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[Display omitted] The adhesion forces of particles in the submicron range play a decisive role in many particle processes such as agglomeration. These forces are influenced by many factors such as particle size, surface roughness, and contact area. The quantification of these influences should be possible by linking adhesion forces, measured with colloidal probe atomic force microscopy (cp-AFM), and surface energy, measured with the capillary rise method, using different adhesion force models. Silica-silica (SiO 2 -SiO 2), polystyrene-polystyrene (PS-PS) and mixed adhesive force contacts of micrometer-sized particles were measured by cp-AFM. The surface energy was determined by the capillary rise method using the Owens-Wendt-Rabel-Kaelble (OWRK) method. Various adhesive force models were used to link the experimentally measured forces. In the adhesive force models, the particle size, the distance between the particles, and the roughness were varied. Adhesion forces measured with the AFM can be linked to the OWRK method via adhesion force models such as the particle–particle Van-der-Waals (VDW) model, Rumpf's roughness model, and a proprietary model for multiple particle–particle interactions with the surface energy. The main influencing factors are the substrate and particle roughness as well as their plastic or elastic behavior, which influences the contact area of the adhesive contact. [ABSTRACT FROM AUTHOR]

In soft lithography roof collapse is frequently observed on the stamp surface consisting of rectangular micro-grooves which yields unwanted contact between the sagged surface and the substrate. Deep understanding of the adhesive contact behavior is a key to design of high-performance collapse-resistant stamp. Both theoretical and numerical studies are presented for a single rectangular micro-groove in the middle of the stamp surface under applied pressure. The JKR-type adhesion solution is established by the principle of superposition and equivalent energy release rate, with a series of closed-form expressions derived which are consistent with previous studies. Finite element analysis (FEA) is performed based on virtual crack closure technique (VCCT) to investigate the roof collapse mechanism with interfacial adhesion and applied pressure considered. Comparison of theoretical, numerical, and experimental results demonstrates that both size effect and constitutive nonlinearity of the stamp on the self-collapse contact width are not obvious for shallow grooves but become prominent for deep grooves, so that the obtained analytical solution is limited to shallow grooves due to its assumptions of half-plane and linear elasticity adopted for the stamp. For deep grooves, the present FEA method shows potential to capture more accurate results. [ABSTRACT FROM AUTHOR]

Nano-mechanical properties of silicone polymer-based nanocomposites loaded with multi-walled carbon nanotube (MWCNT) as nanofillers are probed here using the atomic force microscopy (AFM) technique. Further, the effects of MWCNT concentration and X-ray exposure on the nano-mechanical and topographic properties of the polymer nanocomposites are systematically studied. The root mean square surface roughness is found to increase by ∼123% when the neat polymer matrix is loaded with 2.5 wt.% of MWCNT, whereas, for a fixed MWCNT concentration, the surface roughness is found to increase by ∼171% after X-ray irradiation. This is attributed to the X-ray induced polymer degradation, polymer pull-out and poor filler-matrix interaction leading to the aggregation of MWCNT. Adhesion force and adhesion energy are estimated from the AFM-based force-displacement curves. Both quantities are found to increase with MWCNT concentration. After an exposure to diagnostic X-rays (30–70 keV), the adhesion force is found to increase by ∼4 times, which is attributed to the increased tip-surface contact area due to the higher surface roughness of the X-ray exposed samples. Further, the variation of adhesion energy with filler concentration is found to be in agreement with the theoretical values obtained from the Johnson, Kendall and Roberts (JKR) model. The elastic modulus is found to increase with the filler concentration due to an increase in adhesion force. The obtained results are beneficial for the optimal design of nanofiller loaded polymer nanocomposites for various applications, including the fabrication of lead-free radio-opaque nanocomposites. [ABSTRACT FROM AUTHOR]

