Limit Cycles in Dynamic Adhesion and Friction Processes: A Discussion

Simple equations exist relating adhesion and friction forces. These apply to simple attachment-detachment processes and steady-state (smooth) sliding conditions. However, in the case of more complex, such as polymer, surfaces both the adhesion and friction can be very complex, irreversible, and nonlinear, exhibiting stringing and tack in the former and stick-slip sliding in the latter. We explore possible relationships between such nonlinear adhesion and friction processes. Based on recent experiments we find that certain types of “limit cycles,” relating the (normal) adhesion and (lateral) friction forces, F_⊥ and F_∥, to the relative velocities, V_⊥ and V_∥, of the surfaces during an attachment-detachment process or stick-slip sliding, bear a very similar resemblance to each other. We briefly discuss the theoretical and practical implications of describing such dynamic processes in terms of limit cycles.     

Limit Cycles in Dynamic Adhesion and Friction Processes: A Discussion

Simple equations exist relating adhesion and friction forces. These apply to simple attachment-detachment processes and steady-state (smooth) sliding conditions. However, in the case of more complex, such as polymer, surfaces both the adhesion and friction can be very complex, irreversible, and nonlinear, exhibiting stringing and tack in the former and stick-slip sliding in the latter. We explore possible relationships between such nonlinear adhesion and friction processes. Based on recent experiments we find that certain types of “limit cycles,” relating the (normal) adhesion and (lateral) friction forces, F _⊥ and F _∥, to the relative velocities, V _⊥ and V _∥, of the surfaces during an attachment-detachment process or stick-slip sliding, bear a very similar resemblance to each other. We briefly discuss the theoretical and practical implications of describing such dynamic processes in terms of limit cycles.     

粘接技术的回顾与展望

简要地回顾了胶粘剂应用的发展。对粘接技术的基本科学问题的综述涉及固体表面的湿润问题、现代表面分析技术、粘接机理、粘接接头的强度及粘接的耐久性。展望粘接技术的发展,必须十分重视环境保护和可再生资源的利用;低成本的耐久性粘接技术和多功能胶粘剂是值得注意的方向。最后提出了粘接技术发展的若干前沿难题。     

Studies on Adhesion Between Natural Rubber and Polyethylene and the Role of Adhesion Promoters

Studies on adhesion between natural rubber (NR) and polyethylene (PE) with different levels of interaction (physical and chemical) have been carried out. Ethylene propylene diene rubber (EPDM) and chlorinated polyethylene (CPE) were used as physical promoters and epoxidised natural rubber/modified polyethylene (ENR/PEm) and sulfonated ethylene propylene diene rubber/modified polyethylene (S-EPDM/PEm) were used as chemical adhesion promoters. The failure surfaces were examined with the help of scanning electron microscopy (SEM), optical photography and electron spectroscopy for chemical analysis (ESCA) techniques.

The peel strength between natural rubber and polyethylene as measured in this study is 140 J/m². With the incorporation of physical promoters such as EPDM, the peel strength increases twenty fold because of structural similarity of EPDM with PE and the rubbery nature of EPDM. Similarly, the other promoters show significant improvement in peel strength. At high temperature and low rate of peeling, the nature of failure is mainly “stick-slip” for joints with interaction promoters. The average peel strength increases with increase in test rate and decrease in test temperature for most of the joints. All the data could be shifted onto a master curve indicating that the increase in strength is a result of viscoelastic dissipation. NR/EPDM/PE and NR/CPE/PE systems, however, behave in a different way probably because they alter the nature of crack propagation at or near the interface. ESCA results of the peeled PE surface show a chemical shift of C_1S peak. SEM photographs also indicate interaction at the interface when modifiers are used. An increase in crystallinity of PE from 30% to 64% and modulus increase the peel strength of NR/PE joints by a factor of four. The results of peel strength measurement at 90° are lower than those at 180°. Lap shear results are in line with peel strength.     

Friction and Adhesion Hysteresis between Surfactant Monolayers in Water

We briefly review the model that correlates friction between two surfaces in adhesive contact with the loading–unloading adhesion hysteresis between them. We then examine in light of this model the observed low friction between two mica surfaces coated with a double-chained quaternary ammonium surfactant in intimate adhesive contact in water. This enables us to propose a mechanism for surfactant boundary lubrication in water that is rather different from the classic boundary lubrication in air: in this mechanism, adhesion takes place at the interface between the opposing surfactant hydrocarbon tails, whereas frictional sliding takes place at the interface between the hydrated surfactant headgroups and mica. The implications of our findings to biolubrication processes are discussed.     

