接触角滞后现象的理论分析

在本文的研究中，考虑表面粗糙的影响，通过引入附加“磨擦力”的概念，分别用力学方法和热力学方法导出固体表面上液滴平衡时接触角应满足的条件；并得到了液滴系统自由能－固液接触面积曲线；分析了前进接触角和后退接触角的物理意义；由此给出了接触角滞后现象的一种合理解释．这对于进一步认识接触角的滞后现象，无疑是有积极意义的．     

Changes in wetting and energetic properties of glass caused by deposition of different lipid layers

An investigation of wetting and energetic properties of different lipid layers deposited on the glass surface was carried out by contact angles measurements and determination of the apparent surface free energy. The topography of the lipid layers was also determined with the help of atomic force microscopy (AFM). Two synthetic phospholipids were chosen for these studies, having the same phosphatidylcholine headgroup bound to the apolar part composed either by two saturated chains (1,2-dipalmitoyl-sn-glycero-3-phospshocholine - DPPC) or two unsaturated chains (1,2-dioleoyl-sn-glycero-3-phosphocholine - DOPC) and one lipid (1,2,3-trihexadecanoyl-sn-glycerol - tripalmitoylglycerol - TPG). The lipid layers, from the 1st to the 5th statistical monolayer, were deposited on the glass surface from chloroform solutions by spreading.The apparent surface free energy of the deposited layers was determined by contact angles measurements (advancing and receding) for three probe liquids (diiodomethane, water, and formamide), and then two concepts of interfacial interactions were applied. In the contact angle hysteresis approach (CAH) the apparent total surface free energy was calculated from the advancing and receding contact angles and surface tension of probe liquids. In the Lifshitz-van der Waals/acid-base approach (LWAB) the total surface free energy was calculated from the determined components of the energy, which were obtained from the advancing contact angles of the probe liquids only. Comparison of the results obtained by two approaches provided more information about the changes in the hydrophobicity/hydrophilicity of the layers depending on the number of monolayers and kind of the lipid deposited on the glass surface.It was found that the most visible changes in the surface free energy took place for the first two statistical monolayers irrespectively of the kind of the lipid used. Additionally, in all cases periodic oscillations from layer-to-layer in the lipid surface free energy were observed. The changes in the surface free energy correlated with those in the topography and roughness of lipid layers.     

Two component silicone modified epoxy foul release coatings: Effect of modulus, surface energy and surface restructuring on pseudobarnacle and macrofouling behavior

Two component silicone modified epoxy resin based low surface energy and non-toxic foul release coatings were developed. Silicone modified epoxy resin with 15 and 30% silicone content was used as component A and a polyether diamine (Jeffamine-500) was used as the component B. Free standing films were prepared by casting a mixture of components A and B in stoichiometric proportions. The surface composition, surface topography and wetting properties of the coatings were studied by angle resolved X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and contact angle goniometry respectively. The mechanical properties of the cured films were evaluated by tensile measurements as well as dynamic mechanical analysis. Pseudobarnacles made of aluminium studs were attached to the coated panels and adhesion tests were carried out by a pseudobarnacle tester. Coated panels were exposed in Mumbai harbor for fouling studies for a period of 90 days. Surface restructuring studies of the coatings upon immersion in seawater were carried out by measuring the changes in advancing and receding contact angles by contact angle goniometry. The effect of surface energy, modulus and surface restructuring of the coatings on the macrofouling and pseudobarnacle adhesion properties has been discussed in detail.     

A study of the oscillating corner meniscus in a vertical constrained vapor bubble system

A vertical constrained vapor bubble, VCVB, made of fused silica was used to study the stability and oscillations of an evaporating wetting film of HFE- 7000^® in a corner. The film thickness profile was measured as a function of time and axial position using an advanced form of image analyzing interferometry. The curvature, apparent contact angle, and pressure profiles for the evaporating film were calculated from the measured film thickness profiles. Oscillation of the liquid film was observed and profiles for both the advancing and receding films were obtained. These are the first such detailed profiles obtained for an oscillating meniscus below a thickness of 0.1 μm.The film thickness profiles demonstrated the spreading of the meniscus during advance as well as the presence of a curvature gradient near the contact line region. The maximum curvature decreased for the advancing menisci and increased with time for the receding menisci. An increase in the adsorbed film thickness was associated with the advancing stage and a decrease with the receding stage. Pressure profiles were measured as a function of position indicating the potential for driving the flow of the fluid toward or away from the contact line. As the film advances or recedes, the pressure gradients change as a function of position fueling the next oscillation cycle.     

