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Item type:Publication, Access status: with files , Adhesive contact of elastic spherical shells: Non-monotonic thickness dependence of pull-off forceThe adhesive contact of spherical shells is a critical problem across multiple length scales. In this study, we investigate the adhesive contact between an elastic spherical shell and a rigid plane. Two independent approaches are employed: the JKR theory from an energy perspective, and a simulation-based method incorporating a traction-separation law. Our results reveal a non-monotonic relationship between pull-off force and shell thickness. Specifically, for relatively thick shells, the pull-off force decreases with decreasing thickness; however, below a critical thickness, this trend reverses, and the pull-off force begins to increase. Furthermore, we explore the underlying mechanisms responsible for this behavior and examine the influence of adhesion parameters on the overall response.Publicationtype: Journal Article17 16 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , On the scaling of strength of flat and mushroom-shaped ends of microstructured adhesivesConsiderable research has been conducted on shape of pillar ends for optimal adhesion. In experiments with elastomers, it has been found that mushrooms-ended ones are superior to flat-ended ones, but early experiments have suggested an extremely strong scaling in strength with pillar radius (del Campo et al. Langmuir 23:10235–10243, 2007). We discuss various theories and experimental results on scaling of strength, and in particular, we elaborate recent experiments on single pillars with mushroom ends finding that the scaling on strength is much less surprising. When the ratio of tip to shaft radii increases toward an optimal value, the mode of failure changes from edge to center defects, and the scaling of strength changes from that associated with the presence of the classical linear elastic fracture mechanics stress singularity at the corner of the flat-ended punches caused by large friction at the interface, with that caused by the population of defects immersed on a nearly uniform stress field in the mushroom center. Strong scaling of strength therefore is also accompanied with large scatter of strength.Publicationtype: Journal Article13 14 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , On the dynamic JKR adhesion problemShui et al. (2020) have recently shown that applying high-frequency vibrations, we can increase the mean adhesion between viscoelastic solids. This is due to the fact that oscillating contact area leads to an effect of increased apparent surface energy during the retraction phase which can be described by the well known empirical Gent and Schultz law (GS). However, Shui et al solution surprisingly appears not to depend on GS constants, which would imply perhaps no amplification. Yi et al. (2024) have made similar experiments, and proposed a simpler fitting model, which seems to work however with widely different GS constant when changing the sphere radius. Here, we solve the JKR dynamic adhesion problem for a sphere oscillating on a substrate by imposing an harmonic oscillation of the contact area, which permits to obtain a very simple solution by simply averaging the resulting cycle of indentation. We find that the solution is close to a JKR form for the mean indentation vs mean force, which we find in a simple approximation. Although there is saturation in the amplification when the contact radius shrinks to zero and the problem becomes that of impacts at large amplitudes of vibrations, experiments show that other saturations occurs first, presently unclear. We discuss also the influence of resonances. We find reasonable agreement with experiments conducted on PDMS.Publicationtype: Journal Article22 8 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Enhancement of adhesion strength through microvibrations: Modeling and experimentsHigh-frequency micrometrical vibrations have been shown to greatly influence the adhesive performance of soft interfaces, however a detailed comparison between theoretical predictions and experimental results is still missing. Here, the problem of a rigid spherical indenter, hung on a soft spring, that is unloaded from an adhesive viscoelastic vibrating substrate is considered. The experimental tests were performed by unloading a borosilicate glass lens from a soft PDMS substrate excited by high-frequency micrometrical vibrations. We show that as soon as the vibration starts, the contact area increases abruptly and during unloading it decreases following approximately the JKR classical model, but with a much increased work of adhesion with respect to its thermodynamic value. We find that the pull-off force increases with the amplitude of vibration up to a certain saturation level, which appeared to be frequency dependent. Under the hypothesis of short range adhesion, a lumped mechanical model was derived, which, starting from an independent characterization of the rate-dependent interfacial adhesion, predicted qualitatively and quantitatively the experimental results, without the need of any adjustable parameters.Publicationtype: Journal Article23 43 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Strongly different adhesion reduction for 1D or 2D random fractal roughness, and an extension of the BAM model to anisotropic surfacesThe influence of roughness on adhesion has been studied since the time of Fuller and Tabor, but recently there has been debate about how roughness exactly seems to kill (but sometimes enhance!) adhesion, particularly with reference to the accepted model of fractal roughness. We show that the Persson–Tosatti criterion does not depend on anisotropy of the surface for a typical power law PSD if written in terms of rms roughness and magnification. Instead, a very simple extension of the Bearing Area Model (BAM) of