This study examines the effect of cohesive particle properties and rotary drum operating conditions on heat transfer. Simulations were performed using the Discrete Element Method and Johnson–Kendall–Roberts contact model. Key variables analyzed include material properties (surface energy, particle size, thermal conductivity), fill level, and rotational speed. Angle of repose (AOR) tests were simulated for varying surface energies to establish a correlation between cohesion and bulk properties. Results showed that the AOR was proportional to the particle Bond number and maintaining a constant Bond number across different particle sizes produced similar heating patterns. This enables practical mapping of properties for future modeling and experimentation. Cohesion between particles was found to linearly decrease the rate of heat transfer, whereas adhesion between particles and the drum walls linearly increased it, leading to temperature non-uniformity among particles. Additionally, baffles enhanced heat transfer under cohesive conditions, although their effectiveness decreased with increased adhesion. [Display omitted] • Cohesion increases heating time; adhesion to the drum reduces it. • Increasing rotation rate reduces thermal time, especially for high adhesion. • The impact of surface energy on heat transfer varies with particle size. • Baffles improve heat transfer in cohesive scenarios, less effective with adhesion. • Bond number correlates with the angle of repose, helping estimate heating patterns. [ABSTRACT FROM AUTHOR]

There is a widespread belief that in the 1970s, two conflicting theories of the adhesion of a spherical particle to a substrate were published: the JKR and DMT theories. And that the dispute was resolved when Tabor introduced a parameter μ , such that for small μ , DMT was correct, while for large μ , JKR was correct. We point out that there never were two theories of contact: the dispute was about the magnitude of the pull-off force (with an implication that since the DMT value was obtained by a thermodynamic method, it must be correct). And what Tabor actually said was simply that for large μ , the neglect of surface forces in the JKR theory was acceptable, and that he distrusted the neglect of deformation by the large surface forces immediately outside the Hertzian contact in the DMT theory. We point out the errors in both the DMT/MDT thermodynamic method and the MDT force method (preferred by MDT) but also argue that once Derjaguin and his collaborators (MYD) established that Hertzian geometry does not occur, no theory based on that geometry should be taken seriously. Derjaguin’s well-merited fame rests on more important contributions. [ABSTRACT FROM AUTHOR]

Discrete Element Method (DEM) is a well-established and validated tool for the simulation of bulk materials. However, a lot of questions still need answering in the domain of cohesion and adhesion. This paper will present a calibration approach using the draw down test. This test generates several test results which could be used for the calibration and is one of the standard tests for the calibration of cohesionless materials. For the bulk material wet sand (10% moisture) is used. It is shown how the draw down test can be used to determine a unique set of friction, rolling resistance and cohesion values to reproduce the real behaviour of wet sand. For the DEM simulation the JKR model, one of the most widely used cohesion models, was used. It considers surface energy density and increased particle overlap to explain cohesion. The ambiguity associated with surface energy or interfacial energy is tackled by developing thorough calibration procedures which will help in fine tuning these parameters. Further, the rolling resistance model used for the simulation of cohesive materials is discussed. Calibration results for the very widely used modified elastic-plastic spring dashpot rolling resistance model are compared to an adhesive hysteric rolling resistance model. Unlabelled Image • A procedure is presented to determine an unambiguous parameter set to reproduce the real behaviour of wet sand. • The influence of the rolling resistance model on simulation results is analysed. • Comparison of SJKR model and modified epsd2 model for rolling friction with the JKR model and AdHYS rolling resistance. [ABSTRACT FROM AUTHOR]

