接触角滞后与流体动压润滑的相关性研究

理论研究表明不同润湿性界面对流体动压润滑油膜厚度有着显著地影响,一般采用接触角(CA)来表征固液界面润湿性. 而由热力学原理推导出的界面势能垒理论模型不仅与接触角相关,也是接触角滞后(CAH)的函数. 本文作者通过对不同基体材料的滑块进行表面张力修饰,获得了不同亲和性的界面. 利用干涉法及荧光法分别测量了不同润湿性界面的流体动压润滑膜厚及油膜受剪切的流动特性,研究了接触角及接触角滞后两个界面参数对流体动压润滑油膜厚度的影响,并对势能垒与接触角滞后的关系进行了讨论. 结果表明:接触角与流体动压润滑油膜厚度的相关性较差,接触角滞后可以更好地表征界面效应对流体动压润滑油膜厚度的影响.      

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.     

EFFECT OF TWO-DIMENSIONAL MICROPILLAR ARRAYED TOPOGRAPHY ON SPREADING OF INSOLUBLE SURFACTANT-LADEN DROPLET

针对二维微柱阵列壁面上含不溶性活性剂液滴的铺展过程,采用润滑理论建立了液膜厚度和浓度演化模型,采用数值计算方法得到了液滴的铺展特征及相关参数的影响. 研究表明：活性剂液滴在微柱阵列壁面上铺展时,在壁面凸起处衍生出隆起结构,壁面凹槽处衍生出凹陷结构,随时间持续,隆起和凹陷均向两侧移动,且数量不断增加. 活性剂液膜流经凸起时,隆起高度呈驼峰形变化. 增大预置液膜厚度或活性剂初始浓度,铺展区域隆起和凹陷数量增多,液滴铺展速度加快. 增加凹槽深度或减小斜度会使毛细力作用增强,液膜破断可能性加大;增大凹槽宽度可加速活性剂液滴的铺展,加剧液膜表面波动幅度.     

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。     

二维微柱阵列壁面对活性剂液滴铺展的影响

针对二维微柱阵列壁面上含不溶性活性剂液滴的铺展过程,采用润滑理论建立了液膜厚度和浓度演化模型,采用数值计算方法得到了液滴的铺展特征及相关参数的影响. 研究表明:活性剂液滴在微柱阵列壁面上铺展时,在壁面凸起处衍生出隆起结构,壁面凹槽处衍生出凹陷结构,随时间持续,隆起和凹陷均向两侧移动,且数量不断增加. 活性剂液膜流经凸起时,隆起高度呈驼峰形变化. 增大预置液膜厚度或活性剂初始浓度,铺展区域隆起和凹陷数量增多,液滴铺展速度加快. 增加凹槽深度或减小斜度会使毛细力作用增强,液膜破断可能性加大;增大凹槽宽度可加速活性剂液滴的铺展,加剧液膜表面波动幅度.      

MOVEMENT AND SPLASHING OF A DROPLET IMPACTING ON A WET WALL

壁面液体层的存在对液滴撞击壁面的运动具有重要的影响。采用气液两相流动相界面追踪的水平集和流体体积复合方法和壁面润湿模型，实现了液滴撞击湿润壁面运动的数值求解；在此基础上，开展了液滴撞击湿润壁面运动的研究。研究结果表明：液滴以不同速度撞击湿润壁面时，会呈现出黏附铺展、波动运动、皇冠几何体运动以及飞溅运动等几种不同的运动形态，液滴撞击湿润壁面后的压力分布是不同运动形态形成的主要原因；飞溅运动是一定条件下皇冠几何体运动的一种特殊形态，液滴从皇冠几何体侧壁顶端的飞溅分离满足毛细破碎理论；撞击速度对分离液滴的运动方向影响较小，而对壁面液体层厚度的影响则较大；撞击速度和壁面液体层厚度对分离液滴形态、飞溅分离位置、飞溅速度以及飞溅发生时刻等都具有一定的影响。     

气液两相流动与固壁相互作用耦合求解的研究

气液两相流动与固壁相互作用的研究是液滴撞击壁面运动研究的重要基础.以结合了VOF和Level Set两种方法优点的用于气液相界面追踪的复合Level Set-VOF方法和利用唯象分析方法建立的能够反映接触角滞后性及壁面性质对润湿过程影响的壁面润湿模型为基础,提出了气液两相流动与固壁相互作用耦合求解流程,给出了气液两相流动与固壁相互作用耦合求解过程中接触线速度的计算方法及边界条件的确定方法.通过与已有实验结果的对比,对提出的气液两相流动与固壁相互作用耦合求解方法的有效性进行了验证.     

