Liquid flow over a substrate structured by seeded nanoparticles

Recent experiments have demonstrated that nanoparticles which sparsely distributed over a solid substrate can substantially change the flow conditions at the solid surface in the presence of slip. Inspired by these observations, the flow past tiny particles seeded on a solid substrate is investigated theoretically in the framework of an interface formation model. It has been shown, that even a single seeded nanoparticle can reduce significantly the measurable tangential component of hydrodynamic velocity at the substrate and affect the amount of the observed apparent slippage of the liquid. The effect from the particle manifests in a form of a long relaxation tail defined by the characteristic time of the interface formation process. A comparison with experiments has demonstrated a good agreement between theoretically predicted and experimentally observed values of the relaxation tail length scale.     

非对称纳米通道内流体流动与传热的分子动力学

采用分子动力学方法研究了流体在非对称浸润性粗糙纳米通道内的流动与传热过程,分析了两侧壁面浸润性不对称对流体速度滑移和温度阶跃的影响,以及非对称浸润性组合对流体内部热量传递的影响.研究结果表明,纳米通道主流区域的流体速度在外力作用下呈抛物线分布,但是纳米通道上下壁面浸润性不对称导致速度分布不呈中心对称,同时通道壁面的纳米结构也会限制流体的流动.流体在流动过程中产生黏性耗散,使流体温度升高.增强冷壁面的疏水性对近热壁面区域的流体速度几乎没有影响,滑移速度和滑移长度基本不变,始终为锁定边界,但是会导致近冷壁面区域的流体速度逐渐增大,对应的滑移速度和滑移长度随之增大.此时,近冷壁面区域的流体温度逐渐超过近热壁面区域的流体温度,流体出现反转温度分布,流体内部热流逆向传递.随着两侧壁面浸润性不对称程度增加,流体反转温度分布更加明显.     

Weakly bound buffer layers: A versatile template for metallic nano-clusters growth and film patterning on solid surfaces

Buffer layers composed of weakly bound atoms or molecules on solid surfaces are discussed as a versatile platform for size controlled growth of nano-clusters and for patterning of thin metallic films. Metallic nano-crystals can be prepared and their size and density be controlled by varying the film thickness. Cold metallic or oxide clusters, softly land and deposit on solid substrates via the Buffer Layer Assisted Growth (BLAG) method. Their final structure, therefore, reflects purely the interaction with the substrate with no kinetic constraints that may play a role in direct deposition methods of hot atoms. The nature of the buffer film, being a rare gas or a molecular film, can somewhat affect the shape of clusters as well. Applying laser ablation techniques, these weakly bound films were demonstrated to assist in patterning of metallic films. It operates on practically any cold, flat solid substrate that absorbs the laser light. Parallel stripes at sub-micron width, millimeter long, were obtained experimentally, with line width determined by the ablating laser power. The versatility of these weakly bound films in manipulating the structure of metallic particles and thin films is discussed in its wider potential scope.     

带凹槽的微通道中液滴运动数值模拟

运用改进的耗散粒子动力学方法模拟了液滴在由凹槽所构成的粗糙表面微通道内的运动行为.改进的耗散粒子动力学方法采用新近提出的一种短程排斥、长程吸引相互作用势能函数,从而可以模拟带有自由面的流体,如液滴等.模拟了新势能函数下液滴与固体壁面的静态接触角,并用2次多项式拟合了"接触角-a_wf/a_f"变化曲线.研究了液滴在带凹槽的微通道中运动时,微通道壁面浸润性、外场力、液滴温度对液滴流动特性的影响.研究表明壁面浸润性和外场力对液滴流动特性的影响较大,液滴温度对液滴流动特性的影响较小.研究结果对运用耗散粒子动力学方法模拟并分析微流体在复杂微通道的流动有一定的参考价值.     

基于润湿阶跃的水下大尺度气膜封存方法

超疏水表面水下减阻效果通常与其微结构上封存气膜的厚度和面积正相关,且气膜尺寸越大封存越困难.构造亲疏水相间表面,能在壁面形成润湿阶跃,产生约束固-气-液三相接触线移动的束缚力.通过监测切向水流作用下,润湿阶跃为54.8?,84.7?,103.6?和144.0?的亲疏水相间表面上不同面积和厚度气膜的形态发现,厘米尺度气膜可被长时间稳定封存,且气膜破坏的临界流速随润湿阶跃和气膜厚度的增加而升高,随气膜迎流宽度增加而降低.同时,该方法封存的气膜上能产生显著滑移量,尺寸0.6 cm×0.5 cm×0.15 cm的气膜上即可产生约占主流速度25%的稳定滑移速度.期待该气膜封存方法能进一步提升超疏水表面水下减阻技术性能.     