Ignoring the effect of cohesion between particles in Discrete Element Method (DEM) may incur in errors in the predictions of bulk properties. In this research work, we investigated the role of the cohesion force on macroscopic and microscopic characteristics of monodisperse and bidisperse iron ore granule systems, specifically on the angle of repose and coordination number of the assemblies. In order to simulate the cohesion between particles, a combination of the Hertz contact force and the pull-off force as predicted by the JKR model was employed. The monodisperse systems were made of particles whose sizes ranged between 3 and 10 mm. In order to analyse the effect of bidispersity on the characteristics of the particle assemblies, combinations of 10 mm particles with smaller particles (from 3 to 7 mm) were considered. The results showed that similarly to the rolling friction force, the cohesion force caused a considerable increase in the angle of repose, for the whole particle size range here studied. More importantly by increasing particle size, the rate at which the angle of repose increased as a result of the cohesion force, decreased significantly. This influence became more outstanding by increasing the value of the rolling friction coefficient, signifying the interplay between rolling friction force and cohesion force. In contrast to the case of non-cohesive particles, the effect of particle size on the angle of repose was independent of the rolling friction force in the presence of cohesion force and led to a decrease in the angle of repose for the entire range of rolling friction coefficient values. An empirical analysis of the obtained angles of repose confirmed the strong dependency of the angles on the Bond number and the rolling friction coefficient for cohesive particles. In addition, the coordination number of all studied systems was influenced by both parameters, as the value of coordination number increased by increasing cohesion force but decreased by increasing rolling friction force. Unlabelled Image • Effect of rolling friction and cohesion on angle of repose was studied by DEM. • Cohesion force was more crucial for smaller particles than rolling friction. • Angle of repose always decreased by particle size for cohesive particles. • Coordination number increased by cohesion but decreased with rolling friction. [ABSTRACT FROM AUTHOR]

In this study, we presented an investigation of mechanical properties by AFM nanoindentation on human hepatocellular carcinoma cells treated with fullerenol for 24, 48 and 72 h. AFM nanoindentation was routinely applied to investigate the morphology and biomechanical properties of living carcinoma cells, and adhesion phenomena (negative force) were detected in the obtained force-displacement curves. Conventionally, Hertz contact model has been widely used for determination of cell elasticity, however this contact model cannot account for adhesion. Alternatively, JKR contact model, as expected for adhesion circumstance, has been applied to fit the obtained force-displacement curves. In this investigation, we have derived both the work of adhesion and the elastic modulus of biological cells (human hepatocellular carcinoma) under fullerenol treatment. The results show that the chosen JKR model can provide better fitting results than Hertz contact model. The results show that both Young’s modulus and work of adhesion exhibit significant variation as the treatment time increases. The calculated mechanical properties of elastic modulus and work of adhesion can be used as an effective bio-index to evaluate the effects of fullerenol or other anticancer agents on cancer cells and thus to provide insight into cancer progression in the treatment. [ABSTRACT FROM AUTHOR]

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ABSTRACT We use atomic force microscopy in conjunction with a fluorescence microscope capable of optical sectioning to acquire images of white blood cells while force is applied with the AFM tip. The indentation profile within the cell is compared to the profile of the AFM tip: examples are shown for indentations at the center of the cell which are reasonable matches to the tip profile, and an additional example is shown for an indentation that is on the tilted side of a highly rounded cell and that differs from the tip shape. We also demonstrate that the AFM tip can interact with internal cell structures, we show that the contact area between the cell and the substrate can increase under applied pressure, that the main body of the cell can fuse with the extended lamellipodium, and that the cell can be displaced laterally by the AFM tip. The features illustrated here are relevant to the interpretation of indentation experiments that measure cell elasticity properties, as is discussed briefly. Microsc. Res. Tech. 78:626-632, 2015. © 2015 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Using the concept of stress intensity factors, we suggest a way to include adhesion into boundary elements simulation of contacts. A local criterion concerning the maximum admissible surface stresses decides whether the adhesive bonds in particular grid points fail or not. By taking into account the grid spacing, a robust methodology is found. Validation is done using the theoretically derived cases of JKR adhesion. [ABSTRACT FROM AUTHOR]

Abstract: This paper describes the conditions for contact formation between soft elastic hemispheres and soft elastic substrates micropatterned with pillars. These microstructured substrates were often proposed in the two past decades for the control of adhesion. In this study the hexagonal micropillars are arranged in hexagonal arrays and the width L of the pillars as well as the distance D between the pillars (with L=D for a sample) are both varied from one sample to another in order to keep the overall planar surface fraction of the pillars constant. Additionally, two pillars of heights h=4µm and h=16µm are considered here. As expected from previous studies different contact morphologies are found as a function of the aspect ratio and the contact pressure. The contact may be formed (i) only at the top of the pillar, (ii) both at the top and between the pillars, or (iii) simultaneously at the top of the pillars and in between the pillars at the centre of the contact for which the pressure is the highest. Unexpectedly it is shown in this last case that the same contact morphology is obtained with the same pressure if the surface pattern aspect ratio L/h is varied homothetically, suggesting a scale invariant behaviour of the contact formation between the hemisphere and the soft elastic micropatterned substrate. [Copyright &y& Elsevier]