Friction and Adhesion Hysteresis between Surfactant Monolayers in Water

We briefly review the model that correlates friction between two surfaces in adhesive contact with the loading-unloading adhesion hysteresis between them. We then examine in light of this model the observed low friction between two mica surfaces coated with a double-chained quaternary ammonium surfactant in intimate adhesive contact in water. This enables us to propose a mechanism for surfactant boundary lubrication in water that is rather different from the classic boundary lubrication in air: in this mechanism, adhesion takes place at the interface between the opposing surfactant hydrocarbon tails, whereas frictional sliding takes place at the interface between the hydrated surfactant headgroups and mica. The implications of our findings to biolubrication processes are discussed.     

用黑曜岩泡沫玻璃作紫贻贝附着基的研究

黑曜岩泡沫玻璃配合料由黑曜岩废料、玻璃粉和发泡剂组成。黑曜岩泡沫玻璃用作海水中养殖紫贻贝的附着基。紫贻贝在黑曜岩泡沫玻璃的附着比其他附着基牢固。     

硅橡胶与玻璃布的粘接研究

研究了旨在提高橡胶／玻璃布拉粘接性能的玻璃布的表面处理，并地影响其粘接强度的因素进行了讨论。结果表明，ＫＨ５５０／Ａ１５１、间苯二酚／Ａ１５１是其处理的有效体系。     

Adhesion of Cancer Cells to Endothelial Monolayers: A Study of Initial Attachment Versus Firm Adhesion

For most cancer patients, the ultimate cause of death is not the primary tumor itself, but metastasis, or the spread of cancer from the primary tumor throughout the body. The formation of tumor foci at sites distant from the primary tumor is a multistep process which includes dissemination of the cancer cells through the blood stream and hence, interactions with the endothelium lining the blood vessels walls. At least two theories have been proposed for explaining the interaction between cancer cells and endothelium. According to one theory, the tumor cells roll along the endothelium and the rolling velocity decreases until the cells become firmly attached to the vessel wall. In another theory, the circulating cancer cells must first lodge inside small vessels before they attach to the endothelium. In the latter case, the cells would only metastasize in the smaller vessels where lodging can occur. To gain further insight into the process of metastasis, the adhesion of human breast cancer cells to human umbilical vein endothelial monolayers was investigated in terms of both initial attachment followed by firm adhesion and firm adhesion following incubation in a static environment. The parallel plate flow chamber was employed to perform two different adhesion assays that would simulate these two adhesion mechanisms. Adhesion assays were carried out at a variety of physiological shear stresses found in the microvasculature and both highly metastatic and nonmetastatic cells were investigated. Results showed that initial attachment was only observed at very low shear stresses whereas firm adhesion occurred at a number of physiological shear stresses. These results suggest that the adhesion of the human breast cancer cells used in this study to endothelium most likely takes place via a lodging-firm adhesion mechanism in the capillaries and venules. However, it is important to note that other factors such as pulsatility and vessel compliance may contribute to the attachment. It was also shown that, for these specific breast cancer cells, adhesion did not correlate with metastatic potential. This suggests that while blocking the adhesion of highly metastatic cells may inhibit their ability to metastasize, adhesion properties alone do not provide an indication as to whether a cell is metastatic or nonmetastatic under the conditions studied here.     

Molecular Studies of Adhesion and De-Adhesion on ZnO Nanorod Film-Covered Metals

This paper aims at the investigation of the effect of ZnO nanorod film deposition on the adhesion between a zinc surface and a model epoxy-based adhesive, with and without the adhesion promoter 3-aminopropylphosphonic acid (APPA) treatment. The stability of octadecylphosphonic acid (ODPA) monolayers on ZnO nanorod surfaces was investigated by means of contact angle measurements and FT-IRRAS to simulate wet de-adhesion conditions at the phosphonate-zinc oxide interface. Peel tests were performed under controlled humidity to study the wet de-adhesion of the model epoxy-amine coating from the ZnO nanorod surface. The deposition of ZnO nanorod films resulted in a significant increase of peel forces in comparison with bare zinc. In the case of APPA-treated ZnO nanorod films the increase of the macroscopic adhesion forces was more pronounced. The high surface area ZnO nanorod films provide for the adsorption of polymers as well as adhesion promoters and makes them promising candidates for improving the adhesion properties of engineering metals.     

