Meso-scale dislocations and friction of shape-complementary soft interfaces

The interface between two surfaces patterned with complementary shapes such as arrays of ridge–channel structures or pillars accommodates relative misorientation and lattice mismatch by spontaneous production of dislocation arrays. Here, we show that the relative sliding of such an interface is accomplished by dislocation glide on the interfacial plane. An exception is the singular case where the lattices are perfectly matched across the sample dimension, in which case sliding is accompanied by motion of edge-nucleated defects. These are meso-scale analogues of molecular sliding friction mechanisms between crystalline interfaces. The dislocations, in addition to the long-range elastic energy associated with their Burgers vectors, also cause significant out-of-plane dilation, which props open the interface locally. For this reason, the sliding friction is strongly pressure dependent; it also depends on the relative orientation of the patterns. Sliding friction can be strongly enhanced compared with a control, showing that shape-complementary interfaces can be engineered for strongly enhanced pressure- and orientation-dependent frictional properties in soft solids.     

Surface contact and design of fibrillar ‘friction pads’ in stick insects (Carausius morosus): mechanisms for large friction coefficients and negligible adhesion

Many stick insects and mantophasmids possess tarsal ‘heel pads’ (euplantulae) covered by arrays of conical, micrometre-sized hairs (acanthae). These pads are used mainly under compression; they respond to load with increasing shear resistance, and show negligible adhesion. Reflected-light microscopy in stick insects (Carausius morosus) revealed that the contact area of ‘heel pads’ changes with normal load on three hierarchical levels. First, loading brought larger areas of the convex pads into contact. Second, loading increased the density of acanthae in contact. Third, higher loads changed the shape of individual hair contacts gradually from circular (tip contact) to elongated (side contact). The resulting increase in real contact area can explain the load dependence of friction, indicating a constant shear stress between acanthae and substrate. As the euplantula contact area is negligible for small loads (similar to hard materials), but increases sharply with load (resembling soft materials), these pads show high friction coefficients despite little adhesion. This property appears essential for the pads’ use in locomotion. Several morphological characteristics of hairy friction pads are in apparent contrast to hairy pads used for adhesion, highlighting key adaptations for both pad types. Our results are relevant for the design of fibrillar structures with high friction coefficients but small adhesion.     

铜基面抗拔摩擦摆支座的力学性能研究

为克服现有摩擦摆支座聚四氟乙烯（TEFLON）摩擦面承压能力低和缺乏竖向抗拔功能的不足,对课题组的专利产品—竖向抗拔摩擦摆支座进行了力学性能研究。论文首先介绍了支座的基本构造,并建立了支座的理论模型。为验证理论模型的合理性,设计了竖向承载力3000kN铜基面的摩擦摆支座,并对其进行试验研究。试验测定了竖向抗拔摩擦摆支座滑动面间的摩擦系数,考察了竖向压力和滑移速度对摩擦系数与滞回性能的影响。结果表明：铜基面摩擦摆支座在正弦简谐激励作用下的滞回曲线为平行四边形,与传统摩擦摆支座的滞回曲线具有相同的特征,呈刚塑性;铜基摩擦摆支座的摩擦系数的平均值在0.096-0.109间;滞回曲线的测试结果与数值模拟数据吻合较好,验证了理论模型的合理性。     

Adhesion and sliding response of a biologically inspired fibrillar surface: experimental observations

Inspired by the adhesion mechanisms of several animal species such as geckos, beetles and flies, several efforts in designing and fabricating surface engineering strategies have been made recently to mimic the adhesive and frictional behaviour of biological foot pads. An important feature of such biological adhesion systems is the ability to switch between strong attachment and easy detachment, which is crucial for animal locomotion. Recent investigations have suggested that such a 'switching' mechanism can be achieved by the elastic anisotropy of the attachment pad, which renders the magnitude of the detachment force to be direction dependent. This suggestion is supported by the observations that the fibres of the foot pads in geckos and insects are oriented at an angle to the base and that geckos curl their toes backwards (digital hyperextension) while detaching from a surface. One of the promising bio-inspired architectures developed recently is a film-terminated fibrillar PDMS surface; this structure was demonstrated to result in superior detachment force and energy dissipation compared with a bulk PDMS surface. In this investigation, the film-terminated fibrillar architecture is modified by tilting the fibres to make the surface vertically more compliant and elastically anisotropic. The directional detachment and the sliding resistance between the tilted fibrillar surfaces and a spherical glass lens are measured: both show significant directional anisotropy. It is argued that the anisotropy introduced by the tilted fibres and the deformation-induced change in the compliance of the fibre layer are responsible for the observed anisotropy in the detachment force.     

