Specific adhesion of a soft elastic body on a wavy surface

This paper aims at developing a stochastic-elastic model of a soft elastic body adhering on a wavy surface via a patch of molecular bonds. The elastic deformation of the system is modeled by using continuum contact mechanics, while the stochastic behavior of adhesive bonds is modeled by using Bell's type of exponential bond association/dissociation rates. It is found that for sufficiently small adhesion patch size or stress concentration index, the adhesion strength is insensitive to the wavelength but decreases with the amplitude of surface undulation, and that for large adhesion patch size or stress concentration index, there exist optimal values of the surface wavelength and amplitude for maximum adhesion strength.     

Reducing adhesion of soil against loading shovel using bionic electro-osmosis method

Bionic electro-osmosis, a new method of reducing soil adhesion to soil-engaging components or parts of terrain machines, is presented. It is based on the anti-adhesion mechanism of the body surfaces of soil burrowing animals. The key feature of bionic electro-osmosis is to arrange a series of electric pole plates to create a non-smooth working surface. The static and dynamic effects of bionic electro-osmosis on the reduction of soil adhesion have been examined. The pole plates of the non-smooth surfaces with different dimensions used for bionic electro-osmosis have been designed and tested based on an experimental optimization method. The result showed that the adhesion of soil to a shovel was substantially reduced at a low electro-osmosis voltage. This technique shows promise for practical applications in reducing adhesion of soil against machine parts.     

A theoretical model of reversible adhesion in shape memory surface relief structures and its application in transfer printing

Transfer printing is an important and versatile tool for deterministic assembly and integration of micro/nanomaterials on unusual substrates, with promising applications in fabrication of stretchable and flexible electronics. The shape memory polymers (SMP) with triangular surface relief structures are introduced to achieve large, reversible adhesion, thereby with potential applications in temperature-controlled transfer printing. An analytic model is established, and it identifies two mechanisms to increase the adhesion: (1) transition of contact mode from the triangular to trapezoidal configurations, and (2) explicit enhancement in the contact area. The surface relief structures are optimized to achieve reversible adhesion and transfer printing. The theoretical model and results presented can be exploited as design guidelines for future applications of SMP in reversible adhesion and stretchable electronics.     

Adhesion model of side contact for an extensible elastic fiber

For accurately predicted adhesion laws of fibrillar structures contribute to the rational design of high-performance biomimetic adhesives, an adhesion model is proposed to study the directional adhesion behavior of an extensible elastic fiber that contacts a rigid smooth surface with its side surface under the coupling effect of normal and shear forces, based on the extensible Euler Bernoulli beam theory and the surface energy concept. The deformed configuration of the fiber is obtained analytically, and on the basis of this result, the detachment mode and the normal pull-off force of the fiber for a given shear force are predicted directly. It is also found that, due to the extensibility of the fiber, there exists a maximum normal pull-off force (MNPF) when an optimal shear force is applied. The MNPF will be enhanced by increasing the axial stiffness, and reduced by increasing the bending stiffness. In addition, generating an optimal pre-tension in the adhered part of the fiber will maximize the MNPF. The derived adhesion law is expected to contribute to the optimal design and applications of single-level fibrillar adhesives.     

车轮表面宏观形貌取向对高速轮轨水润滑黏着系数的影响

轮胎和沥青都属于低弹性模量材料,即使运动速度较低,流体动压导致的水膜也足以产生润滑作用. 对于列车轮轨这类高 弹性模量材料,只有当运行速度达到200 km/h 以上,水的润滑作用才体现出来,使轮轨黏着系数大幅降低,给高速列车运 行带来重大安全隐患. 增大表面粗糙度一般能够提高轮轨黏着系数,然而研究表明,在表面粗糙度基本相同的条件下,表面 形貌取向对混合润滑状态下的黏着系数有显著影响. 文中用统一雷诺方程模型,计算了在水润滑状态下,具有纵纹、横纹、菱形等特定形貌取向的车轮在高速运动时(最高500 km/h) 对黏着系数的影响,并将计算结果与平均流量模型计算的结果和已 有的实验结果进行了比较. 结果表明:各种形貌下,轮轨黏着系数都随速度的增大而减小,其中菱形的黏着系数大于横纹 的,而横纹的黏着系数又大于纵纹的,影响黏着系数的主要因素是固体接触压力与总压力之比. 在轮轨点接触椭 圆率$k<1$时, 接触区的侧流效应不可忽略,用平均流量模型计算会导致谬误.      