Adhesion of Cancer Cells to Endothelial Monolayers: A Study of Initial Attachment Versus Firm Adhesion

For most cancer patients, the ultimate cause of death is not the primary tumor itself, but metastasis, or the spread of cancer from the primary tumor throughout the body. The formation of tumor foci at sites distant from the primary tumor is a multistep process which includes dissemination of the cancer cells through the blood stream and hence, interactions with the endothelium lining the blood vessels walls. At least two theories have been proposed for explaining the interaction between cancer cells and endothelium. According to one theory, the tumor cells roll along the endothelium and the rolling velocity decreases until the cells become firmly attached to the vessel wall. In another theory, the circulating cancer cells must first lodge inside small vessels before they attach to the endothelium. In the latter case, the cells would only metastasize in the smaller vessels where lodging can occur. To gain further insight into the process of metastasis, the adhesion of human breast cancer cells to human umbilical vein endothelial monolayers was investigated in terms of both initial attachment followed by firm adhesion and firm adhesion following incubation in a static environment. The parallel plate flow chamber was employed to perform two different adhesion assays that would simulate these two adhesion mechanisms. Adhesion assays were carried out at a variety of physiological shear stresses found in the microvasculature and both highly metastatic and nonmetastatic cells were investigated. Results showed that initial attachment was only observed at very low shear stresses whereas firm adhesion occurred at a number of physiological shear stresses. These results suggest that the adhesion of the human breast cancer cells used in this study to endothelium most likely takes place via a lodging-firm adhesion mechanism in the capillaries and venules. However, it is important to note that other factors such as pulsatility and vessel compliance may contribute to the attachment. It was also shown that, for these specific breast cancer cells, adhesion did not correlate with metastatic potential. This suggests that while blocking the adhesion of highly metastatic cells may inhibit their ability to metastasize, adhesion properties alone do not provide an indication as to whether a cell is metastatic or nonmetastatic under the conditions studied here.     

Effect of the type of plasma on the polydimethylsiloxane/collagen composites adhesive properties

Polydimethylsiloxane (PDMS) films were treated with either oxygen (O₂), nitrogen (N₂) or argon (Ar) plasma between 40 W and 120 W for 5–15 min and their surface properties studied by contact angle measurements, infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and atomic force microscopy (AFM). Lower contact angles and increases in surface roughness, assessed by SEM and AFM, were observed for all used gases when plasma power and time increased, with argon treatment being the one that showed the most significant change in roughness.PDMS/collagen type I composites obtained after treating PDMS with oxygen at 80 W for 13 min or nitrogen and argon at 80 W for 14 min showed a peel strength of 0.1N/mm (oxygen plasma), 0.08 N/mm (nitrogen plasma) and 0.09 N/mm (argon plasma). In all cases, peel strength was higher than that measured for the untreated bilayer composite. An increase in adhesion strength, after oxygen and nitrogen plasma, was mostly attributed to chemical interaction between functional groups introduced on the PDMS surface and the functional groups on collagen as detected by FTIR. In contrast, the high peel strength observed on PDMS treated with argon plasma was attributed to its increased roughness which in turn increased mechanical interlocking. The properties of these composites render them suitable for adhesive free skin substitutes.     

Adhesion Moment of a Particle on a Wall in Gaseous Environment

If a particle adheres to a wall, it gets flattened. When such a particle gets stressed sideways by an external force, it will start to roll. To get the particle into a rolling motion, a resistive moment must be overcome, the so‐called adhesion moment. An experimental setup was designed to measure the force which must be applied to an adhered particle to trigger a rolling motion in the specimen chamber of an environmental scanning electron microscope (ESEM). From this force, the adhesion moment can be calculated. A detailed explanation of the experiments will be given. The results of the experiments are discussed.     

Nano-scale friction of polystyrene in air and in vacuum

Using atomic force microscopy (AFM), we measured friction between an AFM tip and a polystyrene surface at 25 °C, as a function of the sliding velocity and the applied normal load, both in air and under vacuum conditions. The objective was to analyze the influence of humidity on the frictional behavior of polystyrene. Our experimental results as a function of sliding velocity revealed a logarithmic increase of the friction force in air whereas a logarithmic decrease of this force is found in vacuum. Our comparative results unveil that two different dissipation mechanisms are dominating the frictional behavior of polystyrene in air and in vacuum. We propose a tentative explanation.     

