接触角与液固界面热阻关系的分子动力学模拟

本文利用分子动力学方法模拟了液体在固体表面的 接触角及液固界面热阻, 并探讨了二者之间的关系. 通过分别改变液固结合强度和固体的原子性质来分析接触角和界面热阻的关系及变化趋势. 模拟结果显示增强液固间相互作用时, 接触角减小的同时界面热阻也随之单调减小; 而改变固体原子间结合强度和原子质量时, 接触角几乎保持不变, 但界面热阻显著改变. 固体原子间结合强度和原子质量影响界面热阻的原因是其改变了固体的振动频率分布, 导致液固原子间的振动耦合程度发生变化. 本文的结果表明界面热阻不仅与由接触角所表征的液固结合强度有关, 还与液固原子间的振动耦合程度有关. 接触角与界面热阻间不存在单值的对应关系, 不能单一地将接触角作为液固界面热阻的评价标准. 关键词： 液固界面 接触角 界面热阻 分子动力学模拟     

尺寸效应对微通道内固液界面温度边界的影响

采用非平衡分子动力学方法模拟不同浸润性微通道内液体的传热过程,分析了尺寸效应对固液界面热阻及温度阶跃的影响.研究结果表明,界面热阻随微通道尺寸的变化可分为两个阶段,即小尺寸微通道的单调递增阶段和大尺寸微通道的恒定值阶段.随着微通道尺寸的增加,近壁区液体原子受对侧固体原子的约束程度降低,微通道中央的液体原子自由移动,固液原子振动态密度近似不变,使得尺寸效应的影响忽略不计.上述两种阶段的微通道尺寸过渡阈值受固液作用强度与壁面温度的共同作用:减弱壁面浸润性,过渡阈值向大尺寸区域迁移;相较于低温壁面,高温壁面处的过渡阈值更大.增加微通道尺寸,固液界面温度阶跃呈单调递减趋势,致使壁面温度边界和宏观尺度下逐渐符合.探讨尺寸效应有助于深刻理解固液界面能量输运及传递机制.     

Peculiarities of evaporation of a thin water layer in the presence of a solvable surfactant

Evaporation of a thin layer of a polar liquid (water) having a free surface and located on a solid substrate is investigated. A solvable surfactant is placed on the free liquid-vapor interface. The surface tension is a linear function of the surface concentration of the surfactant. The surface energy of the solid-liquid contact line is a nonmonotonic function of the layer thickness and is the sum of the Van der Waals interaction and the specific interaction of the double electric layer on the interface. The effect of the solvable surfactant on the dynamics and stability of the propagation of the evaporation front in the thin liquid film is analyzed in the long-wave approximation in the system of Navier-Stokes equations.     

改性疏水固壁润湿性反转现象的格子Boltzmann方法模拟

基于疏水固壁改性会引起润湿性反转的特点,采用考虑固体与液体间分子力的格子Boltzmann方法,从壁面的线性和瞬时改性两方面对润湿性反转现象进行了数值模拟,并结合流体体积方法处理界面层质量.结果表明:壁面线性改性的过程中润湿性反转变化平稳,润湿所需时间大幅减少,所得到的接触角与固液吸引力系数的关系与其他文献结果一致;壁面瞬时改性幅度越大说明固壁对液滴作用力越强,表现为润湿性变化越明显,瞬时改性后接触角随时间呈指数规律变化,这与现有结论相符合.研究发现:在改性条件下液膜铺展过程中伴随着振荡变化,线性改性的振动峰值与改性幅度相关;瞬时改性的液膜速度会在某一时刻突然增大,这种现象与夹带空气有关.     

微液滴在不同能量表面上润湿状态的分子动力学模拟

微小液滴在不同能量表面上的润湿状态对于准确预测非均相核化速率和揭示界面效应影响液滴增长微观机理具有重要意义. 通过分子动力学模拟, 研究了纳米级液滴在不同能量表面上的铺展过程和润湿形态. 结果表明, 固液界面自由能随固液作用强度增加而增加, 并呈现不同液滴铺展速率和润湿特性. 固液作用强度小于1.6的低能表面呈现疏水特征, 继续增强固液作用强度时表面变为亲水, 而固液作用强度大于3.5的高能表面上液体呈完全润湿特征. 受微尺度条件下非连续、非对称作用力影响, 微液滴气液界面存在明显波动, 呈现与宏观液滴不同的界面特征. 统计意义下, 微小液滴在不同能量表面上铺展后仍可以形成特定接触角, 该接触角随固液作用强度增加而线性减小, 模拟结果与经典润湿理论计算获得的结果呈现相似变化趋势. 模拟结果从分子尺度为核化理论中的毛细假设提供了理论支持, 揭示了液滴气液界面和接触角的波动现象, 为核化速率理论预测结果和实验测定结果之间的差异提供了定性解释.     

