Mechanics of contact and adhesion between viscoelastic spheres: An analysis of hysteresis during loading and unloading

Detailed finite element simulations are carried out to study the adhesive contact of viscoelastic spheres. The spheres are brought into contact by a compressive force that increases at a constant rate. Upon reaching a maximum load, the spheres are unloaded until they separate. We studied in detail the effect of loading and unloading rates on hysteresis and on the pull‐off force for a standard viscoelastic solid. The surface interaction is modeled by the Dugdale–Barenblatt model. Numerical results are compared with analytical models for bonding and debonding, including a recent theory proposed by Johnson. There is excellent agreement between analytical and finite element results for the bonding phase. However, for the debonding phase, current analytical models break down unless the loading and unloading rates are slow in comparison with the material relaxation time. Based on the finite element results, a simple approximate analytical model is proposed to quantify adhesive contact in the debonding phase. We also examine the dependence of hysteresis on interfacial parameters such as the cohesive strength and the intrinsic work of adhesion. Our results show that viscoelastic adhesive contact depends on the details of the surface interaction and cannot be determined solely by the work of adhesion. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 772–793, 2002     

Adhesion in elastic-plastic spherical microcontact

An elastic-plastic adhesion model for a metallic deformable sphere pressed by a rigid flat is presented. Analytical dimensionless expressions for the local separation outside the contact area and for the adhesion force are provided covering a large range of interference values from a point contact to a fully plastic contact. Two main dimensionless parameters of the problem are identified and their effect on the adhesion is investigated. The significance of the adhesion in the elastic-plastic contact force balance is discussed and the regimes where adhesion is important or negligible are pointed out. A comparison of the present results with a previously published approximate elastic-plastic model shows substantial differences in the local separation and in the adhesion force.     

The deformation and adhesion of randomly rough and patterned surfaces

Using a surface forces apparatus (SFA) and an atomic force microscope (AFM) we have studied the effects of surface roughness (root-mean-square (RMS) roughness between 0.3 and 220 nm) on the "contact mechanics", which describes the deformations and loading and unloading adhesion forces, of various polymeric surfaces. For randomly rough, moderately stiff, elastomeric surfaces, the force-distance curves on approach and separation are nearly reversible and almost perfectly exponentially repulsive, with an adhesion on separation that decreases only slightly with increasing RMS. Additionally, the magnitude of the preload force is seen to play a large role in determining the measured adhesion. The exponential repulsion likely arises from the local compressions (fine-grained nano- or submicron-scale deformations) of the surface asperities. The resulting characteristic decay lengths of the repulsion scale with the RMS roughness and correlate very well with a simple finite element method (FEM) analysis based on actual AFM topographical images of the surfaces. For "patterned" surfaces, with a nonrandom terraced structure, no similar exponential repulsion is observed, suggesting that asperity height variability or random roughness is required for the exponential behavior. However, the adhesion force or energy between two "patterned" surfaces fell off dramatically and roughly exponentially as the RMS increased, likely owing to a significant decrease in the contact area which in turn determines their adhesion. For both types of rough surfaces, random and patterned, the coarse-grained (global, meso- or macroscopic) deformations of the initially curved surfaces appear to be Hertzian.     

Use of the JKR model for calculating adhesion between rough surfaces

A simple method for using the JKR model to determine interfacial adhesion between two ideal rough surfaces is demonstrated for individual asperity-asperity and asperity-flat contacts both in air and in water. The model takes into account the effect of a modified contact area at separation due to viscoelastic effects. The equilibrium version of the model significantly underestimates the measured adhesion, whereas the viscoelastic version of the model is much closer to the measured data. The asperity-flat geometry used with the viscoelastic version of the model seems to fit the experimental results best. This was thought to be due to the unlikely occurrence of direct asperity-asperity contacts. Instead, it would seem that the asperities have a far higher chance of fitting between each other on opposing surfaces, leading to correspondingly higher pull-off forces measured on separation. Many possible extensions to the roughness model described here may be made, allowing a much-improved understanding of the contact mechanics between two rough surfaces.     