胶黏剂对纤维桩修复效果的影响

经过几十年的发展,粘接技术已经成为一项重要的口腔临床技术。关于根管桩材料和设计方案的研究已日益成熟。纤维桩树脂核修复被公认为具有较好的修复效果。现在的研究主要集中在桩核的粘接固位方面。为了研究增强在根管内的固位以及桩与树脂的粘接,综述了胶黏剂对纤维桩修复效果的影响。     

A Review of Adhesion Promotion Techniques for Solid Timber Substrates

The use of primers, coupling agents, and other surface treatments to enhance adhesion is now common in the aerospace, automotive, and plastics industries, where they are used to develop highly durable bonds to metals, advanced composites, ceramics, and plastics. However, such treatments are virtually non-existent in the wood products industry although they could solve important adhesion problems. In particular, adhesion promoters can enhance the environmental durability of epoxy bonded joints, and they can enhance the reliability of bonds to timber treated with wood preservatives. A review of current findings is provided that attempts to gather the scarce and disperse information available in the literature about adhesion promotion techniques for bonded solid timber joints. A general overview of the research needs on this topic is also given.     

Adhesion of ultrafine particles—A micromechanical approach

In particle processing and product handling of fine , ultrafine and nanosized particles , the well-known flow problems of dry cohesive powders in process apparatuses or storage and transportation containers include bridging, channelling, widely spread residence time distribution associated with time consolidation or caking effects, chemical conversions and deterioration of bioparticles. Avalanching effects and oscillating mass flow rates in conveyors lead to feeding and dosing problems. Finally, insufficient apparatus and system reliability of powder processing plants are also related to these flow problems. Thus, it is very essential to understand the fundamentals of particle adhesion with respect to product quality assessment and process performance in particle technology.The state-of-the-art in constitutive modelling of elastic, elastic-adhesion, elastic-dissipative, plastic-adhesion and plastic-dissipative contact deformation response of a single isotropic contact of two smooth spheres is briefly discussed. Then the new models are shown that describe the elastic-plastic force-displacement and moment-angle behaviour of adhesive and frictional contacts.Using the model “stiff particles with soft contacts”, a sphere-sphere interaction of van der Waals forces without any contact deformation describes the “stiff” attractive term. A plate-plate model is used to calculate the “soft” micro-contact flattening and adhesion. Various contact deformation paths for loading, unloading and contact detachment are discussed. Thus, the varying adhesion forces between particles depend directly on this “frozen” irreversible deformation. Thus, the adhesion force is found to be load dependent. Their essential contribution on the tangential force in an elastic-plastic frictional contact with partially sticking within the contact plane and microslip, the rolling resistance and the torque of mobilized frictional contact rotation is shown.     

Adhesion of Vacuum Deposited Optical Coatings on PMMA and Polycarbonate

Abstract

The deposition of interference coatings with optical functions (e.g., anti-reflective coatings, beam splitters and filters) on plastic substrates is becoming ever more important. A well-known deposition method for such coatings on polymers is plasma ion-assisted vacuum evaporation (plasma ion-assisted deposition or plasma IAD). However, the industrial production of well-adhering dielectric coatings on unlacquered polymer surfaces has not yet been developed to a satisfactory level. PMMA is known to be extremely difficult to coat with optical layers because of its inadequate adhesion characteristics. Polycarbonate is considered to be less problematic, but in many cases unexpected adhesion failures arise. This paper presents a study of the reasons for the poor adhesion to PMMA of oxide coatings deposited by plasma IAD. We show that in such a coating process it is only the exposure of the substrate to particles and short-wavelength radiation that determines its adhesion to the deposited layer. For polycarbonate, we found a dependence of the adhesion strength of the coating on its porosity. Considering these results, modified plasma IAD processes have been developed that enable the deposition of well-adhering optical coatings on these polymers.     

Adhesion and Debonding of Soft Elastic Films on Rough and Patterned Surfaces

With the help of simulations based on energy minimization, we have studied the effect of roughness of a rigid contactor with sinusoidal and step patterns on the adhesion-debonding cycle of a soft thin elastic film. The surface instability engendered by attractive forces between the contactor and the film produces a regularly spaced array of columns in the bonding phase. The inter-column spacing is governed largely by periodicity of the contactor pattern. Decreased periodicity of the pattern favors intermittent collapse of columns rather than a continuous peeling of contact zones. An increase in the amplitude of roughness decreases the maximum force required for debonding and increases the snap-off distance. The net effect results in a reduced work for debonding. Introduction of noise and increased step-size in simulations decreases the pull-off force and the snap-off distance, as in the case of a smooth contactor. Finally the study reveals that a patterned contactor can be used as a potential template in the patterning of soft interfaces.