Several oleo-based pressure-sensitive adhesives (PSA) were studied as substitutes for current petroleum-based PSAs; however, PSAs derived from plant oils present challenges, such as relatively low biobased contents (20–90%) and complex processes (i.e., purifying, emulsifying, and drying steps). This paper demonstrates the possibility of transparent PSAs from neat acrylated epoxidized soybean oil (AESO) via fast and simple UV free-radical polymerization with 97–100% biobased contents. In this study, we controlled the degree of free radical polymerization by modulating the amount of UV irradiation to obtain a balanced cohesion and adhesion strength for PSA applications. This AESO PSA tape on a polyester film carrier had better peel and significantly higher shear strength on glass surfaces compared with a commercial reusable tape (Post-it®, 3M, St. Paul, MN, USA). The adhesion of the AESO PSA corresponded to its dynamic rheological properties. In addition, the adhesion/cohesion behavior of the PSAs was characterized using the Johnson-Kendall-Roberts (JKR) technique. [ABSTRACT FROM PUBLISHER]

Abstract: This review presents a summary of the current adhesion models available to date, between real rough surfaces, starting from single asperity models and expanding to multiple asperity contacts. The focus is made on multi-asperity contact interactions. Both van der Waals and contact mechanics approaches have been considered and relevant adhesion models are reviewed and discussed. The influence of the meniscus forces on adhesion has been considered, along with a summary of the various meniscus models. The effect of surface geometry, its topography and environmental conditions on meniscus action are also discussed along with its integration into multi-asperity adhesion models. [Copyright &y& Elsevier]

Abstract: In many industrial and medical applications, some strategies to control force of adhesion between materials are necessary as surface forces can enable or hinder the functionality of the devices. All real surfaces present some level of roughness and the contact between two such surfaces is carried by the asperities on the surfaces. A multi-asperity adhesion model based on the JKR and DMT theories has been applied to the contact between silicone or glass and polybutylene terephthalate (PBT). A Monte Carlo method was used to generate a set of asperities for each of the materials. Height and radius of curvature of each asperity were based on the experimentally determined distributions. The normal distribution returned the best fit for asperity height, whilst the Weibull distribution was a better fit for the radius of curvature. The predicted adhesion forces between PBT and silicone or glass were compared with the experimental results performed using atomic force microscopy (AFM). The model predicted higher adhesion forces when the DMT theory was used than the JKR theory for both counter surfaces. The contact area calculated using the JKR model was larger than the DMT for a chosen applied load. Furthermore, the experimental values of the adhesion force were in a good agreement with the simulated data using the JKR theory in case of silicone, whilst the DMT model estimated better adhesion for glass. [Copyright &y& Elsevier]

Professor David A. Dillard and his collaborators invented the constrained blister and the related punch test for characterizing thin film adhesion-delamination and the disjoining pressure at the interface of a clamped membrane and a rigid substrate. A new dimensionless parameter, ψs = [6(1 - ν)a4/γ3Eh]1/4p, relating the membrane dimensions (radius, a, and thickness, h), materials properties (elastic modulus, E, and Poisson's ratio, ν), and adhesion (interface energy, γ, and disjoining pressure, p), is shown to account for the transition from the long range surface force limit with small ψ (Derjaguin-Muller-Toporov, DMT) to the short range limit with large ψ (Johnson-Kendall-Roberts, JKR). This parameter serves as a flexible membrane counterpart to the classical Tabor's parameter for adhesion in bulk solids. An alternative form is defined for thick circular plate under bending, ψb = [6(1 - ν)a4/γEh3]1/2p. Three case studies, namely, the standard pressurized blister, the constrained blister, and the punch tests are investigated for membranes under either pure bending or pure stretching deformations. [ABSTRACT FROM AUTHOR]