Modification of tetrafluoroethylene-hexafluoropropylene copolymer films in direct-current discharge

The effect of dc discharge treatment at the anode and cathode on the surface properties of tetrafluoroethylene-hexafluoropropylene copolymer films was studied. It was found that the modification of the films under conditions that ensure the separation of the discharge active species acting on the polymer materials makes it possible to achieve substantially lower values for the contact angle and higher values for the work of adhesion and surface energy than in the case of other modes of discharge. A change in the composition and structure of the films was studied by means of IR and X-ray photoelectron spectroscopy. It was found that new oxygen-containing groups are formed on the polymer surface as a result of plasma discharge treatment.     

The Role of Viscoelastic Adhesive Contact in the Sintering of Polymeric Particles

The sintering of polymeric particles is analyzed by considering the growth of contact between viscoelastic spheres driven by adhesive intersurface forces. This process is dominant during the initial phase of sintering and is succeeded by a viscous sintering step that is driven by surface tension and accommodated by viscous flow. The intersurface forces in this work are described by a cohesive zone model. A new formulation of adhesive contact that does not require the cohesive zone to be smaller than the contact radius, together with finite element simulations is used to study the growth of contact. The results of this paper establish conditions that determine the dominant mechanism of contact growth during sintering. These conditions are described using a "deformation map". For a Maxwell material, if particle radius R(max),相似文献   

Numerical simulation of interfacial debonding in particulate composites considering material inhomogeneities

A comparative study in the framework of large deformations has been conducted to get insight into micro mechanical response of particulate polymeric composites. The propound model enables finite element predictions of displacement response and resulting debonding of a three-phase material subjected to incremental loading conditions. Predictions employ a unit cell of certain shape under specified loading and constraints. The proposed model involves nonlinear material properties and incorporates a strong or weak non-homogeneous interphase region. The interphase toughness is expressed in terms of the adhesion efficiency between the filler and the matrix, though contact conditions are preserved utilizing special contact elements. Interfacial nonlinear spring joint elements regulate debonding initialization, in relation to the imposed failure criterion. Numerical results are presented and discussed for axial tensile and compressive loadings for a variety of values of the imposed parameters.     

聚合物共混物脆韧转变性能研究 Ⅳ橡胶粒子的分布对PVC/NBR共混物脆韧转变的影响

根据作者已建立的准网络形态模型和推导出的基体层厚度计算公式，从实验上研究了橡胶粒子的分布对聚氯乙烯（ＰＶＣ）／丁氰橡胶（ＮＢＲ）共混物脆韧转变的影响．结果表明，不仅无规形态ＰＶＣ／ＮＢＲ共混物存在脆韧转变主曲线，而且准网络形态ＰＶＣ／ＮＢＲ共混物也存在脆韧转变主曲线．但是两条主曲线明显不重合，表明橡胶粒子的分布对ＰＶＣ／ＮＢＲ共混物脆韧转变有显著影响．而且准网络形态ＰＶＣ／ＮＢＲ共混物的临界基体层厚度比无规形态ＰＶＣ／ＮＢＲ共混物的临界基体层厚度大得多，表明准网络形态比无规形态明显有利于增韧．因此临界基体层厚度不仅是基体的特征参数，还是界面粘结和橡胶粒子分布的函数．     

An elastic-plastic hybrid adhesion model for contacting rough surfaces in the presence of molecularly thin lubricant

The study of adhesion has received considerable attention in recent years, partly due to advances in the design and fabrication of micro/nano devices. Many adhesion investigations are centered on single-spherical-contact models, which include the classic Johnson-Kendall-Roberts (JKR), improved Derjaguin-Muller-Toporov (IDMT), and Maugis-Dugdale (MD) models. Based on the IDMT single-asperity model, adhesive rough surface contact models have also been developed, which are valid for elastic and elastic-plastic contact conditions. A limitation of the IDMT-based models is that they are only valid for application cases with low adhesion parameter values. In this research, a contacting rough surface adhesion model was developed by combining an extended Maugis-Dugdale (EMD) model (which is only valid for elastic contacts) with an IDMT-based elastic-plastic adhesion model. The proposed model, termed the elastic-plastic hybrid adhesion model, is valid for the entire adhesion parameter range and also for elastic-plastic contacts. The proposed model gives results similar to the EMD rough surface model when the contact is primarily elastic. Moreover, the proposed model was compared to an IDMT-based model (ISBL model) and both gave similar results for contacts with low adhesion parameter values. With high adhesion parameter values, the ISBL model fails, whereas the proposed model correctly predicts higher adhesion. Last, based on the stiffness of the external force, the instability for adhesive rough surfaces in contact was also discussed, and it was postulated that a high peak value of the external force stiffness directly relates to the unstable contact process.     