用格子Boltzmann方法模拟壁面微结构对管流特性的影响

采用格子Boltzmann方法和Shan-Chen多相流模型,模拟液滴在常力驱动下在微管道内的流动,研究微孔道壁面润湿性和几何结构对降压增注效果的影响.阐明具有一定粗糙度的疏水表面阻力减小的原因,研究表明,壁面润湿性和粗糙度对管流特性有显著的影响,同时也表明,用Lattice Boltzmann方法模拟,具有相当精确的可预测性,在石油储层微观渗流减阻机制研究方面有很好的应用前景,其可为研究纳米降压增注效果提供有用工具.     

Wetting and coalescence of the liquid metal on the metal substrate

Molecular dynamics(MD) simulations are performed to investigate the wettability of liquid metal on the metal substrate. Results show that there exists different wettability on the different metal substrates, which is mainly determined by the interaction between the liquid and the substrate. The liquid metal is more likely to wet the same kind of metal substrate,which attracts the liquid metal to one side on the hybrid substrate. Exchanging the liquid metal and substrate metal has no effect on the wettability between these two metals. Moreover, the study of metal drop coalescing indicates that the metal substrate can significantly affect the coalescence behavior, in which the changeable wettability of liquid metal plays a predominant role. These studies demonstrate that the wetting behavior of liquid metal can be controlled by choosing the suitable metal substrate.     

Energetic study of ultrasonic wettability enhancement

Many industrial and biological interfacial processes, such as welding and breathing depend directly on wettability and surface tension phenomena. The most common methods to control the wettability are based on modifying the properties of the fluid or the substrate. The present work focuses on the use of high-frequency acoustic waves (ultrasound) for the same purpose. It is well known that ultrasound can effectively clean a surface by acoustic cavitation, hence ultrasonic cleaning technology. Besides the cleaning process itself, many authors have observed an important wettability enhancement when liquids are exposed to low and high (ultrasonic) frequency vibration. Ultrasound goes one step further as it can instantly adjust the contact angle by tuning the vibration amplitude, but there is still a lack of comprehension about the physical principles that explain this phenomenon. To shed light on it, a thermodynamic model describing how ultrasound decreases the contact angle in a three-phase wetting system has been developed. Moreover, an analytical and experimental research has been carried out in order to demonstrate that ultrasound is an important competitor to surfactants in terms of energy efficiency and environmental friendliness.     

高温壁面润湿性对气层稳定性及其壁面滑移性能的分子动力学研究

针对流体在纳米通道的小尺度效应,采用分子动力学方法模拟了传热效应以及流体流动行为,研究在壁面温度影响下,不同润湿性壁面上方气层生成状态以及流体流动时气层的稳定特性和相应的减阻性能.结果表明:当壁面为纯疏水壁面时,不能形成气层;疏水基底+亲水组合壁面形成不规则气层;纯亲水壁面和亲水基底+疏水组合壁面能形成规则气层.当流体流动时,疏水基底+亲水组合壁面气层消失,而纯亲水壁面和亲水基底+疏水组合壁面气层较为稳定.纯疏水壁面主流区域速度较大,而纯亲水壁面主流区域最低.对于壁面滑移速度,存在气层的壁面滑移速度与纯疏水表面相对接近,甚至稍优于纯属疏水表面,而疏水基底+亲水组合壁面滑移速度最小.     

微通道内不混溶气液两相二氧化碳界面动力学行为的格子Boltzmann研究

将高精度的二氧化碳状态方程与气液两相流格子Boltzmann方法中的伪势模型耦合,研究微通道内二氧化碳气液两相流动的界面动力学行为,包括二氧化碳气泡和液滴的分裂、合并、变形,以及气液两相二氧化碳在演化过程中的质量交换.研究发现：当分裂和合并行为达到平衡,并且两相之间不发生质量交换时流动达到稳态.稳态时的流型主要依赖于表面张力,惯性力,管道的润湿性,以及初始体积分数.当表面张力较大时,微通道内形成的二氧化碳气泡或液滴会收缩成圆形,此时二氧化碳气泡或液滴会堵塞微通道,形成段塞流;随着表面张力的减小,形成的气泡或液滴不容易收缩,在微通道内更容易发生变形,出现泡状流或环状流.当壁面润湿性为强疏水性时,二氧化碳在微通道中的流动为环状流,其它润湿性下,流型为段塞流.体积分数较小时,二氧化碳两相流动的流型为段塞流,体积分数较大时,流型为环状流.     

