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K Komvopoulos - One of the best experts on this subject based on the ideXlab platform.
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adhesive contact of an elastic semi infinite solid with a rigid rough Surface strength of adhesion and contact instabilities
International Journal of Solids and Structures, 2014Co-Authors: Z Song, K KomvopoulosAbstract:Abstract 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.
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delamination of an elastic film from an elastic plastic substrate during adhesive contact loading and unloading
International Journal of Solids and Structures, 2013Co-Authors: Z Song, K KomvopoulosAbstract:Abstract Adhesive contact between a rigid sphere and an elastic film on an elastic–perfectly plastic substrate was examined in the context of finite element simulation results. Surface adhesion was modeled by nonlinear springs obeying a force-displacement relationship governed by the Lennard–Jones potential. A bilinear cohesive zone law with prescribed cohesive strength and work of adhesion was used to simulate crack initiation and growth at the film/substrate interface. It is shown that the unloading response consists of five sequential stages: elastic recovery, interface damage (crack) initiation, damage evolution (delamination), film elastic bending, and abrupt Surface Separation (jump-out), with plastic deformation in the substrate occurring only during damage initiation. Substrate plasticity produces partial closure of the cohesive zone upon full unloading (jump-out), residual tensile stresses at the front of the crack tip, and irreversible downward bending of the elastic film. Finite element simulations illustrate the effects of minimum Surface Separation (i.e., maximum compressive Surface force), work of adhesion and cohesive strength of the film/substrate interface, substrate yield strength, and initial crack size on the evolution of the Surface force, residual deflection of the elastic film, film-substrate Separation (debonding), crack-tip opening displacement, and contact instabilities (jump-in and jump-out) during a full load–unload cycle. The results of this study provide insight into the interdependence of contact instabilities and interfacial damage (cracking) encountered in layered media during adhesive contact loading and unloading.
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adhesive contact of elastic plastic layered media effective tabor parameter and mode of Surface Separation
Journal of Applied Mechanics, 2013Co-Authors: Z Song, K KomvopoulosAbstract:Adhesive contact of a rigid sphere with a layered medium consisting of a stiff elastic layer perfectly bonded to an elastic-plastic substrate is examined in the context of finite element simulations. Surface adhesion is modeled by nonlinear spring elements obeying a force-displacement relation governed by the Lennard–Jones potential. Adhesive contact is interpreted in terms of the layer thickness, effective Tabor parameter (a function of the layer thickness and Tabor parameters corresponding to layer and substrate material properties), maximum Surface Separation, layer-to-substrate elastic modulus ratio, and plasticity parameter (a characteristic adhesive stress expressed as the ratio of the work of adhesion to the Surface equilibrium distance, divided by the yield strength of the substrate). It is shown that Surface Separation (detachment) during unloading is not encountered at the instant of maximum adhesion (pull-off) force, but as the layered medium is stretched by the rigid sphere, when abrupt Surface Separation (jump-out) occurs under a smaller force (Surface Separation force). Ductile- and brittle-like modes of Surface detachment, characterized by the formation of a neck between the rigid sphere and the layered medium and a residual impression on the unloaded layered medium, respectively, are interpreted for a wide range of plasticity parameter and maximum Surface Separation. Numerical results illustrate the effects of layer thickness, bulk and Surface material properties, and maximum Surface Separation (interaction distance) on the pull-off and Surface Separation forces, jump-in and jump-out contact instabilities, and evolution of substrate plasticity during loading and unloading. Simulations of cyclic adhesive contact demonstrate that incremental plasticity (ratcheting) in the substrate is the most likely steady-state deformation mechanism under repetitive adhesive contact conditions.