Modelling procedures and properties of rubber in rolling contact

Rubber covered cylinders in rolling contact are studied in two cases; rolling over a flat surface and rolling over a groove. In the first case, two different finite element procedures are compared for the purpose of investigating if computational savings can be made when taking amplitude dependent effects into account by using a modified viscoelastic steady state rolling procedure. This procedure is compared with using a more expensive overlay method with an elastoplastic-viscoelastic material model. The two procedures and material models are shown two give equal results in the flat surface case. For the case of rolling over a groove, it is shown how the non-linear dynamic material characteristics of the rubber layer influence the rolling contact. The groove filling capacity of the rubber is shown to be strongly dependent on the material model. It is shown that amplitude dependent rubber materials have better ability to fill out contact surface irregularities such as a groove.     

Microstructural behavior and failure mechanisms of organic semicrystalline thin film blends

Organic thin film blends of P3HT semiconducting polymers and PCBM fullerenes have enabled large‐scale semiconductor fabrication pertaining to flexible and stretchable electronics. However, molecular packing and film morphologies can significantly alter mechanical stability and failure behavior. To further understand and identify the fundamental mechanisms affecting failure, a multiphase microstructurally based formulation and nonlinear finite‐element fracture methodology were used to investigate the heterogeneous deformation and failure modes of organic semicrystalline thin film blends. The multiphase formulation accounts for the crystalline and amorphous behavior, polymer tie‐chains, and the PCBM aggregates. Face‐on packing orientations resulted in extensive inelastic deformation and crystalline rotation, and this was characterized by ductile failure modes and interfacial delamination. For edge‐on packing orientations, brittle failure modes and film cracking were due to lower inelastic deformation and higher film stress in comparison with the face‐on orientations. The higher crystallinity of P3HT and larger PCBM aggregates associated with larger domain sizes, strengthened the film and resulted in extensive film cracking. These predictions of ductile and brittle failure are consistent with experimental observations for P3HT:PCBM films. The proposed predictive framework can be used to improve organic film ductility and strength through the control of molecular packing orientations and microstructural mechanisms. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 896–907     

Modeling adhesion forces between deformable bodies by FEM and Hamaker summation

A new numerical approach is presented for predicting adhesion forces of particles at flat and rough surfaces. The new hybrid method uses the finite element method (FEM) for the determination of elastic and plastic particle deformation combined with numerical Hamaker summation. In the numerical approach, the influence of the plastic deformation can be fully included. We show how the adhesion force depends on the contact geometry and the material properties. For easy comparison with other models, the force-displacement behavior of the systems is presented. The numerical approach is supported by atomic force microscopy (AFM) measurements. The experimentally observed adhesion force hysteresis is described very well by the new approach. Although calculations in this article are focused on spherical particles, our approach can be extended to particles of arbitrary shapes.     

A modeling approach to describe the adhesion of rough, asymmetric particles to surfaces

A combined theoretical and experimental study of the adhesion of alumina particles and polystyrene latex spheres to silicon dioxide surfaces was performed. A boundary element technique was used to model electrostatic interactions between micron-scale particles and planar surfaces when the particles and surfaces were in contact. This method allows quantitative evaluation of the effects of particle geometry and surface roughness on the electrostatic interaction. The electrostatic interactions are combined with a previously developed model for van der Waals forces in particle adhesion. The combined model accounts for the effects of particle and substrate geometry, surface roughness and asperity deformation on the adhesion force. Predictions from the combined model are compared with experimental measurements made with an atomic force microscope. Measurements are made in aqueous solutions of varying ionic strength and solution pH. While van der Waals forces are generally dominant when particles are in contact with surfaces, results obtained here indicate that electrostatic interactions contribute to the overall adhesion force in certain cases. Specifically, alumina particles with complex geometries were found to adhere to surfaces due to both electrostatic and van der Waals interactions, while polystyrene latex spheres were not affected by electrostatic forces when in contact with various surfaces.     