Ciavarella to anisotropic fractal surface shows a weak but clear dependence on the anisotropy, with higher adhesion in the 1D case, showing better agreement than the Persson–Tosatti criterion to actual numerical results of Afferrante Violano and Dini. However, neither of the two models permit to capture the strong hysteresis found in experiments between loading and unloading, which is very likely to enhance adhesion more as we move from the isotropic to the full 1D case. This suggests the mechanism of load amplification along contact lines and the associated elastic instabilities, is not captured by either the Persson–Tosatti or the BAM model applied to anisotropic surfaces.Publicationtype: Journal Article13 14 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Bulk and fracture process zone contribution to the rate-dependent adhesion amplification in viscoelastic broad-band materials(Elsevier, 2024-09-02); ; The contact between a rigid Hertzian indenter and an adhesive broad-band viscoelastic substrate is considered. The material behavior is described by a modified power law model, which is characterized by only four parameters, the glassy and rubbery elastic moduli, a characteristic exponent n and a timescale τ0. The maximum adherence force that can be reached while unloading the rigid indenter from a relaxed viscoelastic half-space is studied by means of a numerical implementation based on the boundary element method, as a function of the unloading velocity, preload and by varying the broadness of the viscoelastic material spectrum. Through a comprehensive numerical analysis we have determined the minimum contact radius that is needed to achieve the maximum amplification of the pull-off force at a specified unloading rate and for different material exponents n. The numerical results are then compared with the prediction of Persson and Brener viscoelastic crack propagation theory, providing excellent agreement. However, comparison against experimental tests for a glass lens indenting a PDMS substrate shows data can be fitted with the linear theory only up to an unloading rate of about 100μm/s showing the fracture process zone rate-dependent contribution to the energy enhancement is of the same order of the bulk dissipation contribution. Hence, the limitations of the current numerical and theoretical models for viscoelastic adhesion are discussed in light of the most recent literature results.Publicationtype: Journal Article9 74 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , On the interaction of viscoelasticity and waviness in enhancing the pull-off force in sphere/flat contactsMotivated by roughness-induced adhesion enhancement (toughening and strengthening) in low modulus materials, we study the detachment of a sphere from a substrate in the presence of both viscoelastic dissipation at the contact edge, and roughness in the form of a single axisymmetric waviness. We show that the roughness-induced enhancement found by Guduru and coworkers for the elastic case (i.e. at very small detachment speeds) tends to disappear with increasing speeds, where the viscoelastic effect dominates and the problem approaches that of a smooth sphere. This is in qualitative agreement with the original experiments of Guduru’s group with gelatin. The cross-over velocity is where the two separate effects are comparable. Viscoelasticity effectively damps roughness-induced elastic instabilities and makes their effects much less important. Graphical Abstract: [Figure not available: see fulltext.].Publicationtype: Journal ArticleScopus© Citations 3 75 249 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Stickiness of randomly rough surfaces with high fractal dimension: is there a fractal limit?(2021-07); ; Two surfaces are “sticky” if breaking their mutual contact requires a finite tensile force. At low fractal dimensions D, there is consensus stickiness does not depend on the upper truncation frequency of roughness spectrum (or “magnification”). As debate is still open for the case at high D, we exploit BAM theory of Ciavarella and Persson-Tosatti theory, to derive criteria for all fractal dimensions. For high D, we show that stickiness is more influenced by short wavelength roughness with respect to the low D case. BAM converges at high magnifications to a simple criterion which depends only on D, in agreement with theories that includes Lennard-Jones traction-gap law, while Persson-Tosatti disagrees because of its simplifying approximations.Publicationtype: Journal Article71 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , A criterion for the effective work of adhesion in loading and unloading of adhesive soft solids from rough surfacesRecently, 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.Publicationtype: Journal ArticleScopus© Citations 2 157 207 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , On the effect of the loading apparatus stiffness on the equilibrium and stability of soft adhesive contacts under shear loads(2020-11); ; The interaction between contact area and frictional forces in adhesive soft contacts is receiving much attention in the scientific community due to its implications in many areas of engineering such as surface haptics and bioinspired adhesives. In this work, we consider a soft adhesive sphere that is pressed against a rigid substrate and is sheared by a tangential force where the loads are transferred to the sphere through a normal and a tangential spring, representing the loading apparatus stiffness. We derive a general linear elastic fracture mechanics solution, taking into account also the interaction between modes, by adopting a simple but effective mixed-mode model that has been recently validated against experimental results in similar problems. We discuss how the spring stiffness affects the stability of the equilibrium contact solution, i.e. the transition to separation or to sliding.Publicationtype: Journal Article56