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effects of electrical and thermal phenomena on the evolution of adhesion at contact interfaces of electrostatically activated Surface microstructures
Applied Physics Letters, 2007Co-Authors: Shannon J Timpe, K KomvopoulosAbstract:A contact-mode microstructure fabricated by Surface micromachining was used to study the development of adhesion at sidewall contact Surfaces during electrical actuation. Temporary and permanent changes in the adhesion force were monitored for different voltages applied across the contact interface. Relatively low current flow across the interface yielded a significant increase in the adhesion force. A portion of the increase was attributed to thermal heating of the contacting asperities. Current flow through asperity contacts lead to the accumulation of trapped charges in the insulating oxide layer, resulting in electrostatic attraction that was maintained after Surface Separation and with grounded Surfaces. High current flow across asperity contacts due to dielectric breakdown of the native oxide layer at a critical voltage resulted in interfacial bonding that caused permanent adhesion of the sidewall Surfaces.
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contact analysis of elastic plastic fractal Surfaces
Journal of Applied Physics, 1998Co-Authors: W Yan, K KomvopoulosAbstract:Rough Surfaces are characterized by fractal geometry using a modified two-variable Weierstrass–Mandelbrot function. The developed algorithm yields three-dimensional fractal Surface topographies representative of engineering rough Surfaces. This Surface model is incorporated into an elastic-plastic contact mechanics analysis of two approaching rough Surfaces. Closed form solutions for the elastic and plastic components of the total normal force and real contact area are derived in terms of fractal parameters, material properties, and mean Surface Separation distance. The effects of Surface topography parameters and material properties on the total deformation force are investigated by comparing results from two- and three-dimensional contact analyses and elastic and elastic-perfectly plastic material behaviors. For normal contact of elastic-perfectly plastic silica Surfaces and range of Surface interference examined, the interfacial force is predominantly elastic and the real contact area is approximately ...
Jacob Klein - One of the best experts on this subject based on the ideXlab platform.
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Trapped Aqueous Films Lubricate Highly Hydrophobic Surfaces
2018Co-Authors: Irit Rosenhek-goldian, Nir Kampf, Jacob KleinAbstract:Friction at hydrophobic Surfaces in aqueous media is ubiquitous (e.g., prosthetic implants, contact lenses, microfluidic devices, biological tissue) but is not well understood. Here, we measure directly, using a Surface force balance, both normal stresses and sliding friction in an aqueous environment between a hydrophilic Surface (single-crystal mica) and the stable, molecularly smooth, highly hydrophobic Surface of a spin-cast fluoropolymer film. Normal force versus Surface Separation profiles indicate a high negative charge density at the water-immersed fluoropolymer Surface, consistent with previous studies. Sliding of the compressed Surfaces under water or in physiological-level salt solution (0.1 M NaCl) reveals strikingly low boundary friction (friction coefficient μ ≈ 0.003–0.009) up to contact pressures of at least 50 atm. This is attributed largely to hydrated counterions (protons and Na+ ions) trapped in thin interfacial films between the compressed, sliding Surfaces. Our results reveal how frictional dissipation may occur at hydrophobic Surfaces in water and how modification of such Surfaces may suppress this dissipation
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normal and frictional interactions of purified human statherin adsorbed on molecularly smooth solid substrata
Biofouling, 2011Co-Authors: Neale M Harvey, Guy Carpenter, Gordon Proctor, Jacob KleinAbstract:Human salivary statherin was purified from parotid saliva and adsorbed to bare hydrophilic (HP) mica and STAI-coated hydrophobic (HB) mica in a series of Surface Force Balance experiments that measured the normal (F n) and friction forces (F s*) between statherin-coated mica substrata. Readings were taken both in the presence of statherin solution (HP and HB mica) and after rinsing (HP mica). F n measurements showed, for both substrata, monotonic steric repulsion that set on at a Surface Separation D ∼ 20 nm, indicating an adsorbed layer whose unperturbed thickness was ca 10 nm. An additional longer-ranged repulsion, probably of electrostatic double-layer origin, was observed for rinsed Surfaces under pure water. Under applied pressures of ∼ 1 MPa, each Surface layer was compressed to a thickness of ca 2 nm on both types of substratum, comparable with earlier estimates of the size of the statherin molecule. Friction measurements, in contrast with F n observations, were markedly different on the two differ...
