Effective Work Of Adhesion Research Articles

Viscoelasticity, stiffness gradient and their effects on adhesion of an epoxy shape memory polymer

The shape memory polymer based adhesives have demonstrated excellent programmable switchable adhesion, in which the material properties play an important role. Here, the viscoelasticity, stiffness gradient, and their effects on adhesion of an epoxy shape memory polymer (ESMP) were studied. The ESMP sample and polydimethylsiloxane control sample were fabricated firstly by mold casting and curing techniques. The sample was fixed on an electric heating plate with double-sided tape to conduct the adhesion measurements against a hemispherical glass indenter under different temperature and displacement conditions by using an adhesion tester and the temperature measurements on the double-sided tape and ESMP sample surfaces by using a digital thermodetector. Based on the measured force–displacement–time data, the hysteresis behavior and internal dissipation were evaluated; the peak force relaxation was studied; the reduced modulus was calculated and the stiffness gradient features were evaluated. The results show that the ESMP sample exhibits strong viscoelasticity and significantly enhanced adhesion that strongly depends on the compressive displacement in the glass-rubber transition zone near it is glass transition temperature ( Tg ), while weaker viscoelasticity and lower adhesion in the soft rubbery state. The reduced modulus of the ESMP sample obviously decreases with the increasing compressive displacement at a preset temperature below 70 °C. It is found that there exists a linear relation between the pull-off force, effective work of adhesion, reduced modulus, and contact area. This study helps deeply understand the material properties and adhesion mechanism of the ESMP, which can guide the design of switchable adhesives.

Rate effects in detachment of a spherical probe from fibrillar adhesive surfaces

Rate effects in fibrillar adhesive surfaces have received increasing attention recently. However, the requirement for taking into account the viscoelastic behavior of the material, combined with the backing layer coupling, complicates the theoretical treatment. As a result, the role of rate effects, particularly on the array level, remains unclear. This work focuses on a general problem: a rigid spherical probe is detached from a viscoelastic fibrillar adhesive surface at a certain velocity. A theoretical framework is proposed based on the Lee & Radok corresponding principle and the Schapery approximation for crack initiation in viscoelastic solids. The fibril force across the array is explicitly determined during the dynamic detaching process. Simulation results reveal the influence of rate effects on the adhesion response, the preload dependence, and the effective work of adhesion of the fibrillar surface. As expected, the adhesion force monotonically increases with the retraction velocity. An interesting phenomenon is observed at intermediate velocities that modest preload is favorable to the pull-off force while an overlarge one would have an adverse effect. The pull-off force would finally approach a lower steady-state constant at a sufficiently high preload value. Most significantly, it is found that the dependence of the effective work of adhesion on the crack velocity is almost independent of the basic properties of the fibrillar surface when traversing typical parameter space. This indicates that the contribution of the viscoelastic-induced adhesion enhancement can be approximately decoupled as a multiplicative term in the overall effective work of adhesion. For a sufficiently large probe radius, this rate-dependent characteristic can be well captured by the continuum theory of viscoelastic crack propagation.

A Criterion for the Effective Work of Adhesion in Loading and Unloading of Adhesive Soft Solids from Rough Surfaces

Recently, Dalvi and co-authors have shown detailed experimental data of adhesion of soft spheres with rough substrates with roughness measured down to almost the atomic scale, finding that the Persson and Tosatti theory gave satisfactory predictions of the apparent work of adhesion during loading, once the increase of the surface area due to roughness is correctly computed at extremely small scales. We show that unloading data would show similar correlation with the Persson–Tosatti’s simple criterion, but for a much larger effective work of adhesion, which therefore becomes not an “intrinsic” property. This suggests either strong hysteresis even at apparently very low peeling velocities or the need to use a criterion that has different behavior during loading and unloading. We attempt this inspired by the results of Guduru for a simple case of axisymmetric waviness, and a much better fit of the experimental data by Dalvi and co-authors is obtained using the entire set of data at loading and unloading, even assuming a single work of adhesion value. However, we cannot rule out that both (viscoelastic) and (roughness-induced) enhancement effects coexist in these data.

