The Effect of Interfacial Conformity on the Contact, Adhesion, and Sliding Friction of Surfaces with Small Roughness PDF Download

Author: Herbert Mark Stanley
Publisher:
ISBN:
Category :
Languages : en
Pages : 310

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Author: Donald H. Buckley
Publisher: Elsevier
ISBN: 0080875696
Category : Science
Languages : en
Pages : 643

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Surface effects in adhesion, friction, wear, and lubrication

Author: Antoine Sanner
Publisher:
ISBN:
Category :
Languages : en
Pages : 0

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Abstract: At atomic scales, all molecules attract each other, but macroscopic objects usually do not stick.The explanation for this apparent paradox is that most surfaces are rough, so that elastically stiff objects only touch on the top of their asperities. Geckos and insects have compliant fibrillar structures or soft pads at the tip of their feet that conform to surface roughness, sustaining enough adhesion to climb vertical walls. Understanding the role of surface roughness in adhesion is a challenge because surfaces exhibit roughness down to the atomic scale. In this thesis, my collaborators and I investigate the effect of surface roughness on adhesion in both stiff and compliant contact systems. I model adhesion theoretically, and I help experimentalists analyze surface topography over multiple scales. The combination of my new models and of the comprehensive surface topography characterization by Abhijeet Gujrati (University of Pittsburgh), allows us to unravel the role of surface roughness in adhesion experiments. Stiff materials do not stick because roughness prevents most of the surfaces to come into the range of molecular attraction. A recent theory quantifies this effect based on an approximate expression for the distribution of interfacial gaps near the contact edge. Joe Monti (Johns Hopkins University) and I benchmark this expression against gap distributions extracted from finely resolved numerical simulations. The theory is valid provided that adhesive stresses are weak and act over a range shorter than a geometrical parameter determined by small-scale roughness. Elastically soft (jelly-like) objects stick because the elastic penalty to deform into intimate contact is small compared to the gain in surface energy. However, theories based on this simple thermodynamic argument cannot explain the fact that in experiments, the force measured during retraction is often much higher than during indentation. This adhesion hysteresis can be caused by material specific irreversibility or elastic instabilities triggered by surface roughness. The role of these instabilities in adhesion hysteresis remains poorly understood because existing numerical and theoretical models cannot account for realistic roughness in soft contacts. I introduce an efficient crack-perturbation model for the contact of rough spheres, enabling large scale simulations with realistic surface roughness. By clarifying the link between adhesion hysteresis and classic pinning problems (for example fracture of heterogeneous materials and wetting angle hysteresis), this model allows me to derive a simple theoretical model linking adhesion hysteresis to surface roughness. In combination with the characterization of surface roughness over multiple scales, my models shed light on the role of elastic instabilities in adhesion experiments. Surfaces are rough from the macroscopic scale down to the atomic scale, and the lack of comprehensive roughness characterization is the major obstacle towards bringing theory and experiments together. Abhijeet Gujrati and collaborators measured the roughness of four diamond coatings over eight decades of length scales, enabling the application of adhesion theories on experiments performed with these samples. Besides the experimental challenge of determining roughness down to the atomic scale, an additional obstacle to the documentation of roughness is the technical complexity of established multiscale roughness measures such as the power spectral density. My collaborators and I address this problem by introducing the scale-dependent roughness parameters (SDRPs), a new analysis framework that is easy to interpret and to implement. This new analysis, together with several established techniques, is available to use through our web-service contact.engineering. We thereby encourage the community to measure, analyze and publish roughness over multiple length scales. The SDRP analysis computes the fluctuations of slopes and curvatures as a function of the lateral length scale. Slopes and curvatures are important ingredients for rough contact theories, but it remains unclear at which scales they matter. Luke Thimons (University of Pittsburgh) and I show that in macro-scale contacts between ruby spheres and diamond coatings, the roughness that critically affects adhesion is between lateral length scales of 43~nm to 1.8~μm. The large-scale cutoff is related to the finite radius of the spherical indenter, while the unimportance of small scales is due to plastic deformations and the long range of the adhesive interaction (5~nm). To determine the critical range of length scales, as well as the parameters of the adhesive interaction, we analyzed the experimental pull-off forces by combining surface topography characterization and numerical simulations. Adhesion is critical in applications such as microelectromechanical systems (MEMS), soft robotics and skin adhesives. Our insights provide guidance for practitioners which scales of roughness to control in order to tune adhesion, and our framework for surface topography characterization will allow a better overall understanding of surface topography across the community