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layering and shear properties of an ionic liquid 1 ethyl 3 methylimidazolium ethylsulfate confined to nano films between mica Surfaces
Physical Chemistry Chemical Physics, 2010Co-Authors: Susan Perkin, Jacob Klein, Tim AlbrechtAbstract:We report high-resolution measurements of the forces between two atomically smooth solid Surfaces across a film of 1-ethyl-3-methylimidazolium ethylsulfate ionic liquid, for film thickness down to a single ion diameter. For films thinner than ∼2 nm oscillatory structural forces are observed as the Surface Separation decreases and pairs of ion layers are squeezed out of the film. Strikingly, measurements of the shear stress of the ionic liquid film reveal low friction coefficients which are 1–2 orders of magnitude smaller than for analogous films of non-polar molecular liquids, including standard hydrocarbon lubricants, up to ca. 1 MPa pressure. We attribute this to the geometric and charge characteristics of the ionic liquid: the irregular shapes of the ions lead to a low shear stress, while the strong coulombic interactions between the ions and the charged confining Surfaces lead to a robust film which is maintained between the shearing Surfaces when pressure is applied across the film.
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shear and frictional interactions between adsorbed polymer layers in a good solvent
Journal of Physical Chemistry B, 2001Co-Authors: Uri Raviv, And Rafael Tadmor, Jacob KleinAbstract:The forces between layers of poly(ethylene oxide) (PEO), of molecular weights M = 37 × 103 (PEO37) and M = 112 × 103 (PEO112) adsorbed onto smooth, curved solid (mica) Surfaces across the good solvent toluene have been determined using a Surface force balance (SFB). The SFB used is capable of measuring both normal interactions Fn(D) as a function of Surface Separation D and, with extreme sensitivity, shear or frictional forces Fs(D,vs) between them as they slide past each other at velocity vs. The Fn(D) profiles are closely similar to those measured in earlier studies between adsorbed PEO layers. The shear or frictional forces between the sliding PEO-bearing Surfaces are very low up to moderate compressions of the adsorbed layers (local pressures up to ca. 105 N m-2), corresponding to effective friction coefficients μeff = (Fs/Fn) of order 0.003 or less. This is attributed to the fluid interfacial layer between the adsorbed layers resulting from their weak mutual interpenetration. At higher loads Fs incre...
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confinement induced phase transitions in simple liquids
Science, 1995Co-Authors: Jacob Klein, Eugenia KumachevaAbstract:The liquid-to-solid transition of a simple model liquid confined between two Surfaces was studied as a function of Surface Separation. From large Surface Separations (more than 1000 angstroms) down to a Separation corresponding to seven molecular layers, the confined films displayed a liquid-like shear viscosity. When the Surface Separation was further decreased by a single molecular spacing, the films underwent an abrupt, reversible transition to a solid. At the transition, the rigidity of the confined films (quantified in terms of an "effective viscosity") increased reversibly by at least seven orders of magnitude.
Ricardo Garcia - One of the best experts on this subject based on the ideXlab platform.
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force reconstruction from tapping mode force microscopy experiments
Nanotechnology, 2015Co-Authors: Amir Farokh Payam, Daniel Martinjimenez, Ricardo GarciaAbstract:Fast, accurate, and robust nanomechanical measurements are intensely studied in materials science, applied physics, and molecular biology. Amplitude modulation force microscopy (tapping mode) is the most established nanoscale characterization technique of Surfaces for air and liquid environments. However, its quantitative capabilities lag behind its high spatial resolution and robustness. We develop a general method to transform the observables into quantitative force measurements. The force reconstruction algorithm has been deduced on the assumption that the observables (amplitude and phase shift) are slowly varying functions of the tip–Surface Separation. The accuracy and applicability of the method is validated by numerical simulations and experiments. The method is valid for liquid and air environments, small and large free amplitudes, compliant and rigid materials, and conservative and non-conservative forces.