Abstract: In this paper the adhesive contact problem for transversely isotropic materials is discussed. A spherical contact on transversely isotropic, adhesive half-space is studied. Solving an equivalent external circular crack problem in a transversely isotropic body, the modified JKR and DMT theories are developed. The results show that the modified JKR and DMT contact theories for transversely isotropic materials have same form as JKR and DMT expressions for isotropic materials, except for the elastic constant K in the formula, which has a different expression for different materials. [Copyright &y& Elsevier]

Abstract: A particle dynamics simulation model is developed and used to design a mechanical damping device that uses micro or nano powders as the damping medium. A damping device based on a powder has the advantage over the traditional use of a viscous fluid of temperature insensitive operation. Adhesion forces significantly affect the behavior of small particles. The damping mechanism targeted stems from energy dissipated due to the breaking of many adhesive particle contacts. Adhesive contacts can dissipate energy due to hysteresis in the force between particle loading and unloading. In this work adhesion hysteresis is added to the JKR model of adhesive contact and then used in particle dynamics simulations. The effects of adhesive properties and packing densities are studied in two device geometries (shear and piston) using simulation. [Copyright &y& Elsevier]

Abstract: The JKR, DMT, Maugis models and Tabor parameter for contact under normal loading have been developed based mainly on solid–solid (van der Waals) interactions. In this case, the characteristic length scale for the adhesive forces in the Tabor parameter is the equilibrium interatomic spacing. However, for contact in humid environments, where a liquid meniscus may be present, capillary forces with a longer force range related to the Kelvin radius dominate. Fogden and White [J. Colloid Interface Sci. 138 (1990) 414] introduced a parameter that includes the Kelvin radius for the JKR–DMT transition. This topic was also addressed by Maugis and Gauthier-Manuel [J. Adhes. Sci. Technol. 8 (1994) 1311] who included capillary effects within the frame work that Maugis had previously established. The parameters introduced by Fogden and White and Maugis and Gauthier-Manuel can be viewed as a modified Tabor parameter for the JKR–DMT transition. In the present work, the Kelvin equation linking the Kelvin radius and the relative humidity was explicitly included in the modified Tabor parameter. This provided a quantitative description of the JKR–DMT transition in terms of the relative humidity. This parameter was examined via load and contact radius measurements, where the latter were obtained from Bowden and Tabor''s assumption that the friction force . The friction experiments were conducted at two different humidity levels using a newly-developed mesoscale friction tester (MFT), which provides a very wide range of contact radii. The modified Tabor parameter was used to reexamine data from pull-off experiments in water and cyclohexane vapor environments [L.R. Fisher, J.N. Israelachvili, Colloids Surf. 3 (1981) 303 and H.K. Christenson, J. Colloid Interface Sci. 121 (1988) 170]. Finally, guidelines are presented for the appropriate choice of contact mechanics models to be used in interpreting data from SFA and AFM experiments in humid environments. [Copyright &y& Elsevier]

Abstract: Wet and dry adhesion between dextran-coated surfaces were measured aiming to understand the influence of polymer compatibility. The wet adhesion measurements were performed using the atomic force microscope (AFM) colloidal probe technique whereas the dry adhesion measurements were performed using the micro adhesion measurement apparatus (MAMA). Two types of dextrans were used, one cationically modified dextran (DEX) and one that was both cationically and hydrophobically modified (HDEX), leading to three different combinations of polymer-coated surfaces; (1) DEX:DEX, (2) HDEX:DEX, and (3) HDEX:HDEX. DEX increased dry adhesion more than HDEX did, which likely is due to differences in the ability to form specific interactions, especially hydrogen bonding. HDEX gave strong wet adhesion, probably due to its poorer solvency, while DEX contributed to reducing the wet adhesion due to its hydrophilicity. All combinations showed a steric repulsion on approach in aqueous media. Furthermore, when HDEX was adsorbed on either or both surfaces a long range attractive force between the surfaces was detected outside this steric regime. [Copyright &y& Elsevier]