Bio‐Inspired,Smart, Multiscale Interfacial Materials

In this review a strategy for the design of bioinspired, smart, multiscale interfacial (BSMI) materials is presented and put into context with recent progress in the field of BSMI materials spanning natural to artificial to reversibly stimuli‐sensitive interfaces. BSMI materials that respond to single/dual/multiple external stimuli, e.g., light, pH, electrical fields, and so on, can switch reversibly between two entirely opposite properties. This article utilizes hydrophobicity and hydrophilicity as an example to demonstrate the feasibility of the design strategy, which may also be extended to other properties, for example, conductor/insulator, p‐type/n‐type semiconductor, or ferromagnetism/anti‐ferromagnetism, for the design of other BSMI materials in the future.     

Biomimetic interfaces based on S-layer proteins,lipid membranes and functional biomolecules

Designing and utilization of biomimetic membrane systems generated by bottom-up processes is a rapidly growing scientific and engineering field. Elucidation of the supramolecular construction principle of archaeal cell envelopes composed of S-layer stabilized lipid membranes led to new strategies for generating highly stable functional lipid membranes at meso- and macroscopic scale. In this review, we provide a state-of-the-art survey of how S-layer proteins, lipids and polymers may be used as basic building blocks for the assembly of S-layer-supported lipid membranes. These biomimetic membrane systems are distinguished by a nanopatterned fluidity, enhanced stability and longevity and, thus, provide a dedicated reconstitution matrix for membrane-active peptides and transmembrane proteins. Exciting areas in the (lab-on-a-) biochip technology are combining composite S-layer membrane systems involving specific membrane functions with the silicon world. Thus, it might become possible to create artificial noses or tongues, where many receptor proteins have to be exposed and read out simultaneously. Moreover, S-layer-coated liposomes and emulsomes copying virus envelopes constitute promising nanoformulations for the production of novel targeting, delivery, encapsulation and imaging systems.     

Adhesion, friction and wear on the nanoscale of MWNT tips and SWNT and MWNT arrays

The nanotribological characterization of carbon nanotubes is fundamental for the exploration of new sliding applications. In this study, a comprehensive investigation of adhesion, friction and wear of a multiwalled nanotube (MWNT) tip, and SWNT (single-walled nanotube) and MWNT arrays has been carried out. A nonlinear response of the MWNT tip is observed when the tip is brought into and out of contact with various surfaces. A nonlinear response occurs due to the buckling of the nanotube and its subsequent sliding on the surface. In addition to the role of surface chemistry, it can also explain the relatively high value of the coefficient of friction obtained on different surfaces, as compared to that of Si and Si(3)N(4) tips. The adhesion and friction studies carried out on SWNT and MWNT arrays using Si tips show that SWNT arrays, compared to MWNT arrays, exhibit lower values, possibly due to lower van der Waals forces as a result of lower packing density and higher flexibility. The wear tests conducted with the MWNT tip and a Si tip on a gold film, at two normal loads, show less damage of the surface when the MWNT tip is used because of the MWNT acting as a compliant spring, absorbing part of the load. Wear tests conducted with a Si tip on SWNT and MWNT arrays show that the arrays do not wear. The tip wear and the friction force in the SWNT array are lower, because of lower adhesion and higher flexibility of the SWNTs, which causes less opposition to the motion of the tip.     

Sensitivity series and friction surface analysis of non-metallic friction materials

Nine non-metallic friction material formulations contained fibers, fillers and binder without strong abrasives were designed using Golden Section sequence combined with least-square method. Seven ingredients used without strong abrasives were selected based on the combinatorial approach. Wear (w) and coefficient of friction (μ) are expressed as a function of volume fraction of the ingredients selected as and . The formulations were optimized using the sensitivity series obtained from least-square method. An optimized formulation (S-10) was obtained with a total wear loss of 6.69 wt% and an average μ of 0.375. Friction surface of both brake pad and disc was observed using SEM, EDX and XRD. The non-metallic friction materials without abrasives exhibit unique friction performance and phenomena compared with the non-metallic friction materials with abrasives and semi-metallic brake linings. The temperature measured during friction is lower and the oxidation of the cast iron disc is not rigorous.     

Mechanochemical Adhesion and Plasticity in Multifiber Hydrogel Networks

The extracellular matrix (ECM) has force-responsive (i.e., mechanochemical) properties that enable adaptation to mechanical loading through changes in fibrous network structure and interfiber bonding. Imparting such properties into synthetic fibrous materials will allow reinforcement under mechanical load, the potential for material self-adhesion, and the general mimicking of ECM. Multifiber hydrogel networks are developed through the electrospinning of multiple fibrous hydrogel populations, where fibers contain complementary chemical moieties (e.g., aldehyde and hydrazide groups) that form covalent bonds within minutes when brought into contact under mechanical load. These fiber interactions lead to microscale anisotropy, as well as increased material stiffness and plastic deformation. Macroscale structures (e.g., tubes and layered scaffolds) are fabricated from these materials through interfiber bonding and adhesion when placed into contact while maintaining a microscale fibrous architecture. The design principles for engineering plasticity described can be applied to numerous material systems to introduce unique properties, from textiles to biomedical applications.     