粉末热压扩散与应力场耦合的力学模型

以弹性接触应力场为初始条件,建立了热压条件下球形颗粒表面扩散与应力场耦合的力学模型. 引入包含表面能项级数形式的应力函数,以描述随时间演化的表面扩散过程及扩散对应力场演化的影响. 而应力场通过改变化学势梯度,又会促进(或阻止)表面扩散结合的进程.利用该模型分析了压力、温度和界面区应力场演化对致密化参数的影响. 比较了满足粘着或非粘着对结合宽度和应力分布的影响,将考虑粘着的弹性接触应力场作为初始条件,分析了弹性变形和表面扩散共同驱动的粉末冶金热压烧结致密化规律.      

Nonlinear effects of line tension in adhesion of small droplets

Three-phase line tensions may become crucial in the adhesion of miero-nano or small droplets on solid planes. In this paper we study for the first time the nonlinear effects in adhesion spanning the full range of physically possible parameters of surface tension, line tension, and droplet size. It is shown that the nonlinear adhesion solution spaces can be characterized into four regions. Within each region the adhesion behaves essentially the same. Especially, inside the characteristic regions with violent nonlinearities, the co-existence of multiple adhesion states for given materials is disclosed. Besides, two common fixed points in the solution space are revealed. These new results are consistent with numerical analysis and experimental observations reported in the literatures.     

Adhesive contact of an elastic semi-infinite solid with a rigid rough surface: Strength of adhesion and contact instabilities

The effect of adhesion on the contact behavior of elastic rough surfaces is examined within the framework of the multi-asperity contact model of Greenwood and Williamson (1966), known as the GW model. Adhesive surface interaction is modeled by nonlinear springs with a force–displacement relation governed by the Lennard–Jones (LJ) potential. Constitutive models are presented for contact systems characterized by low and high Tabor parameters, exhibiting continuous (stable) and discontinuous (unstable) surface approach, respectively. Constitutive contact relations are obtained by integrating the force–distance relation derived from the LJ potential with a finite element analysis of single-asperity adhesive contact. These constitutive relations are then incorporated into the GW model, and the interfacial force and contact area of rough surfaces are numerically determined. The development of attractive and repulsive forces at the contact interface and the occurrence of instantaneous surface contact (jump-in instability) yield a three-stage evolution of the contact area. It is shown that the adhesion parameter introduced by Fuller and Tabor (1975) governs the strength of adhesion of contact systems with a high Tabor parameter, whereas the strength of adhesion of contact systems with a low Tabor parameter is characterized by a new adhesion parameter, defined as the ratio of the surface roughness to the equilibrium interatomic distance. Applicable ranges of aforementioned adhesion parameters are interpreted in terms of the effective surface separation, obtained as the sum of the effective distance range of the adhesion force and the elastic deformation induced by adhesion. Adhesive strength of rough surfaces in the entire range of the Tabor parameter is discussed in terms of a generalized adhesion parameter, defined as the ratio of the surface roughness to the effective surface separation.     

MEASUREMENT OF ADHESION FORCES IN AIR WITH THE VIBRATION METHOD

The vibration method represents a practical method for the measurement of adhesion forces and adhesion force distributions. This method causes sinusoidally altemating stresses and yields detachment and contact forces between particles and substrate of the same order of magnitude. Alternating contact forces of the vibration method can cause an adhesion force intensification through flattening of asperities. The measuring principle of the vibration method and the analysis of experimental results are described in the article. Normal adhesion forces (pull-off forces) are measured using the vibration method and the colloidal probe technique. The results of both methods show good agreement for small particle sizes. The influence of the detachment force direction is shown by comparing tangential and normal adhesion forces measured using particle reentrainment in a turbulent air flow and the vibration method, respectively. The surface roughness of the substrate and the relative humidity are shown to significantly influence the measured adhesion forces. For the calculation of the adhesion forces, an approach by Rabinovich was combined with approximations of plastic micro asperity flattening. The Rabinovich approach accounts for roughness effects on the van der Waals force by incorporating the rms roughness of the interacting surfaces. rms-values of the particles and substrates were measured with atomic force microscopy at different scanning areas.     