Numerical simulation of an evaporative meniscus on a moving substrate

A hydrodynamic model based on lubrication theory has been developed to describe an evaporative meniscus in a complete wetting configuration, when evaporation takes place in ambient air. We focus on combined effects of evaporation and the substrate motion on the effective contact angle. Numerical simulations show two distinct regimes when varying the substrate velocity on several orders of magnitude. At a small velocity, the effective contact angle is governed by evaporation and is independent of the substrate velocity, while the substrate motion is dominant at a high velocity. In the latter case, a Landau-Levich regime is obtained for the receding contact line, and a Cox-Voinov regime for the advancing contact line. Finally, we use our numerical results to test the simplified model developed by Pham et al. [5,6].     

Influence of (phospho)lipases on properties of mica supported phospholipid layers

The effect of enzymes: lipase from Candida cylindracea (L_Cc), phospholipase A₂ from hog pancreas (PLA₂) and phospholipase C from Bacillus cereus (PLC) to modulate wetting properties of solid supported phospholipid bilayers was studied via advancing and receding contact angle measurements of water, formamide and diiodomethane, and calculation of the surface free energy and its components from van Oss et al. (LWAB) and contact angle hysteresis (CAH) approaches. Simultaneously, topography of the studied layers was determined by Atomic Force Microscopy (AFM). The investigated lipid bilayers were transferred on mica plates from subphase of pure water by means of Langmuir-Blodgett and Langmuir-Schaefer techniques. The investigated phospolipid layers were: saturated DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine), unsaturated DOPC (1,2-dioleoyl-sn-glycero-3-phosphocholine), and their mixture DPPC/DOPC. The obtained results revealed that the lipid membrane degradation by the enzymes caused increase in its surface free energy due to the amphiphilic hydrolysis products, which may accumulate in the lipid bilayer. In result activity of the enzymes may increase and then break down the bilayer structure takes place. It is likely that after dissolution of the hydrolysis reaction products in the bulk phase, patches of bare mica surface are accessible, which contribute to the apparent surface free energy changes. Comparison of AFM images and the free energy changes of the layers gives better insight into changes of their properties. The observed gradual increase in the layer surface free energy allows controlling of the hydrolysis process to obtain the surfaces of defined properties.     

The debate on the dependence of apparent contact angles on drop contact area or three-phase contact line: A review

A sessile drop is an isolated drop which has been deposited on a solid substrate where the wetted area is limited by the three-phase contact line and characterized by contact angle, contact radius and drop height. Although, wetting has been studied using contact angles of drops on solids for more than 200 years, the question remains unanswered: Is wetting of a rough and chemically heterogeneous surface controlled by the interactions within the solid/liquid contact area beneath the droplet or only at the three-phase contact line? After the publications of Pease in 1945, Extrand in 1997, 2003 and Gao and McCarthy in 2007 and 2009, it was proposed that advancing, receding contact angles, and contact angle hysteresis of rough and chemically heterogeneous surfaces are determined by interactions of the liquid and the solid at the three-phase contact line alone and the interfacial area within the contact perimeter is irrelevant. As a consequence of this statement, the well-known Wenzel (1934) and Cassie (1945) equations which were derived using the contact area approach are proposed to be invalid and should be abandoned. A hot debate started in the field of surface science after 2007, between the three-phase contact line and interfacial contact area approach defenders. This paper presents a review of the published articles on contact angles and summarizes the views of the both sides. After presenting a brief history of the contact angles and their measurement methods, we discussed the basic contact angle theory and applications of contact angles on the characterization of flat, rough and micropatterned superhydrophobic surfaces. The weak and strong sides of both three-phase contact line and contact area approaches were discussed in detail and some practical conclusions were drawn.     

