稳定超疏水性表面的理论进展

自然界中很多动植物都具有稳定的超疏水性, 它们既拥有高接触角, 又拥有低滚动角, 且能长期稳定存在.通过对它们的研究, 发现表面的润湿性与表面的化学成分、表面的几何形貌有关, 并且表面几何结构的影响更为显著, 甚至可以实现由亲水性表面向超疏水转变. 虽然目前在这个领域已经有大量的实验验证了表面粗糙结构的重要作用, 但是对于表面微纳米结构对表面疏水性机理的理论研究还并不完善. 本文详细介绍了超疏水表面的基本理论及其适用性、 接触角滞后现象, 分别从经典理论和能量的观点探讨了润湿状态转化发生的条件, 重点介绍了通过仿生理念对表面几何形貌的优化设计, 包括单尺度和多尺度表面结构对于设计稳定超疏水表面的作用. 最后, 对超疏水理论的不足和未来发展进行了展望. 关键词： 超疏水 仿生 接触角 滞后     

液滴在梯度微结构表面上的铺展动力学分析

本文通过改变肋柱宽度和间距, 构造了二级和多级梯度微结构表面, 采用格子-Boltzmann方法对液滴在两种梯度表面上的铺展过程进行了研究, 探析液滴运动的机理和调控方法. 结果表明, 在改变肋柱间距的二级梯度表面上, 当液滴处于Cassie态时, 接触角滞后大小与粗糙度梯度成正比关系; 当液滴从Cassie态转换为Wenzel态或介于两者之间的不稳定态时, 这一正比关系不再遵循. 在改变肋柱宽度的二级梯度表面上, 接触角滞后大小与粗糙度梯度始终成正比关系. 在多级梯度表面上, 随液滴初始半径增大, 接触角滞后减小, 但液滴平衡位置相较于初始位置偏离增大. 对梯度微结构表面上液滴运动和接触角滞后的定量分析, 可为实现梯度微结构表面液滴运动调控提供理论依据.     

正癸烷纳米液滴润湿特性的分子动力学研究

流体液滴在固体表面的浸润性对其润滑性能至关重要.本文利用分子动力学方法研究了正癸烷纳米液滴在铜表面上的润湿特性.结果表明：在平坦光滑表面上,壁面的厚度和分子数目对润湿效果影响不大.随着壁面能量势阱参数εs 增大,接触角线性减小.随着温度升高,液滴的接触角减小.在沟槽粗糙表面上,随着粗糙度因子增大,对于疏液表面,接触角增大到一定值后基本保持不变,符合Cassie理论;中性和亲液表面接触角则会减小,为Wenzel润湿模式.当表面分数增大时,疏液与亲液表面接触角整体呈减小的趋势,对中性表面影响不大.当温度升高时,粗糙疏液表面接触角会增大,润湿效果更差,而粗糙中性和亲液表面液滴润湿性会更好.     

Morphological modification of the surface of polymers by the replication of the structure of anodic aluminum oxide

Anodic aluminum oxide films have been used as stamps to modify polymer materials in order to create superhydrophobic surfaces. Using polyethylene terephthalate, it has been shown that a high replication degree leads to a strong increase in the water wetting contact angle (up to 174°). However, the inverse situation is observed for the case of low replication degrees: hydrophilization of the surface occurs, which can be explained by a change of the Cassie-Baxter wetting mechanism to the Wenzel mechanism.     

Droplets on inclined rough surfaces

The behaviour of liquid droplets on inclined heterogeneous surfaces was simulated by the lattice-Boltzmann method using the Shan-Chen multiphase model. The effect of topography of the surface on the contact angle hysteresis was investigated. It is shown in particular, by using anisotropic rough surfaces, how surface topography and thereby the continuity of the three-phase contact line, affect this hysteresis. Our results clearly indicate that the superhydrophobicity of a surface cannot be judged by the contact angle alone.     