Frictional characteristics of stiff, high aspect ratio microfiber arrays based on cyclic olefin polymers

Cyclic olefin polymer (COP) microfiber arrays with diameters in the range of 0.6–5 μm and up to ~20 μm in length are successfully fabricated using polycarbonate membranes as templates, and their macroscale friction properties are evaluated against smooth glass surfaces. Increasing the aspect ratio of the fibers decreases the effective modulus and increases the effective work of adhesion, which favor a better compliance, resulting in higher friction forces. On the other hand, the shape of the fibers as well as inter-fiber adhesion at high aspect ratio complicates the dependence of friction on fiber geometry. Among the arrays investigated, the 2 and 0.6 μm diameter arrays exhibited excellent friction performance (up to ~6 and 9 N/cm2, respectively) and high fractional contact area (50–60%). These results suggest that arrays with high friction properties can be easily obtained by molding COPs from polycarbonate membranes.

Single asperity nanocontacts: Comparison between molecular dynamics simulations and continuum mechanics models

Using classical molecular dynamics, atomic scale simulations of normal contact between a nominally flat substrate and different atomistic and non-atomistic spherical particles were performed to investigate the applicability of classical contact theories at the nanoscale, and further elucidate the phenomena that govern the perceived breakdown of continuum in nanocontacts. Using a rigorous definition of the real contact area, the corresponding surface energies were calculated via molecular dynamics simulations and validated with values reported in the literature. The contact force and radius, as well as the contact stress distribution and dislocation propagation were then investigated in detail. The Hertz elastic contact model was shown to sufficiently capture the results of molecular dynamics simulations prior to the onset of plastic deformation for non-adhesive contacts (non-atomistic particles). Moreover, the results show that the Lommer–Cottrell locks observed during contact acted as obstacles to dislocation motion, and cross-slipping of atoms at these points was the deformation mechanism responsible for force drops during loading. In the case of the adhesive atomistic particles, the results were compared to the limiting cases of the Johnson–Kendal–Roberts (JKR) and Derjaguin–Muller–Toporov (DMT) continuum models. The atomistic systems showed deviations from the classical models, which could be related to energy loss and changes in the effective work of adhesion, as well as the anisotropic properties of the atomistic systems. The findings of this work support the published literature on the subject, and further contribute to our understanding of discrepancies between atomistic and continuum descriptions of contact.

Analysis on Adhesive Contact of Micro-Columns

This paper analyzes the contact and adhesion between a rigid hemisphere and micro-columns on a substrate. We establish two types of three-dimensional finite element models for the analysis. One uses continuum elements and the other uses spring elements to describe the mechanical behavior of the micro-columns. In the analysis of adhesive contact, the adhesive stress between the surfaces of micro-columns and hemisphere was implemented by a user-defined cohesive element. The validation of the spring element model is tested by comparing the simulation results with those of the continuum element model. The influences of the aspect ratio, column width, and area density of micro-columns and substrate thickness on the effective Young's modulus are discussed, as well as the maximum pull-off force and effective work of adhesion. The numerical results agree mostly with the experimental data in the literature. The comparison also shows that the simple analysis by Hui et al. predicts the effects of micro-columns on effective work of adhesion quite well.

Thermal Fluctuations Limit the Adhesive Strength of Compliant Solids

For compliant solids, the stress required to separate an interface (its adhesive strength) appears to be much lower than that calculated by computing intersurface interactions. We explore the hypothesis that the adhesive strength is limited in value by thermal fluctuations. In a simple model of an interface, molecules bridging the two surfaces are represented by linear entropic springs. Asymptotic and numerical analyses are carried out to evaluate the adhesive strength and effective work of adhesion. For stiff materials, adhesive strength is found to be equal to the intrinsic strength—the maximum value of intersurface stress computed ignoring fluctuations. For compliant materials, adhesive strength is significantly reduced and is on the order of the elastic modulus. The effective work of adhesion agrees with the intrinsic work of adhesion for stiff materials and can decay slowly with increasing compliance.

A simplified approach to the adhesive theory of friction

In the present paper, a simplified approach to the adhesive theory of friction, which encompasses both the concept of work of adhesion and the growth of the real area of contact, is presented. The starting point is to consider the average shear strength of the asperity junctions as dependent on the effective work of adhesion, which is influenced by the thermodynamic work of adhesion and irreversible local phenomena. Simple analytical equations for the average shear strength and friction coefficient are thus obtained, and their validity is checked with reference to experimental data relevant to copper and aluminum alloys dry sliding against high strength steel counterfaces. It is further shown that this approach is able to rationalize the presence and intensity of the oscillations on the friction trace (the so-called stick-slip phenomenon), and the influence of the initial roughness of the mated materials on friction evolution.