Lattice Boltzmann simulations of drops colliding with solid surfaces

Video images of drops colliding with solid surfaces shown by Rioboo et al. (2002) reveal that, for large drop velocities, the drops flatten and form a ring structure before receding and, in some cases, rebounding from the surface. They described the sequence of events in terms of four distinct regimes. During the initial kinematic phase, the dimensionless wetting radius of the drop follows a universal form if the drop Weber and Reynolds numbers are sufficiently large. In the second phase, the drop becomes highly flattened and the values of the Weber and Reynolds numbers influence the time evolution of the dimensionless wetting radius and its maximum value. This is followed by a third phase in which the wetting radius begins to decrease with time and the wettability of the surface influences the dynamics. This paper presents simulation results for the early stages of drop impact and spreading on a partially wetting solid surface. The simulations were performed with a modified version of the lattice Boltzmann method (LBM) developed by Inamuro et al. (2004) for a liquid-gas density ratio of 1000. The Inamuro et al. version of the LBM was modified by incorporating rigid, no-slip boundary conditions and incorporating a boundary condition on the normal derivative of the order parameter to impose the desired equilibrium contact angle.     

液态镓在石墨烯表面的润湿性及形貌特征

液态Ga及其合金的熔点低、毒副作用小、导电率高,使得这类液态金属能像石墨烯一样被广泛应用于微流器件、柔性电子器件中,制备这些器件的关键在于有效控制各生产环节中液态金属在固体界面上的润湿性及形貌特征.基于Lennard-Jones(L-J)势函数,利用分子动力学模拟方法研究了金属Ga在石墨烯表面的润湿性,根据模拟结果拟合的L-J势参数能正确描述Ga原子与衬底之间的相互作用并得到了与实验值极为接近的润湿角,发现衬底与液膜间相互作用的微小改变都会对最终润湿形态产生极大影响,平衡态的润湿角和脱离衬底速度随着Ga-C间势能的减小而增大,并成功获得了不同厚度的Ga液膜在石墨烯表面的形态演变规律,极为符合液态Ga的基本特性.利用所得L-J势函数参数模拟了液态Ga在粗糙度相同但纳米柱尖端形貌不同的C材料表面的润湿演变,发现纳米柱尖端形貌对液态Ga的润湿过程及状态影响极大.     

Adsorption behavior of weak hydrophilic substances on low-energy surface in aqueous medium

Through the methods such as measurements of contact angle and surface tension, calculations of surface energy and interfacial interaction free energy, and four weak hydrophilic substances (WHS) were taken as research objects, some interesting conclusions were obtained as follow. In aqueous medium, the WHS give priority to adsorb on low-energy surface that is low polar or particularly non-polar. There is a clear corresponding relationship between the free energy and Lewis base component γ⁻ or the hydrophile index of low-energy surface, and the specific relationship is obtained. Finally, we find hydrophobic attractive force of the Lewis acid-base interaction is mainly responsible for the absorption of WHS on low-energy surface. In short, an initial insight into adsorption behavior of WHS on low-energy surface is demonstrated in this paper.     

Prediction of contact angle on a microline patterned surface

The Cassie and Baxter’s equation has been modified to predict wetting phenomena on a microline patterned surface with the concept of effective solid-liquid interfacial energy. This interfacial energy was deduced from the total energy barrier at a metastable equilibrium and Helmholtz free energy. The contact angle predicted by the modified equation is reasonably close to the experimental data for the microline patterned surface.     

倾斜均质表面上等径水滴聚合过程的可视化实验

对倾斜均匀表面上等直径水滴的聚合过程及特性进行了可视化实验研究,获得了水滴直径和表面倾角等参数对液滴聚合过程中液滴液桥半径、接触角和接触线变化特性的影响,分析了水滴聚合对其运动的影响.实验结果表明:表面倾角越大,下滑的临界半径越小;液滴的直径越大,液滴聚合后越容易下滑;液滴聚合可以加快液滴的运动,使下滑临界半径减小.     