Shape and eccentricity effects in adhesive contacts of rodlike particles

The effects of shape and eccentricity on adhesion and detachment behavior of long, rodlike particles in contact with a half-space are analyzed using contact mechanics. The particles are considered to have cross sections that are squarish, oblate, or prolate rather than circular. Such cross sections are represented very generally by using superellipses. The contact mechanics model allows deduction of closed-form expressions for the contact pressure, load-contact size relation, detachment load, and detachment contact size. It is found that even relatively small deviations in shape from a cylinder have a significant influence on the detachment load. Eccentricity also affects the adhesive behavior, but to a lesser extent, with oblate shapes requiring larger separation loads than prolate shapes. The load-contact size solution reduces to that for a right-circular, cylindrical rod when the appropriate limit is taken. The detachment behavior of right-circular cylinders is also found to be mimicked by an entire family of rod shapes with different cross sections.     

The adhesion kinetics of sticky vesicles in tension: the distinction between spreading and receptor binding

We investigate the kinetics of spreading and adhesion between polymer vesicles decorated with avidin and biotin, held in micropipettes to maintain fixed tension and suppress membrane bending fluctuations. In this study, the density of avidin (actually Neutravidin) and biotin was varied, but was always sufficiently high so that lateral diffusion in the membrane was unimportant to the adhesive mechanism or rate. For a stunning result, we report a concentration-dependent distinction between adhesion and spreading: At low surface densities of avidin and biotin, irreversible vesicle adhesion is strong enough to break the membrane when vesicle separation is attempted, yet there is no spreading or "wetting". By this we mean that there is no development of an adhesion plaque beyond the initial radius of contact and there is no development of a meaningful contact angle. Conversely, at 30% functionalization and greater, membrane adhesion is manifest through a spreading process in which the vesicle held at lower tension partially engulfs the second vesicle, and the adhesion plaque grows, as does the contact angle. Generally, when spreading occurs, it starts abruptly, following a latent contact period whose duration decreases with increasing membrane functionality. A nucleation-type rate law describes the latency period, determined by competition between bending and sticking energy. The significance of this result is that, not only are membrane mechanics important to the development of adhesion in membranes of nanometer-scale thickness, mechanics can dominate and even mask adhesive features such as contact angle. This renders contact angle analyses inappropriate for some systems. The results also suggest that there exist large regions of parameter space where adhesive polymeric vesicles will behave qualitatively differently from their phospholipid counterparts. This motivates different strategies to design polymeric vesicles for applications such as targeted drug delivery and biomimetic scavengers.     

Combinatorial investigations of interfacial failure

Conventional measurements of interfacial strength focus on a single variable, whereas many variables couple nontrivially and simultaneously to define this property. We present a combinatorial methodology that allows the effects of multivariable environments on interfacial strength to be investigated in a high‐throughput, parallel, and quantitative manner. This technique is largely based on the theory of Johnson, Kendall, and Roberts that quantifies adhesion through the contact and separation of a spherical lens and flat substrate. For our experiments, we fabricated a combinatorial library consisting of a two‐dimensional array of spherical caps and a complementary substrate. The array of spherical caps was brought into contact and subsequently separated from the substrate, whereas the relative displacement and contact area of the individual lenses were recorded. With gradient library‐fabrication methods, two adhesion‐controlling parameters can be continuously varied along the orthogonal axes of the array. In this manner, each lens quantifies the interfacial strength at a unique point in parameter space. We demonstrate this multilens contact‐adhesion test by measuring the effect of temperature and coating thickness on the self‐adhesion of polystyrene thin films. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 883–891, 2003     