Fracture of metal/ceramic interfaces

The present paper examines metal/ceramic interfaces. Energy release rates are calculated with the finite element method for different elastic–plastic material laws of the metal. The local strain field of the metal is measured during a four-point bending test with an optical method and compared with results from the simulations. The aim of the work is to understand the influence of interface strength and material properties on the energy release rate.     

Stress intensity factor analysis of friction sliding at discontinuity interfaces and junctions

A stress intensity factor (SIF) analysis for two-dimensional fractures with frictional contact (crack friction) is presented. This analysis is carried out using the symmetric-Galerkin boundary element method, and a modified quarter-point crack tip element. As in case of non-contact fracture, it is shown that highly accurate SIFs can be obtained, even with the simple Displacement Correlation SIF technique. Moreover, with the modified crack tip element, the mesh on the crack does not need to be excessively refined in order to achieve high accuracy. This meshing advantage is especially important in the context of the nonlinear frictional contact problem, as the computing time for the iterative process strongly depends on the number of elements used. Several numerical examples are presented and the SIF results are compared with available analytical or reference solutions. This research was supported in part by the University of South Alabama Research Council, and by the Applied Mathematical Sciences Research Program of the Office of Mathematical, Information, and Computational Sciences, U.S. Department of Energy under contract DE-AC05-00OR22725 with UT-Battelle, LLC.     

Interaction of dynamic mode-I cracks with inclined interfaces

In this paper, we report on an experimental study of the deflection/penetration behavior of dynamic mode-I cracks propagating at two different crack velocities (slower and faster) toward inclined weak interfaces of three dissimilar angles (α): 30°, 45° and 60°. A simple wedge-loading specimen configuration as proposed by Xu et al. [Xu LR, Huang YY, Rosakis AJ. Dynamic crack deflection and penetration at interface in homogenous materials: experimental studies and model predictions. J Mech Phys Solids 2003;51:461-86], made of brittle Homalite-100, is used. A modified Hopkinson bar setup is used to achieve well-controlled impact loading conditions. Dynamic photoelasticity in conjunction with high-speed photography is used to capture real-time isochromatics associated with deflected/penetrated cracks.     

Adhesion and friction properties of micro/nano-engineered superhydrophobic/hydrophobic surfaces

Hydrophobic micro/nano-engineered surfaces (MNESs) with good adhesion and frictional performances were fabricated by the combination of aluminum-induced crystallization (AIC) of amorphous silicon (a-Si) and octadecyltrichlorosilane (OTS) coating. The AIC of a-Si technique was used to produce silicon micro/nano-textured surfaces, while an OTS self-assembled monolayer was used to lower the surface energies of the textured surfaces. The wetting properties of the MNESs were studied using a video-based contact angle measurement system. The adhesion and friction properties of the MNESs were investigated using a TriboIndenter. This study shows that the adhesion and frictional performances of all MNESs are significantly improved compared to untreated silicon substrate surfaces, and the adhesion and frictional performances of the OTS-modified textured surfaces strongly correlate to their surface wetting property, i.e., the larger the water contact angle, the better the adhesion and frictional performances of the OTS-modified textured surfaces.     

A combined molecular dynamics and Raman spectroscopy approach for designing ice-metal interfaces

Summary Understanding the structure and interatomic interactions of an ice-metal interface plays a fundamental role in the design of deicing coatings. This is demonstrated by a novel approach, combining vibrational results from laser Raman spectroscopy with molecular dynamics simulations to obtain insights into icing on solids which, in turn, lead to design criteria for minimizing adhesion. An atomistic model of ice-copper interaction is constructed based on electronic structure calculations and used to show that reasonable molecular geometry and binding energy at the interface can be obtained. Through molecular dynamics simulations we find that the ice layer adjacent to the copper surface is structurally more disordered than the layers further away, a result which is verified by the Raman spectra of vibrational frequencies. The primary adhesive bond is made by the adsorption of oxygen atoms at the lattice sites of the metal substrate. The information obtained by Raman spectroscopy and molecular dynamics is then exploited to arrive at specific recommendations for designing polymeric deicing coatings and materials.     

