structure of confined films and their response to shearing are affected by atomic in-plane order and smoothness of the confining surfaces, the relative orientation of two crystalline surfaces and the direction of shear. We show a small increase in surface roughness in going from crystalline to amorphous surfaces can lead to a much lower friction. We demonstrate that misaligning (twisting) one surface with respect to the other by 45° results in a much lower effective viscosity. Application of shear for extended times induces alignment of lubricant molecules into a nematic-like order with ultra-low effective viscosity. The magnitude of reduction in the friction and the physical process through which it happens varies for each of these three routes. Depending on the method used, destruction of crystalline bridges, multilayer or fault plane slip provides a route for dramatic reduction in friction.     

An Experimental Validation of the Recently Discovered Scale Effect in Generalized Newtonian EHL

New quantitative numerical simulations of the elastohydrodynamic lubrication (EHL) film forming ability of generalized Newtonian liquids have elucidated a previously unrecognized property of EHL films. The dependency of the film thickness on the scale of the contact is greater when the viscosity is shear dependent within the inlet. Measurements of film thickness were performed in a ball on disc experiment using balls ranging from 5.5 to 35 mm in diameter. Three liquids were investigated with varying shear dependence in the range of stress important to film forming. The experimental results confirm the previous analytical findings. Numerical simulations using the measured viscosities show that the increased scale sensitivity is substantially the result of shear-thinning. However, the smallest scales produced films thinner than even the shear-dependent prediction, possibly indicating molecular degradation. It is quite likely that some machine components, which were designed using the effective viscous properties derived from a larger scale film thickness measurement, are operating with substantially lower film thickness than the designer had intended.     

考虑热效应影响的液黏调速离合器流体剪切转矩预测研究

为了更准确地对液黏调速离合器流体剪切转矩进行预测,以液黏调速离合器摩擦副间的流体为研究对象,建立了考虑热效应影响的三维CFD模型,并考虑了黏温特性的影响,应用计算流体力学软件CFD ACE+对流场进行求解,得到了摩擦副间流体的压力和温度分布以及流体剪切转矩的数值解;通过实验研究对比分析了不同转速和油膜厚度下的流体剪切转矩。结果表明：影响温度分布的主要因素是流体剪切线速度;热效应对摩擦副间流体的压力分布有较小的影响;由于流体温度对黏度的影响,流体剪切转矩随着转速差的增加而缓慢增大。因此,通过与实验数据对比分析,考虑热效应影响的三维CFD模型能够更为准确地对转矩进行预测。     

Nanotribology of Confined Water by Quasistatic Computer Simulations: Effect of Impurities

In our previous simulation study (Pertsin and Grunze, Langmuir 24:4750–4755, 2008), we have shown that water bilayer films confined between structured hydrophilic substrates can acquire the ability to sustain shear stress (i.e., to solidify), while remaining fluidlike in respect of the lateral order and molecular mobility. In this article, the previous simulations are extended to aqueous bilayer films containing simple model impurities. The shear behavior of the films is studied using the grand canonical Monte Carlo technique and quasistatic approach. It is found that the impurities tend to separate into an individual phase or to dissolve in water depending on the strength of the water–impurity interaction. In the former case, the effect of impurity on the shear modulus and yield stress is moderate and nearly linear in the concentration of impurity. In the latter case, the effect is noticeably stronger because the impurity distorts the local order and arrangement of water molecules, thus disturbing the local epitaxial coupling between the aqueous film and confining substrates. At the highest impurity content tried (~13%), the aqueous film loses most of its solidity and becomes practically fluidlike in respect to the shear response.     

薄膜润滑中固—液相变的模拟

本文以分子动力学方法模拟了薄膜润滑中润滑剂的固液相变。模拟系统由两个平行的固体壁面和界于壁面间的流体分子构成。薄膜中流体分子的运动规律可由系统运动方程解得,再通过平均得到薄膜液体的宏观特性。计算结果表明,在薄膜润滑中润滑剂的一些性质会变成与膜厚有关的参量,一定条件下还会发生由壁面诱发的相变。薄膜中润滑剂的相变临界压力随膜厚的减小而降低,当相变压力低于系统给定压力时,润滑剂会发生固化。这种现象与薄膜中有序结构的发展规律有关。随着压力增加,流体分子的有序结构由壁面逐渐向薄膜中部发展,因此较厚的润滑膜需要较高的压力才能形成贯穿全膜的有序结构而产生相变。     

Squeeze fluid film of spherical hydrophobic surfaces with wall slip

Isothermal squeeze film flow of Newtonian fluid between spherical hydrophobic surfaces with wall slip is investigated using a limiting shear stress model and complementary algorithm. Wall slip velocity is controlled by the liquid–solid interface limiting shear stress. It is found that the wall slip dramatically decreases the hydrodynamic support force of the squeeze fluid film. In the case of large wall slip the hydrodynamic support force increases only slightly with the decrease in the film thickness. We find that wall slip decreases with increasing film thickness and limiting shear stress, but increases with increasing fluid viscosity and approaching velocity. An empirical equation is given for prediction of the fluid load support capacity. The possible effect of pressure on wall slip is also discussed. It is found that fluid pressure suppresses wall slip after the proportionality coefficient of limiting shear stress reaches a critical threshold. However, almost no effect is found when it is below this critical threshold. Good agreements exist between the present theoretical predictions and some existing experimental observations.     

Effect of surface roughness on elastohydrodynamic traction: Part 1

The effect of surface roughness on traction performance was experimentally studied using a two‐roller tester. The nature of the contact was investigated using electrical resistance and electrical capacitance methods. Increased shear stress was observed in the viscous region, which could be attributed to a higher average shear rate and to greater viscosity under EHD contact conditions due to pressure fluctuations caused by the surface texture.