纳米通道粗糙内壁对流体流动行为的影响

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.本文采用分子动力学方法以两无限大平行非对称壁面组成的Poiseuille流动为对象,分析了壁面粗糙度与润湿性变化对通道内流体流动的影响.对于不同结构类型的壁面,需要通过水动力位置来确定固液界面位置,准确计算固液界面位置有助于更好地分析界面滑移效应.研究结果表明,上下壁面不对称会引起通道内流场参数分布的不对称,壁面粗糙度及润湿性的变化会影响近壁面附近流体原子的流动特性,由于壁面凹槽的存在,粗糙壁面附近的数密度分布低于光滑壁面一侧.壁面粗糙度及润湿性的变化会影响固液界面位置,肋高变化及壁面润湿性对通道中速度分布影响较大,界面滑移速度及滑移长度随肋高和润湿性的增大而减小;肋间距变化对通道内流体流动影响较小,界面滑移速度和滑移长度基本保持恒定.     

Anomalous Impact of Surface Wettability on Leidenfrost Effect at Nanoscale

Leidenfrost effect is a common and important phenomenon which has many applications,however there is a limited body of knowledge about the Leidenfrost effect at the nanoscale regime.We investigate the impact of substrate wettability on Leidenfrost point temperature(LPT) of nanoscale water film via molecular dynamics simulations,and reveal a new mechanism different from that at the macroscale.In the molecular dynamics simulations,a method of monitoring density change at different heating rates is proposed to obtain accurate LPT under different surface wettability.The results show that LPT decreases firstly and then increases with the surface wettability at the nanoscale,which is different from the monotonous increasing trend at the macroscale.The mechanism is elucidated by analyzing the competitive effect of adhesion force and interfacial thermal resistance,as well as different contributions of gravity on LPT at the nanoscale and macroscale.The investigations can deepen the understanding of Leidenfrost effect at the nanoscale regime and also facilitate to guide the applications of heat transfer and flow transport.     

自润湿流体液滴的热毛细迁移特性

采用数值模拟方法研究了自润湿流体液滴的热毛细迁移特性.基于润滑理论和滑移边界条件建立了二维液滴运动的演化模型,分析了液气界面张力极小值对应温度在壁面上的位置(临界点)与液滴位置间的相对关系对液滴运动特性的影响.结果表明,对于壁面润湿性不随温度变化的情形,随液滴初始位置相对临界点的向左移动,液滴的迁移方向发生改变,但液滴受热毛细力驱动总是向界面张力高的方向移动.对于壁面润湿性随温度变化的情形,无论液滴初始放置于临界点何处,受高温侧壁面润湿性恶化的影响,液滴均向低温区迁移;随液滴初始位置相对临界点的向左移动,液滴受方向向左的热毛细力增大,提高了其向低温区的迁移速率.控制自润湿流体液滴运动可通过调控临界点与液滴位置间的关系来实现,欲抑制液滴向低温区的迁移,则应将液滴放置于临界点右侧.     

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.     

界面润湿性及固相体积分数对颗粒粗化动力学影响的相场法研究

利用多相场模型模拟了液-固两相体系中固相颗粒的粗化过程, 分析了界面润湿性及固相体积分数对粗化指数、粗化速率及颗粒尺寸分布的影响.结果表明, 不同固相体积分数下粗化指数基本不变, 但粗化速率常数及尺寸分布与固相体积分数及界面润湿性密切相关.在完全润湿条件下, 随着固相体积分数的增加, 粗化速率常数逐渐增大; 而非完全润湿条件下, 随着固相体积分数的增加, 粗化速率常数增大速度变缓, 且当润湿性较低、 固相分数较大时, 粗化速率常数还将随体积分数的增加而下降. 此外, 模拟结果表明各种润湿条件下颗粒的尺寸分布均随着固相分数增加而变宽, 分布峰值降低, 但非完全润湿条件下峰值下降变缓.模拟结果为理解不同实验观测结果之间的分歧提供了依据. 关键词： 粗化 相转变 相场法 润湿性     