Wetting transitions of simple liquid films adsorbed on self-assembled monolayer substrates: an ellipsometric study

We report on an ellipsometric experimental study designed to explore the relevance of the wetting phase diagram predicted by liquid state physics of basic models, to the wide class of simple organic liquid films that adsorb from saturated vapour onto planar substrates at room temperature. The wetting properties are explored by measuring adsorption isotherms in the approach to saturation, in particular, for adsorption of n-hexane on a variety of specially constructed substrates (self-assembled monolayers) spanning a wide range of surface energy, and by carrying out the microscopic equivalent of contact angle experiments at saturation. We locate a wetting transition, which in our case is continuous, and then study its properties in detail. The general prediction of the wetting phase diagram, that wetting transitions should be ubiquitous in nature and readily located via control over the substrate field, is supported by our data, but the quantitative nature of the thick film adsorption regime is not in agreement with Lifshitz theory. This conclusion supports the work of a variety of earlier related studies, but contrasts with recent results for adsorption onto the surface of water. In addition, the correlation length determined from our complete wetting adsorption isotherms is mesoscopic, suggesting that equilibrium statistical mechanics of simple models of inhomogeneous fluids cannot explain the data.     

Surface wetting behavior of the poly(styrene-b-isoprene-b-styrene) triblock copolymer with different chemical structures of the polyisoprene block chain

The relationship between the chemical structure of a block copolymer and its surface structure and properties is very important for the careful design of its outer surface layer. For this paper, a series of poly(styrene-b-isoprene-b-styrene) triblock copolymers (SIS), with different chemical structures in the polyisoprene block chain, were synthesized by anionic polymerization and their dynamic wetting behaviors were investigated. The dynamic contact angles of the polyisoprene homopolymer (PI) and the SIS were almost the same when the PI and the corresponding block in the SIS had similar chemical structures. The receding contact angle (θ_r) of SIS depended on the microstructure of the PI block chain, however, the advancing contact angles (θ_a) were almost the same regardless of the PI’s chemical structures. The receding contact angle (θ_r) of SIS containing 3,4-PI was far higher than that of SIS with 1,4-PI. Meanwhile, it gradually approached that of SIS with 1,4-PI as the of 3,4-PI content decreased or as the local temperature increased. Contact angle measurement is one of the most sensitive methods for providing information on the outer few angstroms of a polymer’s surface. Therefore, by designing SIS with different chemical structures in the PI block, it was confirmed that the properties and structure of the outermost layer of the SIS were controlled primarily by the PI block’s chemical structure. This demonstrates the possibility to modulate the surface structure and properties of SIS by adjusting the chemical structure of polyisoprene segment.     

Wettability of Chemically Modified Polymers: Experiment and Theory

The wettabilities of four polymers, polytetrafluoroethylene (PTFE), ultra-high molecular weight polyethylene (UHMW-PE), polyetherurethane (PEU) and polydimethylsiloxane (PDMS) have been studied experimentally. Using PBS as the wetting fluid, advancing and receding contact angles have been measured for virgin surfaces and for materials treated in O₂-, Ar-, N₂- and NH₃-plasmas. In the case of PTFE, further surface modi-fication was produced by storage of virgin and plasma treated specimens in air or PBS for one month. These experimental measurements are presented on wettability plots which facilitate interpretation in terms of current theoretical understanding. Wettability plots show contact angle measurements as a function of a parameter designated X _M which quantifies the surface composition as a weighted average of chemically distinct surface domains. The theoretical wettability of smooth surfaces with molecular scale heterogeneities can be drawn as a line (zerohysteresis) on such wettability plots and provides a helpful datum for analysing the hysteretic wetting behaviour of real surfaces in terms of contact line pinning by surface heterogeneities. An experimental method based on analysis of XPS spectra for estimating X _M is presented. This enables wettability plots (contact angles vs. X _M) to be plotted for the four materials studied. These plots elucidate the variation of the wettability of a given material as a function of surface treatment, and the comparative behaviour of different materials.     

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.     