Using a homemade setup, we investigated the adhesion between soft elastic substrates bearing surface microstructures (array of caps [resp., holes] of height [resp., depth] h) and a smooth surface of the same rubber. In the framework of the classical model developed by Johnson, Kendall, and Roberts, we show the following. (i) The existence of a critical height hc for the microstructures, resulting from a competition between the adhesion energy and the elastic deformation energy necessary to invade the pattern: for h < hc, the bead and the substrate are in intimate contact even when the applied force is zero, and for h > hc, an air film remains intercalated in the microstructure, and the contact is limited to the top of the caps or between the holes. The transition between these two states can be induced by increasing the squeezing force. (ii) The adhesion energy, W, of intimate contacts (h < hc) decreases as the height increases. Suspended contacts correspond to a low adhesion and a nearly Hertzian behavior. Using simple scaling arguments and a two-level energetic description (single microstructure and whole contact) we propose a semiquantitative description of these observations. [ABSTRACT FROM AUTHOR]

The thermodynamic work of adhesion between micron-sized zirconia or carbon spheres and polyester-melamine surfaces was determined using the Johnson-Kendall-Roberts (JKR) relationship, after first validating the JKR approach for this system. The calculated works of adhesion for both zirconia and carbon were similar for any given substrate composition and were found to be approximately 35 mJ/m2 at low melamine concentrations, close to theoretical predictions. The apparent work of adhesion decreased with increasing melamine concentration, most likely due to the presence of a glassy melamine-rich surface layer, which is not representative of the bulk. The value found for low melamine concentration was assumed to be a true value for cocondensed polyester-melamine, and this was used to estimate surface modulus and the amount of excess melamine in the surface as a function of bulk composition. [ABSTRACT FROM AUTHOR]

The gecko has incredible climbing ability for its nanoscale hierarchical fibrillar structure foot-hair. Can we fabricate a new type of adhesive by mimicking the gecko mechanism? In this paper, we consider a simplified case of an elastic cylinder with hemispherical shape terminals adhering to a rigid substrate. By using the classical JKR model to calculate the adhesive strength, we find that the strong size effect arises when the cylinder's radius is in nanoscale. This finding may help us to design a biomemetic adhesive. [ABSTRACT FROM AUTHOR]

Abstract: The Johnson–Kendall–Roberts (JKR) theory of elastic contact, extended to take viscoelastic effects into account, is used to evaluate work of adhesion and modulus of elastomeric films. In this paper, we present a comparison of five approaches to analyze quasi-static and dynamic JKR force curve data obtained using instrumented indentation. The load-displacement experiments were performed using a 200-μm radius borosilicate glass sphere against poly(dimethyl siloxane) (PDMS). By applying a small oscillation to the tip during indentation, dynamic stiffness vs load data were also obtained for frequencies between 25 and 160 Hz. Direct curve fitting as well as simplified 2- and 3-point analysis methods were used to compare modulus values obtained from load-displacement and stiffness-load data. Fit methods not requiring determination of the initial point of tip–sample contact (“zero” displacement) provided modulus values closest to those obtained by direct curve fitting. The dynamic stiffness-load data revealed a frequency dependent modulus; load-displacement measurements obtained simultaneously were consistent with the relaxed, or low-frequency, modulus of the PDMS sample. These experiments demonstrate that both the frequency dependent and relaxed modulus can be obtained from a single experiment. [Copyright &y& Elsevier]

Abstract: The cycle of loading and unloading of a spherically-tipped probe against an adhesive, viscoelastic plane specimen is studied by numerical integration of the relations between crack speed and apparent surface energy previously found for a linear 3-element viscoelastic solid with a Maugis–Dugdale law of force across the crack. It is found that even when the rate of loading is so slow that the loading and unloading curves almost coincide, suggesting purely elastic behaviour, the pull-off force can be appreciably greater than the elastic (JKR) value. When the normal force is modulated with a small amplitude sinusoidal variation during unloading—in order to find the contact stiffness—the contact radius barely changes, and the stiffness is close to that for a rigid flat punch instead of having the expected JKR value. [Copyright &y& Elsevier]