Author: Ying Bai
Publisher:
ISBN: 9781303658921
Category :
Languages : en
Pages : 185

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Enhanced and selective adhesion, and controlled friction between contact surfaces are highly desirable mechanical properties for high-level functional materials. There are many instances in nature where such properties have been obtained by design of near-surface architecture. Inspired by many highly functional biological systems, we have explored bio-mimetic materials with different surface patterning, with the goal of designing surfaces that have unique combinations of contact mechanical properties. In the studies presented here, we show how: (a) highly selective adhesion can be achieved by complementarity of patterned charge and shape, and (b) how friction can be modulated by spatial variation in stiffness, and how structured surfaces interact with surface roughness. We consider how adhesion selectivity can be accomplished by complementarity of shape and inter-surface forces. We have studied an example each of charge and shape complementarity for selective adhesion between extended surfaces. First, we studied theoretically how surfaces patterned with stripes of charge interact with each other, and exhibit strong selectivity on rigid surfaces. However, deformability of the surfaces plays a crucial role in modulating adhesion by accommodating mismatches. To achieve shape complementarity, we designed and fabricated patterned elastomeric surfaces with lines of channels and complementary ridges with dimensions at the micrometer scale. We show that such surfaces have highly enhanced effective adhesion for shape complementary pairs and low adhesion between surfaces with a shape mismatch. We find that the pillar/channel combinations form defects to accommodate interfacial misalignment. These defects are interfacial dislocations. Adhesion between complementary surfaces is enhanced by crack trapping and friction, and attenuated due to the energy released by dislocation structures. In addition to enhanced adhesion, we studied the deliberate control of friction through near-surface micro-structures. Friction measurements on elastomeric surfaces patterned with periodic variation in stiffness show that it undergoes an "auto-roughening" transition under shear and this process can strongly attenuate overall sliding friction. Friction reduction is due to reduction of real contact area, as the initially full contact breaks up into partial contact at the interface. Finite element analysis demonstrates how auto-roughening depends on the modulus mismatch, frictional stress and normal displacement. A surface with random roughness is used to study sliding friction against micro-channel structures under fixed normal force. In contrast to a smooth surface, against which structured surfaces all have highly reduced sliding friction, the roughened surface can exhibit significantly larger frictional force on a structured surface. The enhancement of sliding friction is governed by channel depth, spacing and applied normal force.

Author: Siddhesh Narayan Dalvi
Publisher:
ISBN:
Category : Polydimethylsiloxane
Languages : en
Pages : 143