Stress field and interaction forces of dislocations in anisotropic multilayer thin films

Utilizing Fourier transforms, the elastic field of three-dimensional dislocation loops in anisotropic multilayer materials is developed. Green's functions and their derivatives, obtained first in the Fourier domain and then in the real domain by numerical inversion, are used in integrals to determine the elastic field of dislocation loops. The interaction forces between dislocations and free surfaces or interfaces in multilayer thin films are then investigated. The developed method is based on rigorous elasticity solutions for dislocations approaching to within one to two atomic planes from the interface. For a dislocation in one layer, the interface image force is determined mainly by the elastic moduli and thicknesses of neighbouring layers. When a dislocation approaches an interface between two layers, within 10–20 atomic planes, the image force changes rapidly. Interaction forces are then kept constant up to the interface. The model shows that, when a dislocation crosses an interface from a soft to a hard layer, additional external forces must be applied to overcome an elastic mismatch barrier. The developed method extends the concept of the Kohler barrier in 2D, and shows that the interface force barrier not only depends on the relative ratio of the elastic moduli of neighbouring layers, but also on the 3D shape of the dislocation, the number of interacting adjacent layers, and on layer thicknesses.     

Effect of contact line curvature on solid-fluid surface tensions without line tension

For sessile droplets partially wetting a solid surface, it has been observed experimentally that the value of the contact angle depends on the contact line curvature and this dependence has been attributed to tension in the contact line. But previous analyses of these observations have neglected adsorption at the solid-liquid interface and its effect on the surface tension of this interface. We show that if this adsorption is taken into account the relation between the contact angle and contact line curvature is completely accounted for without introducing line tension. Further, from the observed relation between the contact angle and contact line curvature, the adsorption at the solid-liquid interface can be determined, as can the surface tensions of the solid-liquid and solid-vapor interfaces.     

Nanoscale deformation of a liquid surface

We study the interaction between a solid particle and a liquid interface. A semianalytical solution of the nonlinear equation that describes the interface deformation points out the existence of a bifurcation behavior for the apex deformation as a function of the distance. We show that the apex curvature obeys a simple power-law dependency on the deformation. Relationships between physical parameters disclose the threshold distance at which the particle can approach the liquid before capillarity provokes a "jump to contact." A prediction of the interface original position before deformation takes place, as well as the attraction force measured by an approaching probe, are produced. The results of our analysis agree with the force curves obtained from atomic force microscopy experiments over a liquid puddle.     

润湿性梯度驱动液滴运动的格子Boltzmann模拟

运用考虑了固体与液体间分子作用力的格子Boltzmann方法,数值研究了由于固液界面上表面张力梯度引起的Marangoni效应驱动的液滴运动.当表面张力梯度较小时,计算结果和前人的理论预测符合较好.而表面张力梯度较大时,由于液滴不变形和准平衡态等假设不再满足,理论预测的液滴运动速度高于数值模拟的结果.计算结果显示,在向亲水端运动过程中液滴内部出现旋涡结构,当润湿性梯度较大时,其前进速度和接触角随时间变化出现振荡. 关键词： 润湿性 格子Boltzmann方法 Marangoni效应 液滴     

润湿性梯度驱动液滴运动的格子Boltzmann模拟

运用考虑了固体与液体间分子作用力的格子Boltzmann方法,数值研究了由于固液界面上表面张力梯度引起的Marangoni效应驱动的液滴运动.当表面张力梯度较小时,计算结果和前人的理论预测符合较好.而表面张力梯度较大时,由于液滴不变形和准平衡态等假设不再满足,理论预测的液滴运动速度高于数值模拟的结果.计算结果显示,在向亲水端运动过程中液滴内部出现旋涡结构,当润湿性梯度较大时,其前进速度和接触角随时间变化出现振荡.     

Molecular dynamics simulation of Ga penetration along grain boundaries in Al: a dislocation climb mechanism

Many systems where a liquid metal is in contact with a polycrystalline solid exhibit deep liquid grooves where the grain boundary meets the solid-liquid interface. For example, liquid Ga quickly penetrates deep into grain boundaries in Al, leading to intergranular fracture under very small stresses. We report on a series of molecular dynamics simulations of liquid Ga in contact with an Al bicrystal. We identify the mechanism for liquid metal embrittlement, develop a new model for it, and show that is in excellent agreement with both simulation and experimental data.     