Suspended penetration wetting state of droplets on microstructured surfaces

When a water droplet on a micropillar-structured hydrophobic surface is submitted to gradually increased pressure, the CassieBaxter wetting state transforms into the Wenzel wetting state once the pressure exceeds a critical value. It has been assumed that the reverse transition(Wenzel-to-Cassie-Baxter wetting state) cannot happen spontaneously after the pressure has been removed.In this paper, we report a new wetting-state transition. When external pressure is exerted on a droplet in the Cassie-Baxter wetting state on textured surfaces with high micropillars to trigger the breakdown of this wetting state, the droplet penetrates the micropillars but does not touch the base of the surface to trigger the occurrence of the Wenzel wetting state. We have named this state the suspended penetration wetting state. Spontaneous recovery from the suspended penetration wetting state to the initial Cassie-Baxter wetting state is achieved when the pressure is removed. Based on the experimental results, we built models to establish the penetration depth that the suspended penetration wetting state could achieve and to understand the energy barrier that influences the equilibrium position of the liquid surface. These results deepen our understanding of wetting states on rough surfaces subjected to external disturbances and shed new light on the design of superhydrophobic materials with a robust wetting stability.     

Influence of surface roughness on water- and oil-repellent surfaces coated with nanoparticles

Various rough surfaces coated with titanium oxide nanoparticles and perfluoroalkyl methacrylic copolymer were conducted to explore the influence of surface roughness on the performance of water- and oil-repellence. Surface characteristics determined from nitrogen physisorption at −196 °C showed that the surface area and pore volume increased significantly with the extent of nanoparticle ratio, indicating an increase of surface roughness. Due to the surface nano-coating, the maximum contact angles of water and ethylene glycol (EG) droplets increased up to 56 and 48%, respectively, e.g. from 105° to 164° for water droplets and from 96° to 144° for EG droplets. The excellent water- and oil-repellence of the prepared surfaces was ascribed to this increase of surface roughness and fluorinated-contained surface. Compared with Wenzel model, the Cassie model yielded a fairly good fit to the simulation of contact angle with surface roughness. However, a derivation of 3°–10° at higher roughness still existed. This phenomenon was very likely due to the surface heterogeneity with different pore size distributions of the fractal surfaces. In this case, it was unfavorable for super repellency from rough surface with larger mesopore fraction because of its capillary condensation, reflecting that micropore provided more air resistance against wettability.     

Hydrophobic properties of a wavy rough substrate

The wetting/non-wetting properties of a liquid drop in contact with a chemically hydrophobic rough surface (thermodynamic contact angle e>/2) are studied for the case of an extremely idealized rough profile: the liquid drop is considered to lie on a simple sinusoidal profile. Depending on surface geometry and pressure values, it is found that the Cassie and Wenzel states can coexist. But if the amplitude h of the substrate is sufficiently large the only possible stable state is the Cassie one, whereas if h is below a certain critical value hcr a transition to the Wenzel state occurs. Since in many potential applications of such super-hydrophobic surfaces, liquid drops often collide with the substrate (e.g. vehicle windscreens), in the paper the critical drop pressure pW is calculated at which the Cassie state is no longer stable and the liquid jumps into full contact with the substrate (Wenzel state). By analyzing the asymptotic behavior of the systems in the limiting case of a large substrate corrugation, a simple criterion is also proposed to calculate the minimum height asperity h necessary to prevent the Wenzel state from being formed, to preserve the super-hydrophobic properties of the substrate, and, hence, to design a robust super-hydrophobic surface.     

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.     

The apparent state of droplets on a rough surface

The factors influencing the state and wetting transition of droplets on a rough surface are both complex and obscure. The change in wetting is directly reflected by changes under the contact condition of the droplets with the surface. The recent study about the wettability of the superhydrophobic surface under the condensing condition arouses the new understanding about the apparent state of droplets on a rough surface. In this work, to validate the existence of droplets in an intermediate state, a microscale pillar topological polydimethylsiloxane (PDMS) surface was manufactured and its wettability under various conditions was studied. According to the experimental data, it is proposed that the wetting state of a rough surface may be embodied using the contact area ratio of a solid/liquid/gas droplet with the projective plane. A general calculation model for the apparent contact angle of droplets is given and expressed diagrammatically. It is found that the measured apparent contact angles of droplets at different states on the surface falls within the range predicted by our proposed equation. Supported by the National Natural Science Foundation of China (Grant No. 50606025)     