一类功能梯度材料表面接触力学研究进展

针对一类杨氏模量梯度变化的功能梯度材料, 考虑摩擦、微动磨损和黏附等因素, 综述了梯度材料有限尺寸、梯度变化规律、梯度涂层厚度、界面摩擦热、压头形状等对表面接触力学性能的影响; 根据不同接触模型中接触界面应力场分布, 分析多种因素影响下功能梯度材料表面抵抗磨损的能力; 最后给出了功能梯度材料接触力学研究中仍存在的主要科学问题及进一步研究展望.      

Bio-inspired mechanics of reversible adhesion: Orientation-dependent adhesion strength for non-slipping adhesive contact with transversely isotropic elastic materials

Geckos and many insects have evolved elastically anisotropic adhesive tissues with hierarchical structures that allow these animals not only to adhere robustly to rough surfaces but also to detach easily upon movement. In order to improve our understanding of the role of elastic anisotropy in reversible adhesion, here we extend the classical JKR model of adhesive contact mechanics to anisotropic materials. In particular, we consider the plane strain problem of a rigid cylinder in non-slipping adhesive contact with a transversely isotropic elastic half space with the axis of symmetry oriented at an angle inclined to the surface. The cylinder is then subjected to an arbitrarily oriented pulling force. The critical force and contact width at pull-off are calculated as a function of the pulling angle. The analysis shows that elastic anisotropy leads to an orientation-dependent adhesion strength which can vary strongly with the direction of pulling. This study may suggest possible mechanisms by which reversible adhesion devices can be designed for engineering applications.     

Retraction and Adhesion of a Single Droplet Normal Impact on the Solid Surface

The impact behavior of individual biomass oil droplets was investigated on solid surfaces having different structures (flat, cylindrical, and spherical) using the high-speed video technique. This makes it possible to compare the evolution of the droplet impact on various surface structures. The impact behaviors of retraction–oscillation and adhesion are analyzed for different hydrophobic surfaces. The influence of the Weber number (We), the surface structure, and the surface curvature is further examined by focusing on the retraction and stable adhesion (thickness, adhesion, and contact angle) for different biomass oil droplets. The results show that the retraction factor gradually increases as We increases to some critical value, beyond which the increase rate slows down or the retraction factor begins to decrease. The largest retraction factor is observed on the flat surface and the smallest one appears on the spherical surface. The adhesion thickness of the liquid film oscillates periodically over time, and its oscillation amplitude gradually decreases with a constant frequency, which is smaller for the more hydrophobic surfaces. The curvatures of the cylinder and sphere have little influence on the stable adhesion behavior. For the different droplet types, the adhesion diameter on the flat surface gradually increases as We rises, whereas the adhesion thickness gradually decreases with increase in We. These results are helpful for understanding the impact behaviors of biomass oil droplets with high viscosity and small surface tension on solid surfaces.

     

Nano-tribological study on a super-hydrophobic film formed on rough aluminium substrates

A novel super-hydrophobic stearic acid (STA) film with a water contact angle of 166^o was prepared by chemical adsorption on aluminum wafer coated with polyethyleneimine (PEI) film. The micro-tribological behavior of the super-hydrophobic STA monolayer was compared with that of the polished and PEI-coated Al surfaces. The effect of relative humidity on the adhesion and friction was investigated as well. It was found that the STA monolayer showed decreased friction, while the adhesive force was greatly decreased by increasing the surface roughness of the Al wafer to reduce the contact area between the atomic force microscope (AFM) tip and the sample surface to be tested. Thus the friction and adhesion of the Al wafer was effectively decreased by generating the STA monolayer, which indicated that it could be feasible and rational to prepare a surface with good adhesion resistance and lubricity by properly controlling the surface morphology and the chemical composition. Both the adhesion and friction decreased as the relative humidity was lowered from 65% to 10%, though the decrease extent became insignificant for the STA monolayer. The project supported by the National Natural Science Foundation of China (50375151, 50323007, 10225209) and the Chinese Academy of Sciences (KJCX-SW-L2)     

Tabor数、粘着数与微尺度粘着弹性接触理论

微电子机械系统（ＭＥＭＳ）等领域的飞速发展,促使我们迈进了一个表面效应在许多现象 中占主导地位的研究领域．本文重点介绍在ＭＥＭＳ中经常遇到的微尺度粘着弹性接触的相关理论． 通过对两个无量纲数——Ｔａｂｏｒ数μ（以及其相应形式）和粘着数θ的分析,以及考虑它们对于粘 着力的影响,指出了粘着弹性接触理论中所隐含的尺度效应,随着特征尺度的减小,粘着弹性接触中 的表面效应愈加明显．     