动脉粥样硬化晚期斑块局部应力的流固耦合分析及体外反搏作用干预机制的研究

生物机械力被普遍认为在动脉粥样硬化晚期斑块进程及最终破裂中起着重要的作用. 本文的目的是研究血流灌注、动脉内压、斑块组织和材料特性等因素对斑块局部流动切应力 及斑块结构应力 水平的影响,同时评价临床中的非介入辅助循环疗法 —— 体外反搏 对斑块局部应力水平的干预作用. 采用结合猪动物模型在体测量及三维流固耦合数值仿真的研究方法. 结果显示,当斑块狭窄率一定时 (50%),斑块的流动切应力水平主要由血流灌注决定;而斑块结构应力主要取决于动脉内压及纤维帽 厚度. 只有在纤维帽足够薄的情况下,斑块的材料特性才对斑块结构应力有显著影响;当纤维帽最薄同时脂质池材料最软时,临界斑块壁面应力 因子达到极值的 257.72 kPa (正常生理状态) 及 300.20 kPa (体外反博状态). 由于最大壁面应力、临界斑块壁面应力 及全局最大斑块壁面应力 三个应力因子中,只有临界斑块壁面应力 明显受纤维帽厚度和脂质池材料特性的影响,因此 其可能与斑块进程的关联最为紧密. 此外,体外反博作用明显提高了晚期斑块的应力水平,这是否会给斑块进程及重构带来慢性的影响,需要作更深入的研究.      

动脉粥样硬化晚期斑块局部应力的流固耦合分析及体外反搏作用干预机制的研究

生物机械力被普遍认为在动脉粥样硬化晚期斑块进程及最终破裂中起着重要的作用.本文的目的是研究血流灌注、动脉内压、斑块组织和材料特性等因素对斑块局部流动切应力及斑块结构应力水平的影响,同时评价临床中的非介入辅助循环疗法——体外反搏对斑块局部应力水平的干预作用.采用结合猪动物模型在体测量及三维流固耦合数值仿真的研究方法.结果显示,当斑块狭窄率一定时(50%),斑块的流动切应力水平主要由血流灌注决定;而斑块结构应力主要取决于动脉内压及纤维帽厚度.只有在纤维帽足够薄的情况下,斑块的材料特性才对斑块结构应力有显著影响;当纤维帽最薄同时脂质池材料最软时,临界斑块壁面应力因子达到极值的257.72 k Pa(正常生理状态)及300.20 k Pa(体外反博状态).由于最大壁面应力、临界斑块壁面应力及全局最大斑块壁面应力三个应力因子中,只有临界斑块壁面应力明显受纤维帽厚度和脂质池材料特性的影响,因此其可能与斑块进程的关联最为紧密.此外,体外反博作用明显提高了晚期斑块的应力水平,这是否会给斑块进程及重构带来慢性的影响,需要作更深入的研究.     

含空心球涂层粒子复合材料界面的脱粘应力分析

在球对称拉伸载荷作用下针对空心球涂层复合材料分析了空心球涂层粒子增强复合材料的局部应力场,得到了界面临界脱粘应力的解析表达式.讨论了各相几何参数对非均匀涂层空心球粒子临界脱粘应力的影响,比较了均匀涂层和非均匀涂层的脱粘应力.结果表明:在球对称拉伸下界面脱粘更容易发生在涂层相与基体相界面间,空心球的壁厚和涂层厚度是影响界面临界脱粘应力的重要因素,因而选择适当的空心球、涂层厚度和提高界面粘结能将有利于提高界面的临界脱粘应力.     

Microscale heat transfer in an evaporating moving extended meniscus

The evaporative heat flux distribution in the leading edge region of a moving evaporating thin liquid film of pentane on quartz was obtained by analyzing the measured thickness profile for thicknesses, δ < 2 μm. The profiles in a constrained vapor bubble were obtained using image analyzing interferometry. Although the evaporating meniscus appeared to be benign (i.e., without additional observed motion beyond creeping), high heat fluxes were obtained. Significantly higher heat fluxes are possible. The interfacial slope, curvature, interfacial shear stress, and liquid pressure profiles were also obtained. Results obtained using a continuum model were consistent with those obtained using a control volume model. The measured pressure field profile of the isothermal extended meniscus agreed with the constant pressure field predicted by the augmented Young–Laplace model. For the non-isothermal case, measured thickness gradients lead to disjoining pressure and curvature gradients for fluid flow and evaporation. The experimental results demonstrate that disjoining pressure at the contact line controls fluid flow within an evaporating completely wetting thin curved film and is, therefore, a useful boundary condition. However, in small interfacial systems, non-idealities can have a dramatic effect.     