Molecular simulation study of the adhesion work for water droplets on water monolayer at room temperature

The wetting phenomenon of water droplets coexisting with the ordered water monolayer termed an unexpected phenomenon of "water that does not wet a water monolayer" at room temperature has been found on several solid surfaces.Although the hydrogen bond saturation inside the monolayer can qualitatively describe this phenomenon, whether the Young–Dupré equation still holds under this unconventional wetting framework is still not answered. In this work, we have calculated the contact angle values of the droplets as well as the work of adhesion between the droplets and the monolayer based on an extended phantom-wall method. The results show that similar to the conventional solid–liquid interface,classical Young–Dupré equation is also applicable for the interface of liquid water and ordered water monolayer.     

A Possible Mechanism for the Formation of Unwettable Dry Spots on the Heated Surface under Nucleate Boiling: Part I. Basic Models and Characteristics of Heterogeneous Nucleate Pool Boiling at a Low Pressure

The problems associated with the physics of heterogeneous pool boiling at a low pressure on a flat horizontal surface are considered. The examples of parametric mismatch between the trends are connected with the thermophysical properties and wall surface microgeometry, with the size of the working area, with the affected zone of a growing bubble, and with the contact angle at the interface between the liquid, solid, and vapor phases. A conclusion is drawn concerning the possible causes of ambiguity in the results of the simulation of nucleate pool boiling modes under calculation of the individual contribution of each of the heat transfer mechanisms (convection, liquid microlayer evaporation, and rearrangement of the thermal boundary layer after bubble detachment). It is emphasized that the problem should be solved in the 3D conjugate formulation.     

The behaviour of liquid alkanes near interfaces

Simulations of thick films of liquid alkanes supported on a wax-like substrate were carried out at a number of temperatures in order to investigate the structure and dynamics of molecules near the solid-liquid and the liquid-vapour interfaces. Films of butane, octane and a mixture were investigated. Near the solid surface the liquids were found to be structured and molecular diffusion slowed. However, there was no evidence of a frozen layer at this interface even near the bulk freezing temperature. The mixed liquid showed considerable segregation at both interfaces with preferential absorption of butane at the liquid-vapour interface and octane at the liquid-solid interface.     

Dynamics of collapse of air films in drop impact

Liquid drops hitting solid surfaces deform substantially under the influence of the ambient air that needs to be squeezed out before the liquid actually touches the solid. Nanometer- and microsecond-resolved dual wavelength interferometry reveals a complex evolution of the interface between the drop and the gas layer underneath. For intermediate impact speeds (We～1…10) the layer thickness can develop one or two local minima-reproduced in numerical calculations-that eventually lead to the nucleation of solid-liquid contact at a We-dependent radial position, from a film thickness >200 nm. Solid-liquid contact spreads at a speed involving capillarity, liquid viscosity and inertia.     

Non-Contact to Contact Transition： Direct Measurements of Interaction Forces between a Solid Probe and a Planar Air-Water Interface

The interaction force between a solid probe and a planar air-water interface is measured by using an atomic force microscope. It is demonstrated that during the approach of the probe to the air-water interface, the force curves decline all the time due to the van der Waals attraction and induces a stable profile of water surface raised. When the tip approaches very close to the water surface, force curves jump suddenly, reflecting the complex behaviour of the unstable water surface. With a theoretical analysis we conclude that before the tip touches water surface, two water profiles appear, one stable and the other unstable. Then, with further approaching, the tip touches water surface and the non-contact to contact transition occurs.     

Mg-Al合金熔体中固液界面结构的分子动力学研究

本文采用分子动力学研究了Mg-3%Al合金熔体中固液界面结构及界面附近原子的扩散行为.计算结果表明,该二元合金的固液界面为粗糙界面.垂直于界面方向的数密度分布,表现出复杂波动的特征,这种波动一直延伸到液体中.在界面附近的区域,扩散系数的三个不同方向的分量表现出了明显的各向异性,并且这种各向异性一直持续到液相当中.对界面二维结构的分析表明,界面附近液相原子的二维排列呈现出从长程有序逐渐过渡到短程有序的变化.     

Thermodynamic analysis of liquid bridge for fixed volume in atomic force microscope

In ambient condition,capillary forces are the major contributors to the adhesive forces between the tip of an atomic force microscope(AFM) and the sample.In general,capillary forces are thought to be related to water film thickness,contact time and relative humidity and so on.In this paper,an original analysis regarding the liquid bridge,based on the surface and interface thermodynamic theory,is proposed.The cases covered in the study include the capillary forces and temperature of liquid bridge for quickly drawn liquid bridge,and for nonvolatile liquid bridge.The study results show that variation in temperature may occur in the liquid bridge when it is stretched.