边界膜剪切弹性模量对于微接触性能的影响

建立接触模型,理论分析了微接触中边界膜剪切弹性模量对于接触性能的影响. 接触区由两平行平面形成,属一维接触. 上接触表面为粗糙表面,具有矩形微凸体. 下接触表面为光滑平面. 两接触表面均处理成刚性表面. 微接触区中充满流体. 它分成两个子区,在微接触的出口区由于极小的接触间隙充满边界膜,在微接触的入口区由于接触间隙较大充满流体膜. 边界膜和流体膜行为决定整个微接触性能. 当膜厚较大时,这里边界膜可看成纳米级薄膜. 由于上接触表面处有限的剪应力承受能力,边界膜可于上接触表面滑移. 设下接触表面处剪应力承受能力很大而边界膜在下接触表面不滑移. 由于边界膜-接触表面间相互作用,边界膜黏度、密度和剪切弹性模量均沿膜厚变化,在理论分析中使用它们的等效值,这些值与边界膜厚度有关. 流体膜在两个接触表面均不发生滑移,分析中不考虑流体膜剪切弹性模量. 流体膜采用传统分析法. 给出了理论分析和若干变工况参数下的计算结果.      

微纳米结构壁面对流场及动压润滑的影响研究

固体边界具有的微纳米结构将影响流体在近壁面处的流动行为,进而由于尺度效应改变流体在整个微间隙的流动或润滑规律.将壁面可渗透微纳米结构等效为多孔介质薄膜,采用Brinkman方程来描述流体在近壁面边界渗透层内的流动,并将其与自由流动区域的不可压缩流体Navier-Stokes控制方程耦合,在界面处的连续边界条件下求解和分析了速度分布规律和压力变化规律.针对恒定法向承载力的油膜润滑条件,进一步讨论了静止表面或运动表面的微纳米结构对近壁面流动行为的影响;并揭示了考虑壁面微纳米结构的流体动压润滑的油膜厚度和摩擦系数的变化规律.论文结果为具有可渗透微结构表面的微间隙流动与润滑提供了理论参考.     

纳米尺度边界滑移的分子动力学模拟研究

利用分子动力学模拟方法研究了纳米尺度超薄膜润滑的边界滑移现象,分别模拟考察了固液作用势、固液密度差异和温度对滑移长度的影响.结果表明:在固液作用势较强的情况下,滑移长度随着温度的增加而增大;当固液作用势较弱时,滑移长度随着温度的增加反而下降;滑移界面上、下的层状有序化差异程度是导致滑移的主要原因;应用所建立的方法可以较好地解释不同物理参数条件下的壁面滑移问题.     

滑滚表面间的流体膜壁面滑移和流体动力学

经典雷诺润滑理论建立在无壁面滑移的假设基础之上。近年来许多试验报告了发生在流体膜流动的壁面滑移证据。本文研究了两固体表面间的流体膜流动特性和流体动力学,发现壁面滑移显著影响膜的流体动力学问题,流体动压力不仅受黏度和几何间隙的影响,而且还由壁面滑移和表面运动强力控制,通过控制表面的吸附性质,甚至可以得到零摩擦表面。另一方面,如果两个表面具有相同的滑移特性,存在一个临界滑动速度使得流体动压效应完全消失;但是在纯滚动条件下,即使界面极限剪应力很小,仍然有相当可观的流体动压效应。     

微液滴在PDMS软基体表面的动态摩擦行为研究

通过固液界面摩擦力测试装置研究了微液滴在PDMS软基体表面运动时的动态摩擦学行为,并对微液滴体积、滑动速度及软基体力学性能对固液界面动态摩擦行为的影响进行了分析. 结果表明:微液滴在软基体表面运动时表现出最大静摩擦力和动态摩擦力. 最大静摩擦力与微液滴黏度和速度梯度呈正比,动态摩擦力与微液滴体积、滑动速度和基体力学性能有关. 随着微液滴体积的增加,三相接触线长度增加,动态摩擦力增加;随着相对滑动速度增加,三相接触线长度及接触角滞后增加,动态摩擦力增加;随着软基体弹性模量降低,固液界面黏附力增加,固液界面运动能量耗散增加,动态摩擦力增加. 研究结果可为PDMS软基体表面微液滴的精确驱动和运动参数优化提供理论指导,也可进一步丰富固液界面摩擦理论.      