Boundary film shear elastic modulus effect in hydrodynamic contact. Part II: influence of operational parameters

The present paper is the subsequent research of the first part (Theor Comput Fluid Dyn, 2009). It investigates the boundary film shear elastic modulus effect in a hydrodynamic contact in different operating conditions. The hydrodynamic contact is one-dimensional, composed of two parallel plane surfaces, which are respectively rough rigid with rectangular micro projections in profile periodically distributed on the surface and ideally smooth rigid. The whole contact consists of cavitated area and hydrodynamic area. The hydrodynamic area consists of many micro Raleigh bearings which are discontinuously and periodically distributed in the contact. The hydrodynamic contact in a micro Raleigh bearing consists of boundary film area and fluid film area which, respectively, occur in the outlet and inlet zones. In boundary film area, the film slips at the upper contact surface due to the limited shear stress capacity of the film–contact interface, while the film does not slip at the lower contact surface due to the shear stress capacity of the film–contact interface large enough. In boundary film area, the viscosity, density, and shear elastic modulus of the film are varied across the film thickness due to the film–contact interactions, and their effective values are used in modeling which depends on the film thickness. In fluid film area, the film does not slip at either of the contact surfaces, and the shear elastic modulus of the film is neglected. It is found from the simulation results that the boundary film shear elastic modulus influences are normally negligible on the mass flow through the contact, the carried load of the contact and the overall film thickness of the contact, and the boundary film shear elastic modulus would normally influence the local film thickness in an elastic contact when the local film thickness is on the film molecule diameter scale. It is also found that the boundary film shear elastic modulus effect has the tendency of being increased with the reduction of the width of a micro contact. It is increased with the reduction of the boundary film–contact interfacial shear strength or with the increase of the critical boundary film thickness, while it is strongest at certain values of the contact surface roughness, the width ratio of fluid film area to boundary film area, and the lubricant film shear elastic modulus.

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Film thickness measurement with an ultrasonic transducer

A technique for measuring condensate film thickness using an ultrasonic transducer is described. In the experiment, the condensate film thickness with R-113 and FC-72 (a fluorinert compound developed by the 3M Company) condensing on the horizontal lower surface of a rectangular duct was measured at several locations. From the measured values a power law relation between the condensate film thickness and the axial distance from the leading edge of the condensing surface was derived by regression analysis. Assuming a linear temperature profile in the condensate film, local and average heat transfer coefficients were computed from the condensate film thickness. The average heat transfer coefficients were compared with the values obtained by measuring the heat transfer rate to the coolant. The two values were within ±12% of each other. As yet there is no satisfactory analytical model to predict the local heat transfer coefficient even in the annular condensation regime. One of the main difficulties in modeling the condensation is the lack of a suitable model to predict the interfacial shear stress. With the measurement of the film thickness it is possible to determine the interfacial shear stress. It is hoped that the shear stresses so determined will lead to the development of a satisfactory model for interfacial shear stress with condensation.     

SQUEEZE FILM FLOW WITH NONLINEAR BOUNDARY SLIP

A nonlinear boundary slip model consisting of an initial slip length and a critical shear rate was used to study the nonlinear boundary slip of squeeze fluid film confined between two approaching spheres. It is found that the initial slip length controls the slip behavior at small shear rate, but the critical shear rate controls the boundary slip at high shear rate. The boundary slip at the squeeze fluid film of spherical surfaces is a strongly nonlinear function of the radius coordinate. At the center or far from the center of the squeeze film, the slip length equals the initial slip length due to the small shear rate. However, in the high shear rate regime the slip length increases very much. The hydrodynamic force of the spherical squeeze film decreases with increasing the initial slip length and decreasing the critical shear rate. The effect of initial slip length on the hydrodynamic force seems less than that of the critical shear rate. When the critical shear rate is very small the hydrodynamic force increases very slowly with a decrease in minimum film thickness. The theoretical predictions agree well with the experiment measurements.     

Modelling the wetting of a solid occlusion by a liquid film

We investigate in this work how the presence of an occlusion affects the dynamics of the wetting front of a liquid film draining down a vertical surface. This numerical study is developed in the context of the lubrication approximation. Through a parametric study, we show that depending on the asymptotic film thickness and the fluid properties, there exists a critical substrate contact angle below which separation of the contact line from the occlusion wall is observed which results in the appearance of a dry zone in the wake of the occlusion. In analogy with external aerodynamics, we also show that a sharp corner in the occlusion can induce this contact line separation. Our numerical results also highlight the importance of the occlusion wettability on the morphology of the wetting front suggesting a possible mechanism to control and mitigate the often undesirable fingering instability.     