粘着行为中预压紧力和分离速度及相对湿度对分离力影响的实验研究

在改变粘着试验中接触副预压紧力、接触副分离速度以及环境相对湿度的条件下,利用表面力仪实时测量了新鲜解理云母接触副分离过程的分离力,并观察了接触后解理云母接触区的状态．结果表明：在保持弹性接触时,预压紧力对分离力影响不大,接触材料的表面能决定分离力的强度;当预压紧力超过临界值时,接触表面发生塑性变形,在一定范围内分离力随预压紧力的增加而增大;在干接触时分离速度与分离力无直接联系,但在存在大粘度中间层液体时,分离速度的增加可使分离力显著增大;分离力随相对湿度的增加而增大．     

A model for computational investigation of elasto-plastic normal and tangential contact considering adhesion effect

With the rapid development of Micro-Electro-Mechanical System (MEMS), we enter a field in which the surface effects have dominated many of the micro-scale phenomena, and the adhesive contact is one of the focuses. In this paper, a feasible model for finite element computation is presented via a macroscopic and microscopic combination approach, in which the adhesive forces are simulated by some non-linear spring elements considering the softening stage. Two basic problems concerning the adhesion effect were considered; through specific quantitative analysis, the results show a consistency with the current elastic continuum theories of adhesion and a brief investigation into the effects of adhesion on plastic deformation and tangential contact will be carried out as well. The project supported by the National Natural Science Foundation of China (10172050, 90205022) and Key Grant Project of Chinese MoE (0306)     

Nanoscale viscoelastic properties and adhesion of polydimethylsiloxane for tissue engineering简

It has shown that altering crosslink density of biopolymers will regulate the morphology of Mesenchymal Stem Cells （MSCs） and the subsequent MSCs differentia- tion. These observations have been found in a wide range of biopolymers. However, a recent work published in Nature Materials has revealed that MSCs morphology and differen- tiation was unaffected by crosslink density of polydimethyl- siloxane （PDMS）, which remains elusive. To understand such unusual behaviour, we use nanoindentation tests and modelling to characterize viscoelastic properties and sur- face adhesion of PDMS with different base：crosslink ratio varied from 50：1 （50D） to 10：1 （10D）. It has shown that lower crosslink density leads to lower elastic moduli. De- spite lower nanoindentation elastic moduli, PDMS with lowest crosslink density has higher local surface adhesion which would affect cell-biomaterials interactions. This work suggests that surface adhesion is likely another important physical cue to regulate cell-biomaterials interactions.     

壁虎粘附微观力学机制的仿生研究进展

本文针对壁虎粘附系统最小单元的真实形状, 类似于有限尺寸纳米薄膜的铲状纤维, 综述了对其微观粘附力学机制主要影响因素的多个研究, 主要考虑了有限尺寸纳米薄膜长度、厚度、撕脱角等对撕脱力的影响; 物体表面粗糙度以及环境湿度等对粘附的影响因素; 包括实验、理论及数值模拟的研究及结果比较. 最后给出仿生粘附力学方向仍然存在的主要科学问题及进一步的研究展望.     

EFFECT OF HEAD SHAPE ON THE ADHESION CAPABILITY OF MUSHROOM-LIKE BIOLOGICAL ADHESIVE STRUCTURES

Theoretical studies have been extensively performed to reveal the mechanism of the biological adhesive structure with a flat head.However,most biological adhesive structures have a big head.Here we investigate the effect of the head shape on the adhesion capability of a mushroomlike adhesion structure based on a finite element model.We show that a mushroom-like head produces significant adhesion enhancement.We investigate also the effect of the elastic modulus of the adhesive structure on the adhesion.Our results are consistent with existing experimental results.     

Contact radius of stamps in reversible adhesion

A mechanics model is developed for the contact radius of stamps with pyramid tips in transfer printing. This is important to the realization of reversible control of adhesion, which has many important applications, such as climbing robots, medical tapes, and transfer printing of electronics. The contact radius is shown to scale linearly with the work of adhesion between the stamp and the contacting surface, and inversely with the plane-strain modulus of the stamp. It also depends on the cone angle and tip radius of the stamp, but is essentially independent of details of the tip geometry.