固液润湿性对流体动压润滑薄膜的影响

利用自行开发的微型面接触润滑油膜测量系统,研究了固液润湿性对流体动压润滑油膜厚度的影响.试验中以静止的微型滑块平面和旋转的光学透明圆盘平面形成润滑副.固液的润湿性通过接触角判定,不同材料的微滑块平面和润滑液体形成不同的界面.在保持载荷和面接触楔形角不变的条件下对油膜厚度-速度关系进行了测量.结果表明:对于固液润湿性强的界面,形成的油膜厚度与经典润滑理论有较好的一致性;当固液润湿性明显降低时,测量得到的油膜厚度减小.对于试验中观察到的界面效应,应用界面滑移理论进行了初步分析.     

压力相关的壁面滑移在滑动间隙流体动压力产生中的作用

近年来,壁面滑移在纳米流变学、微流体力学、薄模润滑和微机电系统(MEMS)等领域越来越引起关注。以前大部分研究集中于表面初始极限剪应力对薄模润滑的壁面滑移和流体动力学的影响。本文通过一个极限剪切应力比例系数主要研究了与压力相关的壁面滑移滑动间隙流体动压力产生中的作用,发现极限剪切应力比例系数以相反的两种方式影响着流体膜的流体动力学:在高初始剪应力区使流体动力增加,但在低初始剪应力区使流体动力减小,这意味着就极限剪切应力比例系数影响流体动压力而言,存在一个初始极限剪切应力的转换点。但是在界面滑移存在时,较小的极限剪切应力比例系数总是产生较小的摩擦阻力。     

微/纳流控系统电渗流研究进展

简要介绍了微/纳流控系统中双电层和电渗流的基本原理、当前研究热点以及最新进展.从胶体界面理论出发,基于流体连续性模型和微纳米尺度流动的多物理场数值分析法,介绍了电渗流特性, 如焦耳热效应、 反离子效应、 表面电场调控双电层、双流体输运和周期电渗输运等电渗流控制方法.最后对电渗流测量作简要介绍.      

微纳尺度流动与界面流动专题序

<正>微纳尺度流动与界面流动是流体力学的重要分支学科^[1],其中微纳尺度流动关注微米、亚微米乃至纳米尺度下流体运动及物质输运规律,界面流动关注不同流体或流固界面之间的相互作用及流动规律.随着微流控芯片和微纳机电系统的发展,微纳尺度流动与界面流动已成为流体力学学科的重要前沿领域,并且涉及了流体力学与生命科学、材料科学、医疗健康、能源环境、先进制造等多学科领域的交叉融合.     

改性对硅胶纳米通道润湿性的影响规律研究

研究了多孔硅胶在改性前后不同的壁面特征对纳米通道润湿性的影响规律。采用非平衡分子动力学方法研究了水分子在改性前羟基化及改性后不同链长硅烷化壁面的纳米通道内密度、速度的分布状况,并分析了氢键的径向分布函数。结果表明:羟基化壁面通道的固液界面处流体出现高密度层,几乎无速度滑移,流体密度最大值为1.24g/cm~3,滑移速度仅为0.056?/ps;链长为C_4、C_8、C_(12)的硅烷化壁面通道的固液界面处水的密度分别为1.18g/cm~3、1.12g/cm~3、1.01g/cm~3,滑移速度分别为0.402?/ps、1.211?/ps、1.810?/ps;随烷基链长的增加,固液界面处水的密度接近纯水的密度1.0g/cm~3,固液界面处出现了速度滑移;羟基化通道壁面由于较强的固液相互作用势呈强亲水性;改性后的硅烷化壁面呈疏水性,并随改性层烷基链长度的增加而增强,且固液界面处出现"滑移效应";羟基化通道壁面与水分子存在较强的氢键作用,而改性后硅烷化通道壁面与水分子无氢键的形成,改性层烷基链长的增加减弱了两者的相互作用势,进一步揭示了其疏水机理。     

Quadratic programming algorithm for wall slip and free boundary pressure condition

Wall slip is often observed in a highly sheared fluid film in a solid gap. This makes a difficulty in mathematical analysis for the hydrodynamic effect because fluid velocity at the liquid–solid interfaces is not known a priori. If the gap has a convergent–divergent wedge, a free boundary pressure condition, i.e. Reynolds pressure boundary condition, is usually used in the outlet zone in numerical solution. This paper, based on finite element method and parametric quadratic programming technique, gives a numerical solution technique for a coupled boundary non‐linearity of wall slip and free boundary pressure condition. It is found that the numerical error decreases with the number of elements in a negative power law having an index larger than 2. Our method does not need an iterative process and can simultaneously gives rise to fluid film pressure distribution, wall slip velocity and surface shear stress. Wall slip always decreases the hydrodynamic pressure. Large wall slip even causes a null hydrodynamic pressure in a pure sliding solid gap. Copyright © 2005 John Wiley & Sons, Ltd.     