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measuring phase shifts and energy dissipation with amplitude modulation atomic force microscopy
Nanotechnology, 2006Co-Authors: Nicolas F Martinez, Ricardo GarciaAbstract:By recording the phase angle difference between the excitation force and the tip response in amplitude modulation AFM it is possible to image compositional variations in heterogeneous samples. In this contribution we address some of the experimental issues relevant to perform phase contrast imaging measurements. Specifically, we study the dependence of the phase shift on the tip?Surface Separation, interaction regime, cantilever parameters, free amplitude and tip?Surface dissipative processes. We show that phase shift measurements can be converted into energy dissipation values. Energy dissipation curves show a maximum (~10?eV/cycle) with the amplitude ratio. Furthermore, energy dissipation maps provide a robust method to image material properties because they do not depend directly on the tip?Surface interaction regime. Compositional contrast images are illustrated by imaging conjugated molecular islands deposited on silicon Surfaces.
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unifying theory of tapping mode atomic force microscopy
Physical Review B, 2002Co-Authors: Alvaro San Paulo, Ricardo GarciaAbstract:We propose a general method for describing tapping-mode atomic-force microscopy. The combined participation of attractive and repulsive interactions determines the multivalued nature of the resonance curve. This, in turn, implies the coexistence of two different stable oscillations for some excitation frequencies. The coexistence of two stable oscillations depends on the driving force and tip-Surface Separation. Increasing the driving force inhibits the low-amplitude oscillation state. Because resolution depends on the oscillation state, we propose that the absence of the low amplitude solution is responsible for the inconsistencies observed in high-resolution imaging of biomolecules.
L P Demejo - One of the best experts on this subject based on the ideXlab platform.
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contact electrification and the interaction force between a micrometer size polystyrene sphere and a graphite Surface
Langmuir, 1997Co-Authors: B Gady, R Reifenberger, Donald Saul Rimai, L P DemejoAbstract:Two independent techniques are used to measure the interaction force between a single 3 μm radius polystyrene sphere and an atomically flat, highly oriented pyrolytic graphite substrate. The variation of the interaction force with the Surface-to-Surface Separation between the sphere and plane is determined using both a static and a dynamic atomic force technique. The measured interaction force is dominated at long range by an electrostatic force arising from localized charges triboelectrically produced on the sphere when it makes contact with the substrate. For small sphere−substrate Separations, evidence for a van der Waals force is observed. The data provide consistent estimates for both the Hamaker coefficient and the triboelectrically produced charge which can be measured to an accuracy of ±10 electrons.
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identification of electrostatic and van der waals interaction forces between a micrometer size sphere and a flat substrate
Physical Review B, 1996Co-Authors: B Gady, D Schleef, R Reifenberger, Donald Saul Rimai, L P DemejoAbstract:The interaction force gradient between a micron-size polystyrene sphere and an atomically flat highly oriented pyrolytic graphite substrate has been analyzed as a function of Surface-to-Surface Separation distance z0 using an oscillating cantilever technique. The interaction force gradient was found to have two contributions. For z0>30 nm, an electrostatic force due to charges trapped on the polystyrene sphere dominates. For z0<30 nm, a van der Waals interaction, characteristic of a sphere near a flat plane, is observed. Fits to the data are in good agreement with theoretical expectations and allow estimates of the Surface charge density triboelectrically produced on the sphere’s Surface.
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identification of electrostatic and van der waals interaction forces between a micrometer size sphere and a flat substrate
Physical Review B, 1996Co-Authors: B Gady, D Schleef, R Reifenberger, Donald Saul Rimai, L P DemejoAbstract:The interaction force gradient between a micron-size polystyrene sphere and an atomically flat highly oriented pyrolytic graphite substrate has been analyzed as a function of Surface-to-Surface Separation distance ${\mathit{z}}_{0}$ using an oscillating cantilever technique. The interaction force gradient was found to have two contributions. For ${\mathit{z}}_{0}$\ensuremath{\ge}30 nm, an electrostatic force due to charges trapped on the polystyrene sphere dominates. For ${\mathit{z}}_{0}$\ensuremath{\le}30 nm, a van der Waals interaction, characteristic of a sphere near a flat plane, is observed. Fits to the data are in good agreement with theoretical expectations and allow estimates of the Surface charge density triboelectrically produced on the sphere's Surface. \textcopyright{} 1996 The American Physical Society.