Abstract: Adhesion of micron-scale probes with model poly(dimethylsiloxane), PDMS, elastomers was studied with a depth-sensing nanoindenter under oscillatory loading conditions. For contacts between diamond indenters (radius or 10 μm) and PDMS, force–displacement curves were highly reversible and consistent with Johnson–Kendall–Roberts (JKR) behavior. However, our experiments have revealed striking differences between the experimental measurements of tip–sample interaction stiffness and the theoretical JKR stiffness. The measured stiffness was always greater than zero, even in the reflex portion of the curve (between the maximum adhesive force and release), where the JKR stiffness is negative. This apparent paradox can be resolved by considering the effects of viscoelasticity of PDMS on an oscillating crack tip in a JKR contact. Under well described conditions determined by oscillation frequency, sample viscoelastic properties, and the Tabor parameter (with variables R, reduced elastic modulus, , and interfacial energy, Δγ), an oscillating crack tip will neither advance nor recede. In that case, the contact size is fixed (like that of a flat punch) at any given point on the load–displacement cycle, and the experimentally measured stiffness is equal to the equivalent punch stiffness. For a fixed oscillation frequency, a transition between JKR and punch stiffness can be brought about by an increase in radius of the probe or a decrease in PDMS modulus. Additionally, varying the oscillation frequency for a fixed , R, and Δγ also resulted in transition between JKR and punch stiffness in a predictable manner. Comparisons of experiments and theory for an oscillating viscoelastic JKR contact are presented. The storage modulus and surface energy from nanoscale JKR stiffness measurements were compared to calculated values and those measured with conventional nanoindentation and JKR force–displacement analyses. [Copyright &y& Elsevier]

Defined surface structures of various shapes and length scales can impact the adhesion of polymer interfaces. Most recent research has focused on the use of high-aspect-ratio post-like structures to modify the formation and separation mechanisms for a contacting interface. These mechanisms for controlling adhesion were inspired by the natural mechanisms present in examples such as the gecko and jumping spider. Although post-like structures are effective, numerous other geometries can impact adhesion properties through different mechanisms. In this article, we present and discuss the impact of surface hole patterns on the adhesion of soft, elastomeric interfaces. The surface holes are shallow, cylindrical depressions that decorate the surface of the elastomer and alter the separation mechanisms from rigid surfaces. We specifically focus on the effect of hole radius and nearest hole spacing on the overall adhesion descriptors. Using contact adhesion experiments largely based on the theory of Johnson, Kendall, and Roberts (JKR), we demonstrate that surface holes can alter the stability of the contact edge during interfacial separation. This stability does not affect the overall work of adhesion for a crosslinked polydimethylsiloxane interface, but it does permit tuning of the maximum separation force. These results emphasize the importance of measuring the interfacial area during adhesion testing, the significance of the contact edge, and the opportunities for using surface holes to tune the dissipative processes of interfaces that involve viscoelastic materials. These conclusions are particularly significant in the context of characterization techniques that do not allow for the direct measurement of contact area, such as nanoindentation or scanning probe microscopy measurements. [ABSTRACT FROM AUTHOR]

Abstract: A custom-built adhesion-testing device (ATD) is described in this paper, which was developed to study energetics of various solid (polymeric) interfaces. A review is also given of the main techniques of adhesion and adherence measurements, including non-destructive and destructive methods, with major emphasis on the evolution and applications of contact mechanics techniques. Using the Johnson–Kendall–Roberts (JKR) theory of contact mechanics in the elastic deformation regime, the interfacial energy of solid surfaces can be obtained by measuring the contact radius, loading force, and vertical displacement between an (elastic) sphere (lens) and a flat surface (one of which, or both, coated with the sample of interest). The parameters needed for JKR analyses were determined by our custom-built device. Based on the JKR theory, the values of work of adhesion, combined elastic modulus and interfacial energy were determined from the loading and unloading curves on poly(dimethylsiloxane)–poly(dimethylsiloxane) (PDMS) systems. Cumulative adhesion hysteresis and elastic modulus were also calculated. The results obtained agree well with literature data measured by different methods. These measurements on compliant PDMS–PDMS model systems can also serve as validation and verification of the adhesion-testing devices described in this study. [Copyright &y& Elsevier]