Molecular dynamics simulation of liquid argon flow at platinum surfaces

The micro Poiseuille flow for liquid argon flowing in a nanoscale channel formed by two solid walls was studied in the present paper. The solid wall material was selected as platinum, which has well established interaction potential. We consider the intermolecular force not only among the liquid argon molecules, but also between the liquid argon atoms and the solid wall particles, therefore three regions, i.e. the liquid argon computation domain, the top and bottom solid wall regions are included for the force interaction. The present MD (Molecular Dynamics) simulation was performed without any assumptions at the wall surface. The objective of the study is to find how the flow and the slip boundaries at the wall surface are affected by the applied gravity force, or the shear rate. The MD simulations are performed in a nondimensional unit system, with the periodic boundary conditions applied except in the channel height direction. Once the steady state is reached, the macroscopic parameters are evaluated using the statistical mechanics approach. For all the cases tested numerically in the present paper, slip boundaries occur, and such slip velocity at the stationary wall surface increases with increasing the applied gravity force, or the shear rate. The slip length, which is defined as the distance that the liquid particles shall travel beyond the wall surfaces to reach the same velocity as the wall surface, sharply decreases at small shear rate, then slightly decreases with increasing the applied shear rate. We observe that the liquid viscosity remains nearly constant at small shear rates, and the Newtonian flow occurs. However, with increasing the shear rate, the viscosity increases and the non-Newtonian flow appears.     

Direct simulation of multiphase flows with modeling of dynamic interface contact angle

We describe a modeling technique for dynamic contact angle between a phase interface and a solid wall using a generalized Navier boundary condition in the context of a front-tracking-based multiphase method. The contact line motion is determined by the generalized Navier slip boundary condition in order to eliminate the infinite shear stress at the contact line. Applying this slip boundary condition only to the interface movement with various slip ratios shows good agreement with experimental results compared to allowing full fluid slip along the solid surface. The interface slip model performs well on grid convergence tests using both the slip ratio and slip length models. A detailed energy analysis was performed to identify changes in kinetic, surface, and potential energies as well as viscous and contact line dissipation with time. A friction coefficient for contact line dissipation was obtained based on the other computed energy terms. Each energy term and the friction coefficient were compared for different grid resolutions. The effect of varying the slip ratio as well as the contact angle distribution versus contact line speed was analyzed. The behavior of drop impact on a solid wall with different advancing and receding angles was investigated. Finally, the proposed dynamic contact model was extended to three dimensions for large-scale parallel calculations. The impact of a droplet on a solid cylinder was simulated to demonstrate the capabilities of the proposing formulation on general solid structures. Widely different contact angles were tested and showed distinctive characteristic behavior clearly.     

Wall slip phenomena in concentrated ionic liquid-based magnetorheological fluids

Ionic liquids (ILs) have been recently proposed as carrier for magnetorheological (MR) fluids. Their special properties, such as very low vapor pressure and high thermal stability, make ILs highly suitable dispersion media to increase the broad range of technological applications that magnetorheological fluids already have. It has been just reported that using ILs as carriers in MR fluids an improvement in the colloidal stability and suspension redispersibility is obtained. In this work, the magnetorheological behavior of highly concentrated suspensions in ILs is studied. Two kinds of suspensions were analyzed: using an ionic liquid of low conductivity and a mineral oil as carriers. In both cases, silica-coated iron microparticles were used as solid phase, being the solid volume concentration of 50% vol. A complete magnetorheological analysis focused on the wall slip phenomenon was performed. Steady-state and oscillatory experiments were carried out. In order to study wall slip effects, all experiments were performed with a plate–plate system, using both smooth and rough measuring surfaces. A significant effect of wall slip was observed when the experiments were performed using smooth surfaces. The novelty of this paper is mainly based on (1) the use of an ionic liquid as carrier to prepare magnetic suspensions, and?(2) the analysis of wall slip phenomena in MR fluids with a particle content close to the maximum packing fraction.     