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Understanding the mechanism of adhesion and friction in soft materials is critical to the fields of transportation (tires, wiper blades, seals etc.), prosthetics and soft robotics. Most surfaces are inherently rough and the interfacial area between two contacting bodies depends largely on the material properties and surface topography of the contacting bodies. Johnson, Kendall and Roberts (JKR) derived an equilibrium energy balance for the behavior of smooth elastic spherical bodies in adhesive contact that predicts a thermodynamic work of adhesion for two surfaces in contact. The JKR equation gives a reversible work of adhesion value during approach and retraction. However, viscoelastic dissipation, surface roughness and chemical bonding result in different work of adhesion values for approach and retraction. This discrepancy is termed adhesion hysteresis. Roughness is undermined as a cause of hysteresis in adhesion studies. Recently, a continuum mechanics model has been developed that predicts the work of adhesion on rough surfaces with known roughness in the form of power spectral density (PSD) function. To test the above mentioned theoretical model, we have conducted JKR experiments between highly cross-linked smooth polydimethylsiloxane (PDMS) of four different elastic moduli and diamond surfaces of four different crystal sizes and roughness.The rough diamond surfaces are characterized for topography using stylus profilometry, atomic force microscopy and in-situ transmission electron microscopy combined to give a comprehensive PSD. Results suggest that the observed work of adhesion during approach is equivalent to energy required to stretch the PDMS network at the surface and in the bulk to form the real rough contact area. However, in retraction work of adhesion is found to be proportional to the ratio of excess energy spent in the loading-unloading cycle and the true contact area obtained from topography indicating conformal contact matching fracture mechanics behavior. Thus, the study resolves adhesion hysteresis discrepancy on rough surfaces.It is known that adhesion hysteresis increases interfacial friction on rough surfaces. However, an experimentally proven quantitative model is still missing. Previous studies on smooth surfaces have shown that shear stress increases with velocity initially, reaching a maximum and then either plateaus out or decreases depending upon the modulus of the sliding elastomer. We have performed shear measurements with velocities ranging from nm/sec to cm/sec between PDMS elastomers and diamond surfaces. Data suggests higher shear stresses at lower velocities for rough surfaces and thus a shift for the peak previously observed on smooth surfaces. Additionally, there are states such as steady-state sliding, stick-slip and detachment waves with increasing stress in the same order. These states are found to occur at a critical stress and their onset is linearly proportional to the elastic modulus of the sliding rubber. The stress predictions using existing theories do not decouple adhesion and deformation energy losses during friction observed ex- perimentally on rough surfaces and further investigation is required in order to obtain a better friction model.

Author: Waldemar Karwowski
Publisher: CRC Press
ISBN: 9780415251617
Category : Technology & Engineering
Languages : en
Pages : 712

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Author: Informa Healthcare
Publisher: CRC Press
ISBN: 1482298538
Category : Technology & Engineering
Languages : en
Pages : 1980

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The first encyclopedia in the field, the International Encyclopedia of Ergonomics and Human Factors provides a comprehensive and authoritative compendium of current knowledge on ergonomics and human factors. It gives specific information on concepts and tools unique to ergonomics. About 500 entries, published in three volumes and on CD-ROM, are pre

Author: Matthew Arnold O'Hara
Publisher:
ISBN:
Category : Gas-lubricated bearings
Languages : en
Pages : 178

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Author: J.T. Davies
Publisher: Academic Press
ISBN: 0323148344
Category : Science
Languages : en
Pages : 507

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Interfacial Phenomena examines the fundamental properties of various liquid interfaces. This book discusses the physics of surfaces; electrostatic and electrokinetic phenomena; and adsorption at liquid interfaces. The properties of monolayers; reactions at liquid surfaces; diffusion through interfaces; and disperse systems and adhesion are also deliberated. Other topics include the vapor pressures over curved surfaces; electrical capacity of the double layer; applications of electrophoresis; and thermodynamics of adsorption and desorption. The experimental methods of spreading films at the oil-water interface; penetration into monolayers; experiments on dynamic systems; and spontaneous emulsification are likewise covered in this text. This book is beneficial to chemical engineers and students concerned with interfacial phenomena.

Author: Bi-min Zhang Newby
Publisher:
ISBN:
Category : Adhesion
Languages : en
Pages : 526

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Conventionally, the energy required to fracture an interface is predicted to be directly proportional to the thermodynamic work of adhesion. It has been observed, however, that on some low energy surfaces, the fracture energy of an adhesive deviates significantly from this rule. In viscoelastic systems, it is well known that most of the energy is dissipated by the viscous bulk materials near the crack tip region. What is, perhaps, not so widely known is that for a crack propagating at an asymmetric interface, a substantial amount of energy can be dissipated through a frictional mechanism near the crack tip region. This energy dissipation due to friction at and near the crack tip is being proposed to account for the discrepancies observed with adhesion on low energy surfaces. The objective of this research is to investigate the contribution of interfacial friction and associated slip processes to the total fracture energy.