The JKR technique was applied to study the influence of interfacial reactions on the adhesion between functional elastomer gels and functional solid substrates. The gelation chemistry of poly (dimethyl siloxane) (PDMS) gels cured by hydrosilylation reactions was purposely adjusted to produce an excess of either silane or vinyl functionality. Hemispherical lenses of these materials were then contacted under load with a variety of functionalized solid substrates: poly(styrene-b-butadiene) copolymers with vinyl functionality, vinyl-terminated trimethoxysilane self-assembled monolayers, and α,ω-functional PDMS brushes terminated with either monomethoxy or hydroxyl groups. To rule out chain interpenetration effects, the molecular weights were kept below the entanglement molecular weight or immiscible polymers were employed on opposite sides of the interface. Significant adhesion enhancement was observed for most systems, indicating that a variety of different interfacial reactions can occur across the interface between PDMS elastomers and solid polymeric substrates. The overall nature of the adhesion enhancement found is consistent with the predictions of the Lake-Thomas theory for failure of elastomers, increasing linearly with the length and areal density of covalent linker chains that span the interface. [ABSTRACT FROM AUTHOR]

Self-adhesion hysteresis has been investigated in crosslinked poly(dimethylsiloxane) (PDMS) lenses using the Johnson, Kendall, and Roberts technique. The experimental conditions involved relatively short contact times for which interchain penetration effects across the interface are minimal. Only lenses that had been extracted in toluene displayed self-adhesion hysteresis. The same lenses demonstrated no adhesion hysteresis when pressed against tethered polystyrene substrates, indicating that hysteresis was caused by surface interactions and not bulk viscoelastic effects. Extraction produces hysteresis by removing the free chains, which normally lubricate the interface, inhibiting the adhesion mechanism. Self-adhesion hysteresis was only observed for networks with a high molecular weight between crosslinks. More tightly crosslinked networks did not display self-adhesion hysteresis, even at extended contact times under load. By inhibiting the hydrosilylation reaction between residual vinyl and silane groups in the PDMS lenses, self-adhesion hysteresis was prevented, suggesting that the formation of chemical crosslinks across the interface caused the observed hysteresis. The molecular weight dependence of the hysteresis can be interpreted in terms of the Lake-Thomas model [1] for fracture in elastomers. [ABSTRACT FROM AUTHOR]

This work presents an experimental analysis of the friction response of an ice-rubber interface over five decades of sliding velocity and temperature down to − 20 ∘C, combining in-situ contact visualisation and simultaneous force measurements. Viscoelastic properties of the rubber were varied in terms of glassy temperature transition and elastic modulus. Based on the in-situ contact area measurements, the adhesive and viscoelastic contributions were identified. Even though a bell-shape friction-velocity curve was observed, the classical WLF transform did not allow a description of the friction behaviour. A simple analytical model accounting for the thermal dissipation induced by friction was thus proposed and a dimensionless master curve was obtained with the sliding velocity, regardless of the temperature and the material properties. From this master curve, a predictive friction model was proposed, in which both friction contributions, adhesion-viscoelasticity and thermal dissipation, were multiplicative rather than simply additive. Friction mastercurve, for G varying from 1 to 1.7 MPa, T g ranging from − 40 ∘C and − 50 ∘C and environmental temperature between − 15 ∘C and 0 ∘C [Display omitted] • Ice-rubber friction over five decades of sliding speeds and temperature down to − 20 ∘C. • Investigation of the adhesive and viscoelastic contributions. • Analytical modelling of the thermal dissipation. • Friction mastercurve, regardless of viscoelasticity and environmental temperature. • Multiplicative viscoelastic-thermal contributions to friction mechanisms. [ABSTRACT FROM AUTHOR]