Adhesion curves between two viscoelastic materials by a point‐contact method in a crossed‐cylinder geometry

We investigated an evaluation method of adhesion between two cylindrical viscoelastic materials by a point contact in a crossed‐cylinder geometry. The shape of the adhesion curve obtained in this technique is characterized not only by the maximum adhesion force, F_A, but also by the adhesion force at complete separation, F_S. To clarify the factors that determine the characteristic properties of the adhesion curve, the adhesion forces of a highly crosslinked polydimethylsiloxane were measured as a function of the separation velocity. As a result, F_A and F_S strongly depended on the separation velocity. To understand the experimental results, a simulation of the separation behavior was carried out using the Generalized Maxwell model, which could qualitatively reproduce the experimental observations. From these results, we discussed the factors that determine the adhesion curve and clarified the uniqueness and advantages of this evaluation method. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1778–1788, 2009     

Effect of fibril arrangement on crack trapping in a film‐terminated fibrillar interface

We present a three‐dimensional (3D) model for adhesion enhancement due to crack trapping in a film‐terminated fibrillar structure. Adhesion enhancement occurs due to trapping of the interfacial crack in the region between fibrils. Energy release to the crack tip is attenuated because, between fibrils, it has to pass through the compliant terminal film. Using perturbation theory and a finite element method, we solve for the shape of crack front, which is unknown. Our model thus also allows us to study how adhesion enhancement depends on the arrangement of fibrils. For example, our model explains why, for a fixed area density of fibrils and for similar crack orientations, hexagonal arrays have higher adhesion than square arrays. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2368–2384, 2009     

Colloidal adhesion of phospholipid vesicles: high-resolution reflection interference contrast microscopy and theory

High-resolution reflection interference contrast microscopy (HR-RICM) was developed for probing the deformation and adhesion of phospholipid vesicles induced by colloidal forces on solid surfaces. The new technique raised the upper limit of the measured membrane–substrate separation from 1 to 4.5 μm and improved the spatial resolution of the heterogeneous contact zones. It was applied to elucidate the effects of wall thickness, pH and osmotic stress on the non-specific adhesion of giant unilamellar vesicles (ULV) and multilamellar vesicles (MLV) on fused silica substrates. By simultaneous cross-polarization light microscopy and HR-RICM measurements, it was observed that ULV with the wall thickness of a single bilayer would be significantly deformed in its equilibrium state on the substrate as the dimension of its adhesive–cohesive zone was 29% higher than the theoretical value of a rigid sphere with the same diameter. Besides, electrostatic interaction was shown as a significant driving force for vesicle adhesions since the reduction in pH significantly increased the degree of deformation of adhering ULV and heterogeneity of the adhesion discs. The degree of MLV deformation on the solid surfaces was significantly less than that of ULV. When the wall thickness of vesicle increased, the dimension of contact zone was reduced dramatically due to the increase of membrane bending modulus. Most important, the adhesion strength of colloidal adhesion approached that of specific adhesion. Finally, the increase of osmotic stress led to the collapse of adhering vesicles on the non-deformable substrate and raised the area of adhesive contact zone. To interpret these results better, the equilibrium deformation of adhering vesicle was modeled as a truncated sphere and the adhesion energy was calculated with a new theory.     

纳米银米表面等离子激元增强上转换发光

上转换纳米发光材料因发光效率普遍较低限制了其在应用领域的进一步发展。利用贵金属的局域表面等离子体共振特性调控发射体的局域场成为提高上转换纳米粒子发光性能的有效途径之一。在这个工作中,我们利用合成工艺简单的贵金属米状结构,通过调控上转换纳米颗粒周围的局域场,实现了上转换发光强度的大幅提高。结合三维有限元方法研究了在实际薄膜中Ag米之间可能出现的不同接触方式对所产生的局域场增强效果的影响,同时证实这种局域等离激元耦合所引起的电磁场增强是提高上转换发光性能的重要因素,并且获得了上转换发光将近百倍的增强效果。最后,通过调控等离子层和发光层的厚度,实现上转换发光性能的进一步优化。