Simulation and measurement of 3D shear-driven thin liquid film flow in a duct

Three-dimensional flow behavior of thin liquid film that is shear-driven by turbulent air flow in a duct is measured and simulated. Its film thickness and width are reported as a function of air velocity, liquid flow rate, surface tension coefficient, and wall contact angle. The numerical component of this study is aimed at exploring and assessing the suitability of utilizing the FLUENT-CFD code and its existing components, i.e. Volume of Fluid model (VOF) along with selected turbulence model, for simulating the behavior of 3D shear-driven liquid film flow, through a comparison with measured results. The thickness and width of the shear-driven liquid film are measured using an interferometric technique that makes use of the phase shift between the reflections of incident light from the top and bottom surfaces of the thin liquid film. Such measurements are quite challenging due to the dynamic interfacial instabilities that develop in this flow. The results reveal that higher air flow velocity decreases the liquid film thickness but increases its width, while higher liquid flow rate increases both its thickness and width. Simulated results provide good estimates of the measured values, and reveal the need for considering a dynamic rather than a static wall contact angle in the model for improving the comparison with measured values.     

纳米尺度下烷烃润滑薄膜分层现象和速度滑移机制的分子模拟研究

采用非平衡分子动力学模拟方法,研究了纳米尺度下十六烷润滑薄膜的分层现象和速度滑移现象,重点考察了剪切速度对速度滑移现象的影响规律,并对其中的微观机理作出了解释.研究结果表明:在铁壁面的限制情况下润滑薄膜出现了分层现象,当润滑薄膜厚度超过50?时,其中间区域呈现出体相均质流体的特征.润滑薄膜层间滑移和界面滑移的临界剪切速度分别为5.5和7.5?/ps,随着剪切速度增加,界面滑移程度增强,而层间滑移程度减弱.润滑薄膜第1和2层十六烷分子层间桥接分子数目决定层间滑移程度,随着剪切速度增加,桥接分子数目也相应增多,层间滑移程度随之减弱.     

Pure steam condensation model with laminar film in a vertical tube

A new physical model for calculating the liquid film thickness and condensation heat transfer coefficient in a vertical condenser tube is proposed by considering the effects of gravity, liquid viscosity, and vapor flow in the core region of the flow. To estimate the velocity profile in the liquid film, the liquid film was assumed to be in Couette flow forced by the interfacial velocity at the liquid–vapor interface. For simplifying the calculation procedures, the interfacial velocity was estimated by introducing an empirical power-law velocity profile. The resulting film thickness and heat transfer coefficient from the model were compared with the experimental data and the results obtained from the other condensation models. The results demonstrated that the proposed model described the liquid film thinning effect by the vapor shear flow and predicted the condensation heat transfer coefficient from experiments reasonably well.     

Boundary film shear elastic modulus effect in hydrodynamic contact. Part I: theoretical analysis and typical solution

Boundary film shear elastic modulus effect is analyzed in a hydrodynamic contact. The contact is one-dimensional composed of two parallel plane surfaces, which are, respectively, rough rigid with rectangular micro projections in profile periodically distributed on the surface and ideally smooth rigid. The whole contact is consisted of cavitated area and hydrodynamic area. The hydrodynamic area consists of many micro Raleigh bearings which are discontinuously and periodically distributed in the contact. Analysis is thus carried out for a micro Raleigh bearing in this contact. The hydrodynamic contact in this micro Raleigh bearing consists of boundary film area and fluid film area which, respectively, occur in the outlet and inlet zones. In boundary film area, the film slips at the upper contact surface due to the limited shear stress capacity of the film–contact interface, while the film does not slip at the lower contact surface due to the shear stress capacity large enough at the film–contact interface. In boundary film area, the viscosity, density and shear elastic modulus of the film are varied across the film thickness due to the film–contact interactions, and their effective values are used in modeling, which depend on the film thickness. The analytical approach proposed by Zhang (J Mol Liq 128:60–64, 2006) and Zhang et al. (Int J Fluid Mech Res 30:542–557, 2003) is used for boundary film area. In fluid film area, the film does not slip at either of the contact surfaces, and the shear elastic modulus of the film is neglected. Conventional hydrodynamic analysis is used for fluid film area. The present paper presents the theoretical analysis and a typical solution. It is found that for the simulated case the boundary film shear elastic modulus effects on the mass flow through the contact, the overall film thickness of the contact and the carried load of the contact are negligible but the boundary film shear elastic modulus effect on the local film thickness of the contact may be significant when the boundary film thickness is on the 1 nm scale and the contact surfaces are elastic. In Part II will be presented detailed results showing boundary film shear elastic modulus effects in different operating conditions.

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Three-Phase Effective Contact Angle in a Model Pore

We consider a model pore (2D) in which a sharp interface between two fluids contact a third fluid which wets the solid boundary. If the configuration is capillary dominated, the geometry can be determined analytically in terms of the effective contact angle. This angle depends not only on the interfacial tensions, but also on the capillary pressures. However, if the height of the cusp formed by the wetting fluid is much smaller than the pore width, the effective contact angle is a simple function of the interfacial tensions. It turns out to be the same function as in the case of an undeformed wetting layer of molecular thickness. The analytical expression for the effective contact angle has been confirmed by a numerical technique, known as the lattice-Boltzmann method. This method, in turn, has been validated with Neumann's law for the three-phase contact angles.