Nanodroplets on rough hydrophilic and hydrophobic surfaces

We present results of Molecular Dynamics (MD) calculations on the behavior of liquid nanodroplets on rough hydrophobic and hydrophilic solid surfaces. On hydrophobic surfaces, the contact angle for nanodroplets depends strongly on the root-mean-square roughness amplitude, but it is nearly independent of the fractal dimension of the surface. Since increasing the fractal dimension increases the short-wavelength roughness, while the long-wavelength roughness is almost unchanged, we conclude that for hydrophobic interactions the short-wavelength (atomistic) roughness is not very important. We show that the nanodroplet is in a Cassie-like state. For rough hydrophobic surfaces, there is no contact angle hysteresis due to strong thermal fluctuations, which occur at the liquid-solid interface on the nanoscale. On hydrophilic surfaces, however, there is strong contact angle hysteresis due to higher energy barrier. These findings may be very important for the development of artificially biomimetic superhydrophobic surfaces.     

A multicomponent multiphase lattice Boltzmann model with large liquid–gas density ratios for simulations of wetting phenomena

A multicomponent multiphase(MCMP) pseudopotential lattice Boltzmann(LB) model with large liquid–gas density ratios is proposed for simulating the wetting phenomena. In the proposed model, two layers of neighboring nodes are adopted to calculate the fluid–fluid cohesion force with higher isotropy order. In addition, the different-time-step method is employed to calculate the processes of particle propagation and collision for the two fluid components with a large pseudoparticle mass contrast. It is found that the spurious current is remarkably reduced by employing the higher isotropy order calculation of the fluid–fluid cohesion force. The maximum spurious current appearing at the phase interfaces is evidently influenced by the magnitudes of fluid–fluid and fluid–solid interaction strengths, but weakly affected by the time step ratio.The density ratio analyses show that the liquid–gas density ratio is dependent on both the fluid–fluid interaction strength and the time step ratio. For the liquid–gas flow simulations without solid phase, the maximum liquid–gas density ratio achieved by the present model is higher than 1000:1. However, the obtainable maximum liquid–gas density ratio in the solid–liquid–gas system is lower. Wetting phenomena of droplets contacting smooth/rough solid surfaces and the dynamic process of liquid movement in a capillary tube are simulated to validate the proposed model in different solid–liquid–gas coexisting systems. It is shown that the simulated intrinsic contact angles of droplets on smooth surfaces are in good agreement with those predicted by the constructed LB formula that is related to Young's equation. The apparent contact angles of droplets on rough surfaces compare reasonably well with the predictions of Cassie's law. For the simulation of liquid movement in a capillary tube, the linear relation between the liquid–gas interface position and simulation time is observed, which is identical to the analytical prediction. The simulation results regarding the wetting phenomena of droplets on smooth/rough surfaces and the dynamic process of liquid movement in the capillary tube demonstrate the quantitative capability of the proposed model.     

A new model for the formation of contact angle and contact angle hysteresis

The formation mechanism of the contact angle and the sliding angle for a liquid drop on a solid surface plays an important role in producing hydrophobic surfaces. A new half soakage model is established in this paper as a substitute for Wenzel (complete soakage) and Cassie (no soakage) models. The model is suited to many solid surfaces, whether they are hydrophilic or hydrophobic, or even superhydrophobic. Based on the half soakage model, we analyse two surfaces resembling lotus, i.e. taper-like surface and corona-like surface. Furthermore, this new model is used to establish a quantitative relationship between the sliding angle and the parameters of surface morphology.     

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.     

Wetting property of smooth and textured hydrophobic surfaces under condensation condition

Static and dynamic wetting behaviors of sessile droplet on smooth, microstructured and micro/nanostructured surface under condensation condition are systematically studied. In contrast to the conventional droplet wetting on such natural materials by dropping, we demonstrate here that when dropwise condensation occurs, the sessile droplet will transit from the Cassie-Baxter wetting state to the Wenzel wetting state or partial Cassie-Baxter wetting state on the microstructured surface or the micro/nanostructured surface, which leads to a strong adhesion between the droplet and the substrate. In contrast, the apparent contact angle and the sliding angle on the smooth surface changes a little before and after the condensation because of small roughness. Theoretical analysis shows that the roughness factor controls the adhesion force of the droplet during condensation, and a theoretical model is constructed which will be helpful for us to understand the relationship between the adhesion force and the geometry of the surface.     