Thoughts on some outstanding issues in the physics of equilibrium wetting and conceptual understanding of contact lines

Equilibrium wetting is a fundamental phenomenon, relevant to many scientific areas. Since the pioneering work on equilibrium wetting of Thomas Young (1805) [1], researchers strived to advance our understanding of this fundamental problem. Despite its apparent simplicity, equilibrium wetting phenomenon still holds many unanswered questions and represents a challenge to modern physicists and engineers. The relationship between quantities amenable to measurements, like macroscopic wetting contact angle, and other surface ener- gies and physical properties remains to be fully elucidated. Wetting is a physical problem which spans over two length scales, inner region (“microscopic”) length scale and outer region (“macroscopic”). The three-phase contact line, where the macroscopic region meets the micro- scopic one, and underlying surface forces, represents a challenge to fully understand and model. In this paper, a brief review of the basics of wetting and existing concepts is first presented. Then two important questions are discussed in the light of the latest experimental findings: first the relevance of the continuum concept when describing interfaces near the three-phase contact line, and second the effect of adsorption on interfacial energies and its use to explain some interesting observations like the dependence of equilibrium contact angle on pressure and size of droplets. These recent observations raise some fundamental questions about how the three-phase contact line is conceptualised.     

亲水性微观粗糙表面润湿状态转变性能研究

以亲水性微观粗糙表面上不同几何形貌及分布的微柱阵列为对象, 讨论了液滴在亲水性粗糙表面上的润湿过程以及润湿状态的转变阶段. 从能量角度分别考察了微观粗糙结构几何形貌及分布、微柱几何参数、固体表面亲水性、接触角滞后作用等因素对液滴润湿状态转变的影响规律. 研究发现: 在亲水粗糙表面, 正方形微柱呈正六边形阵列分布时, 液滴更容易形成稳定的Cassie状态, 或者液滴仅发生Cassie状态向中间浸润状态的转变; 与此同时, 减小微柱间距、增大方柱宽度或圆柱直径、增大微柱高度、增强固体表面的亲水性将有利于液滴处于稳定的Cassie状态, 或阻止润湿状态向伪-Wenzel或Wenzel状态转变; 然而, 当液滴处于Cassie状态时, 较小的固-液界面面积分数或减弱固体表面亲水性能均有利于增大液滴的表观接触角, 因此在亲水表面设计粗糙结构时应综合考虑润湿状态稳定性和较大表观接触角两方面因素; 此外, 接触角滞后作用对于液滴状态的稳定性以及疏水性能的实现具有相反作用的影响. 研究结果为液滴在亲水表面获得稳定Cassie状态的粗糙结构设计方法提供了理论依据. 关键词： 亲水表面 微观粗糙结构 表面自由能 润湿状态转变     

浸润接触线的摩擦性质与固体表面张力的Wenzel行为

依据Adam和Jessop关于固-液-气三相接触线具有静摩擦性的观点，能从浸润接触角数据推算 出固体的表面张力系数，但结果显然会与摩擦条件的具体形式有关.以报道的不锈钢和聚丙 烯实验数据为例，通过对比Mises和Amonton两种摩擦定律给出的表面张力系数，后者给出的 张力才具有总是随表面粗糙性的增大而增加的Wenzel效应，以及当固-液，固-气界面的张力 系数之差等于液-气界面的张力系数时，退后角将变为零的全浸润条件.这似乎表明用Amonto n定律描写接触线的静摩擦要更为合理. 关键词： 浸润角 静摩擦 固体表面张力     

Wetting behaviour of plasma sprayed oxide coatings

Wetting behaviour of several plasma sprayed oxide surfaces were characterised using contact angle measurements. Since surfaces contained pores and cracks, the evaluation of wetting angles led only to rough estimation of surface free energies. In order to find out the effect of atmospheric contamination the wetting behaviour of plasma-etched surfaces was followed as a function of time.It was found out that the sample preparation method had great influence on the contact angle of plasma sprayed oxide surfaces. The contact angle of plasma-etched surfaces increased when the surfaces were exposed to air. The probable reason for that was adsorption of low surface free energy contaminants to the sample surfaces.