疏液壁面上亲液杂质对滑移特性的影响

纳米流动系统具有高效、经济等优势,在众多领域具有广泛的应用前景.因该类系统具有极高的表面积体积比,致使界面滑移效应对流动具有显著影响.论文采用非平衡分子动力学模拟方法,研究了疏液壁面表层混入少量亲液原子时纳米通道内液体的滑移特性,并基于分子动理论解释了其影响机制.研究发现,亲液杂质(均布或集中)对液体法向密度振荡程度影响较弱,但会显著改变壁面附近类固体层的分布和液体滑移规律;随亲液杂质占比增加,液体类固体现象更趋明显,壁面处液体接触密度也线性增大,但通道内液体平均速度逐渐降低,滑移长度也迅速减小;相比于集中的亲液杂质,均匀分布亲液杂质对滑移的弱化效应更强.如当亲液杂质占比为28%时,其滑移长度比单纯疏液表面的降低率从50%扩大至56%.基于分子动理论的分析发现,亲液杂质会导致杂质原子附近第一液体层内的原子发生跃迁的能垒加大,即弱化了液体原子的流向跃迁行为,从而降低了滑移量;相比于集中杂质,均匀分布的杂质还会降低固液间的非公度性,致使滑移特性破坏更严重.     

Effect of Slow and Fast Moving Liquid Zones on Solute Transport in Porous Media

A conceptual model is developed in this article that accounts for the effect of slow and fast moving liquid zones on solute transport in porous media. The liquid phase within the porous media is divided into three zones—immobile, slow moving, and fast moving. Slow moving liquids surround the solid particles in thin layers and have lower velocity in flow. Fast moving liquids have higher velocity and are not in contact with the solid particles. Solute mass transfer occurs between the slow and fast liquids, and the slow and immobile liquids. The immobile and slow moving liquids interact with the solid matrix in the media through the mechanism of sorption and desorption. Implicit finite-difference methods are used to solve the partial differential equations that describe the slow and fast movement of solute in the porous medium. The model was validated for a laboratory column experimental data. Sensitivity analyses were conducted to ascertain the effects of the model parameters on solute movement. The effect of each parameter on retardation of the solute movement was analyzed. It was observed that the maximum retardation of solute occurs when there is high adsorption coefficient, high mass transfer rates, and high volume of slow moving liquid in the porous media.     

On flows of third-grade fluids with non-linear slip boundary conditions

We prove the existence and uniqueness of steady flows of incompressible fluids of grade three subject to slip and no-slip boundary conditions in bounded domains. The slip boundary condition is a non-linear generalization of the Navier slip boundary condition and permits situations in which the solid boundary undergoes non-rigid tangential motion. The existence proof is based on a fixed point method in which the boundary-value problem is decomposed into four linear problems.     

Numerical simulations of fluid–structure interaction based on Cartesian grids with two boundary velocities

This work proposes an innovative numerical method for simulating the interaction of fluid with irregularly shaped stationary structures based on Cartesian grids. Instead of prescribing an artificial force to enforce the no‐slip boundary condition at the solid–fluid interface, this work imposes two boundary velocities, referred to as the solid and mass‐conserving boundary velocities, to satisfy the no‐slip boundary condition and mass conservation in the ghost cells around the immersed solid boundary. Both the traditional level set method [41] and the hybrid particle level set method [45] were used to represent the solid boundary and the complex free‐surface evolution, respectively. Consequently, the boundary velocities close to the immersed solid boundary can be determined in terms of the level set function and the neighboring fluid velocity. The projection method is further modified to incorporate the solid and mass‐conserving boundary velocities into the solution algorithm. A series of numerical experiments were conducted to demonstrate the feasibility of the proposed method. They involved uniform flow past a stationary circular cylinder and the propagation of water waves over a submerged trapezoidal breakwater. Comparisons between the numerical results and experimental data showed very good agreement in all cases of interest. Copyright © 2015 John Wiley & Sons, Ltd.     

Dynamics of a diverging liquid meniscus in a capillary,taking into account the specific properties of thin films

The theory of the diverging meniscus of a Newtonian liquid for capillary flow conditions at low meniscus velocities, in which the thermodynamic and Theological features of thin wetting films appear, is set forth. Two cases are considered: thermodynamically stable wetting film with high viscosity in the boundary layer on a completely wetted solid surface and a thermodynamically unstable film on a conditionally wetted solid surface exhibiting a liquid slip effect.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 3, pp. 72–82, May–June, 1976.The author wishes to express his appreciation to A. G. Grivtsov and V. S. Yushchenko for valuable assistance in the computer calculations.