The effect of nanoparticles on the surface hydrophobicity of polystyrene

The surface hydrophobicity of polystyrene-nanoparticle nanocomposites has been investigated as a function of the nanoparticle content. The addition of hydrophobically coated nanoparticles in polystyrene increased the contact angle θ of a water drop with respect to that on polystyrene surface due to change of surface composition and/or surface roughness. When the nanoparticles dispersed well in the polymer, cos θ decreased linearly with increasing amount of nanoparticles indicating a composite surface consisting of smooth polystyrene regions and rough nanoparticle regions. In case of formation of nanoparticle aggregates in polystyrene, cos θ decreased sharply at a critical concentration of nanoparticles. The observed behaviour was modeled in terms of a transition from Wenzel regime to Cassie-Baxter regime at a critical roughness length scale below which the Laplace pressure prevented the penetration of the water drop into the surface undulations. We argue that multiple length scales are needed below the critical roughness length scale to increase the contact angle further by decreasing the fraction of surface area of solid material (increasing the fraction of surface area of air) underlying the water drop.     

微粗糙结构表面液滴浸润特性的多体耗散粒子动力学研究

超疏水表面因其优异的自洁作用一直是表面科学领域关注的重点.本文使用多体耗散粒子动力学(many-body dissipative particle dynamics, MDPD)方法模拟研究了不同粗糙结构下液滴的浸润特性, 并与Cassie-Baxter理论进行了对比. 研究使用了一种具有长吸短斥作用的流固作用函数来获得不同的液滴浸润性, 并利用一种简洁的数值方法来测量接触角. 模拟结果表明本研究方法能够稳定地捕捉到液滴在粗糙表面的静态和动态特性. 模拟了粗糙结构对三相接触线运动的黏滞作用, 与物理实验结果相符合, 表明Cassie-Baxter理论在实际应用中尚存在一定局限性. 研究分析了在动态铺展过程中的能量转化关系, 并指出在低值表面容易引起液滴反弹.     

Wetting characteristics of ZnO smooth film and nanowire structure with and without OTS coating

ZnO is an important material that is used in a variety of technologies including optical devices, sensors, and other microsystems. In many of these technologies, wettability is of great concern because of its implications in numerous surface related interactions. In this work, the effects of surface morphology and surface energy on the wetting characteristics of ZnO were investigated. ZnO specimens were prepared in both smooth film and nanowire structure in order to investigate the effects of surface morphology. Also, a hydrophobic octadecyltrichlorosilane (OTS) coating was used to chemically modify the surface energy of the ZnO surface. Wettability of the surfaces was assessed by measuring the water contact angle. The results showed that the water contact angle varied significantly with surface morphology as well as surface energy. OTS coated ZnO nanowire specimen had the highest contact angle of 150°, which corresponded to a superhydrophobic surface. This was a drastic difference from the contact angle of 87° obtained for the smooth ZnO film specimen. In addition to the initial contact angle, the evolution of the water droplet with respect to time was investigated. The wetting state of water droplet was analyzed with both Wenzel and Cassie-Baxter models. Spontaneous and gradual spreading, together with evaporation phenomenon contributed to the changing shape, and hence the varying contact angle, of the water droplet over time.     

Dentinal tubules driven wetting of dentin: Cassie-Baxter modelling

We investigate the wetting properties of dentin surfaces submitted to a phosphoric acid etching followed by an air drying procedure, as in clinical situations of adhesive dentistry. The surface topography of the etched surfaces was characterized by AFM, and the wetting properties of water on these rough and heterogeneous surfaces were studied, by contact angle measurements. We showed that the contact angle increases with the acid exposure time and consequently with both surface roughness and the organic-mineral ratio of the dentin components. From the whole results, obtained on dentin and also on synthesized hydroxyapatites samples, we inferred a water contact angle of ∼ 133^° on the dentinal tubule. These experimental results may be described by the Cassie-Baxter approach, and it is suggested that small air pockets could be formed inside the dentinal tubules.