Strength statistics of adhesive contact between a fibrillar structure and a rough substrate

Equal distribution of load among fibrils in contact with a substrate is an important characteristic of fibrillar structures used by many small animals and insects for contact and adhesion. This is in contrast with continuum systems where stress concentration dominates interfacial failure. In this work, we study how adhesion strength of a fibrillar system depends on substrate roughness and variability of the fibril structure, which are modelled using probability distributions for fibril length and fibril attachment strength. Monte Carlo simulations are carried out to determine the adhesion strength statistics where fibril length follows normal or uniform distribution and attachment strength has a power-law form. Our results indicate that the strength distribution is Gaussian (normal) for both the uniform and the normal distributions for length. However, the fibrillar structure having normally distributed lengths has higher strength and lower toughness than one having uniformly distributed lengths. Our simulations also show that an increase in the compliance of the fibrils can compensate for both the substrate roughness and the attachment strength variation. We also show that, as the number of fibrils n increases, the load-carrying efficiency of each fibril goes down. For large n, this effect is found to be small. Furthermore, this effect is compensated by the fact that the standard deviation of the adhesive strength decreases as 1/ square root n.     

Classical atomistic simulations of surfaces and heterogeneous interfaces with the charge-optimized many body (COMB) potentials

Interest in atomic scale computational simulations of multi-phase systems has grown as our ability to simulate nanometer-sized systems has become commonplace. The recently developed charge optimized many body potential (COMB) potentials have significantly enhanced the atomic-scale simulation of heterogeneous material systems, including chemical reactions at surfaces and the physical properties of interfaces. The COMB formalism, which merges variable charge electrostatic interactions with a classical analytical potential, has the capacity to adaptively model metallic, covalent, ionic and van der Waals bonding in the same simulation cell and dynamically determine the charges according to the local environment. Presented here is the theoretical background and evolution of the COMB potential family. The parameterization of the potential is described for several metals, ceramics, a semiconductor, and hydrocarbons, with the intent that the final parameter sets are consistent among materials. The utility of this approach is illustrated with several examples that explore the structure, stability, and mechanical and thermal properties of metallic systems and metal-ceramic and semi-conductor oxide interfaces, including surfaces and/or interfaces of copper and cuprite, copper and silica, silicon and silica, silicon and hafnia, and copper and zinc oxide. The potential is also applied to the simulation of atomic scale processes such as early stage oxidation of copper surfaces, tensile test of polycrystalline zirconium, and hyper-thermal deposition of ethyl radicals on selected copper surfaces.     

A general strategy for self-assembly of nanosized building blocks on liquid/liquid interfaces

A family of water/oil interfaces is introduced to provide effective platforms for rapid fabrication of large-area self-assembled nanofilms composed of various nanosized building blocks, including nanoparticles (NPs), nanocubes (NC), nanowires (NWs), and nanosheets, at room temperature. As a general interfacial assembly method, NWs and NPs are co-assembled at the liquid/liquid interface. The as-prepared co-assembled Ag NW and Ag NC films show high surface-enhanced Raman spectroscopy (SERS) intensity, the SERS performance being strongly dependent on the number ratio of the two kinds of nanosized building blocks. The results demonstrate that this interfacial system provides a general method for the assembly of various nanosized building blocks with different shapes and dimensionalities, and thus paves an alternative pathway for further applications of macroscopic assemblies with different functionalities.     

Mechanically driven accumulation of microscale material at coupled solid–fluid interfaces in biological channels

The accumulation of microscale materials at solid–fluid interfaces in biological channels is often the initial stage of certain growth processes, which are present in some forms of atherosclerosis. The objective of this work is to develop a relatively simple model for such accumulation, which researchers can use to qualitatively guide their analyses. Specifically, the approach is to construct rate equations for the accumulation at the solid–fluid interface as a function of the intensity of the shear stress. The accumulation of material subsequently reduces the cross-sectional area of the channel until the fluid-induced shear stress at the solid–fluid interface reaches a critical value, which terminates the accumulation rate. Characteristics of the model are explored analytically and numerically.     

含环氧-SiC复合微/纳米纤维的层合板制备及其力学性能

应用同轴静电纺丝技术制备环氧包覆纳米SiC 复合微/ 纳米纤维, 将该复合微/ 纳米纤维收集成无纺布薄膜引入层合板层间界面并固化成型, 研究其对层合板力学性能的影响。采用扫描电子显微镜(SEM) 和透射电子显微镜( TEM) 分析了微/ 纳米纤维的形貌和结构, 并测试了微/ 纳米纤维薄膜的拉伸性能。应用三点弯曲、短梁剪切和简支式冲击实验测定了层合板的弯曲性能、层间剪切强度和冲击韧性。结果表明, 一定厚度及一定SiC 含量的微/ 纳米纤维无纺布薄膜对层合板的力学性能无显著影响。      

摩擦材料中树脂对其性能的影响

阐述了硼桐油双改性酚醛树脂的改性原理,对以此为基体的编织型摩擦材料的磨损率和摩擦系数进行了测试和分析,并与其他几种酚醛树脂基摩擦材料进行了对比。结果表明,对酚醛树脂进行硼桐油双改性后应用于摩擦材料的效果理想。