Substrate-assisted laser patterning of indium tin oxide thin films

Maskless laser patterning of indium tin oxide (ITO) thin films was studied by the use of a diode-pumped Q-switched Nd:YLF laser. The ITO films were sputter-deposited either on lime glass, the standard substrate material for flat panel display applications, or fused quartz so that the efficiency of laser patterning as a function of substrate absorption could be studied. The laser wavelength was varied among infrared (5=1047 nm), visible (5=523 nm), and ultraviolet (5=349 nm and 5=262 nm). It is observed that strong light absorption by the substrate is a crucial requirement for a residue-free patterning of the ITO film. Observations and numerical calculations of the laser-induced surface temperature indicate that material removal occurs via thermal vaporization and that other mechanisms such as photochemical decomposition or spallation can be neglected.     

Dynamical model for chemically driven running droplets

We propose coupled evolution equations for the thickness of a liquid film and the density of an adsorbate layer on a partially wetting solid substrate. Therein, running droplets are studied assuming a chemical reaction underneath the droplets that induces a wettability gradient on the substrate and provides the driving force for droplet motion. Two different regimes for moving droplets--reaction-limited and saturated regime--are described. They correspond to increasing and decreasing velocities with increasing reaction rates and droplet sizes, respectively. The existence of the two regimes offers a natural explanation of prior experimental observations.     

Mechanisms for liquid slip at solid surfaces

One of the oldest unresolved problems in fluid mechanics is the nature of liquid flow along solid surfaces. It is traditionally assumed that across the liquid-solid interface, liquid and solid speeds exactly match. However, recent observations document that on the molecular scale, liquids can slip relative to solids. We formulate a model in which the liquid dynamics are described by a stochastic differential-difference equation, related to the Frenkel-Kontorova equation. The model, in agreement with molecular dynamics simulations, reveals that slip occurs via two mechanisms: localized defect propagation and concurrent slip of large domains. Well-defined transitions occur between the two mechanisms.     

Solid-state wetting on nanopatterned substrates

We review some recent results on the morphology of heteroepitaxial overlayers deposited on nanopatterned substrate surfaces. We mainly focus on modeling based on kinetic Monte Carlo (KMC) simulations. On arrays of parallel grooves or arrays of pillars, we find three main classes of states similar to those known for liquids: (i) Cassie–Baxter states, where solid islands sit on top of the substrate structure; (ii) Wenzel states, where solid islands are in contact with the bottom of the substrate; and (iii) solid imbibition (also denoted as capillary filling for parallel grooves), where the solid forms a film in the substrate surface structure. This multi-stability is controlled by the wettability parameter χ, and shape transitions exhibit hysteresis. We discuss the dynamics of the collapse of a Cassie–Baxter to Wenzel state, and the collapse of the Wenzel state to an imbibition film. We also discuss the possibility to grow crystals in the CB state on nanopatterned substrates. During growth, the crystals formed in CB state merge, leading to a continuous polycrystalline overlayer where the size of the grains is controlled by the distance between pillars.     

Effects of wettability and interfacial nanobubbles on flow through structured nanochannels: an investigation of molecular dynamics

Solid–fluid boundary conditions are strongly influenced by a number of factors, including the intrinsic properties of the solid/fluid materials, surface roughness, wettability, and the presence of interfacial nanobubbles (INBs). The interconnected nature of these factors means that they should be considered jointly. This paper employs molecular dynamics (MD) simulation in a series of studies aimed at elucidating the influence of wettability in boundary behaviour and the accumulation of interfacial gas. Specifically, we examined the relationship between effective slip length, the morphology of nanobubbles, and wettability. Two methods were employed for the promotion of hydrophobicity between two structured substrates with similar intrinsic contact angles. We also compared anisotropic and isotropic atomic arrangements in the form of graphite and Si(100), respectively. A physical method was employed to deal with variations in surface roughness, whereas a chemical method was used to adjust the wall–fluid interaction energy (?_wf). We first compared the characteristic properties of wettability, including contact angle and fluid density within the cavity. We then investigated the means by which variations in solid–fluid interfacial wettability affect interfacial gas molecules. Our results reveal that the morphology of INB on a patterned substrate is determined by wettability as well as the methods employed for the promotion of hydrophobicity. The present study also illustrates the means by which the multiple effects of the atomic arrangement of solids, surface roughness, wettability and INB influence effective slip length.