Z Song - One of the best experts on this subject based on the ideXlab platform.
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adhesive contact of an elastic semi infinite solid with a rigid rough Surface strength of adhesion and contact instabilities
International Journal of Solids and Structures, 2014Co-Authors: Z Song, K KomvopoulosAbstract:Abstract 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.
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delamination of an elastic film from an elastic plastic substrate during adhesive contact loading and unloading
International Journal of Solids and Structures, 2013Co-Authors: Z Song, K KomvopoulosAbstract:Abstract Adhesive contact between a rigid sphere and an elastic film on an elastic–perfectly plastic substrate was examined in the context of finite element simulation results. Surface adhesion was modeled by nonlinear springs obeying a force-displacement relationship governed by the Lennard–Jones potential. A bilinear cohesive zone law with prescribed cohesive strength and work of adhesion was used to simulate crack initiation and growth at the film/substrate interface. It is shown that the unloading response consists of five sequential stages: elastic recovery, interface damage (crack) initiation, damage evolution (delamination), film elastic bending, and abrupt Surface Separation (jump-out), with plastic deformation in the substrate occurring only during damage initiation. Substrate plasticity produces partial closure of the cohesive zone upon full unloading (jump-out), residual tensile stresses at the front of the crack tip, and irreversible downward bending of the elastic film. Finite element simulations illustrate the effects of minimum Surface Separation (i.e., maximum compressive Surface force), work of adhesion and cohesive strength of the film/substrate interface, substrate yield strength, and initial crack size on the evolution of the Surface force, residual deflection of the elastic film, film-substrate Separation (debonding), crack-tip opening displacement, and contact instabilities (jump-in and jump-out) during a full load–unload cycle. The results of this study provide insight into the interdependence of contact instabilities and interfacial damage (cracking) encountered in layered media during adhesive contact loading and unloading.
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adhesive contact of elastic plastic layered media effective tabor parameter and mode of Surface Separation
Journal of Applied Mechanics, 2013Co-Authors: Z Song, K KomvopoulosAbstract:Adhesive contact of a rigid sphere with a layered medium consisting of a stiff elastic layer perfectly bonded to an elastic-plastic substrate is examined in the context of finite element simulations. Surface adhesion is modeled by nonlinear spring elements obeying a force-displacement relation governed by the Lennard–Jones potential. Adhesive contact is interpreted in terms of the layer thickness, effective Tabor parameter (a function of the layer thickness and Tabor parameters corresponding to layer and substrate material properties), maximum Surface Separation, layer-to-substrate elastic modulus ratio, and plasticity parameter (a characteristic adhesive stress expressed as the ratio of the work of adhesion to the Surface equilibrium distance, divided by the yield strength of the substrate). It is shown that Surface Separation (detachment) during unloading is not encountered at the instant of maximum adhesion (pull-off) force, but as the layered medium is stretched by the rigid sphere, when abrupt Surface Separation (jump-out) occurs under a smaller force (Surface Separation force). Ductile- and brittle-like modes of Surface detachment, characterized by the formation of a neck between the rigid sphere and the layered medium and a residual impression on the unloaded layered medium, respectively, are interpreted for a wide range of plasticity parameter and maximum Surface Separation. Numerical results illustrate the effects of layer thickness, bulk and Surface material properties, and maximum Surface Separation (interaction distance) on the pull-off and Surface Separation forces, jump-in and jump-out contact instabilities, and evolution of substrate plasticity during loading and unloading. Simulations of cyclic adhesive contact demonstrate that incremental plasticity (ratcheting) in the substrate is the most likely steady-state deformation mechanism under repetitive adhesive contact conditions.
