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Item type:Publication, Access status: with files , Influence of material and geometrical parameters on the adhesive performance of vibration-modulated soft contactsVibroadhesion, the modulation of interfacial adhesion by superimposing small-amplitude, high-frequency vibrations on an adhesive contact, offers a simple and fast strategy to rapidly tune contact forces. A systematic understanding of how key geometrical and material parameters govern this behavior is still lacking. Here, we present a comprehensive experimental investigation of vibroadhesion in viscoelastic PDMS–glass Hertzian contacts, examining the influence of substrate thickness 𝑡, radius of the indenter 𝑅, material properties like modulus and thermodynamic surface energy, unloading rate 𝘳 and preload 𝘍cₒ on the stickiness performance of vibration-modulated interfaces such as: pull-off force 𝘍ₚₒ contact radius 𝒶, mean stress at pull-off σₚₒ, maximum enhancement with respect to the static adhesion test, range of vibration amplitudes that sustain adhesion enhancement. We find that across all tests, vibration activation triggers an abrupt expansion of the contact area, which upon unloading, results in a substantial increase of the pull-off force up to a saturation level always coinciding with the vibration level causing interfacial instabilities at the interface (wrinkles). Although the absolute value of the pull-off force may be strongly influenced by geometrical and material parameters, the maximum adhesion enhancement with respect to the reference static test remained consistently close to a 13-fold increase. We obtained the maximum mean stress at pull-off of 186 kPa when indenting with the smallest sphere. The unloading rate r weakly affects the adhesive performance and the preload Fcₒ is nearly irrelevant in determining the pull-off force 𝘍ₚₒ. Taken together, these results provide both physical insights and practical guidelines for designing vibration-modulated adhesive interfaces for rapid, reversible, and tunable adhesion.Publicationtype: Journal Article14 15 - 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 , 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: without files , Viscoelastic amplification of the pull-off stress in the detachment of a rigid flat punch from an adhesive soft viscoelastic layerThe problem of the detachment of a flat indenter from a plane adhesive viscoelastic strip of thickness “b” is studied. For any given retraction speed, three different detachment regimes are found: (i) for very small “b” the detachment stress is constant and equal to the theoretical strength of the interface, (ii) for intermediate values of “b” the detachment stress decays approximately as b−1/2, (iii) for thick layers a constant detachment stress is obtained corresponding to case the punch is detaching from a halfplane. By using the boundary element method a comprehensive numerical study is performed which assumes a linear viscoelastic material with a single relaxation time and a Lennard-Jones force-separation law. Pull-off stress is found to consistently and monotonically increase with unloading rate, but to be almost insensitive to the history of the contact. Due to viscoelasticity, unloading at high enough retraction velocity may allow punches of macroscopic size to reach the theoretical strength of the interface. Finally, a corrective term in Greenwood or Persson theories considering finite size effects is proposed with good agreement between theoretical and numerical results.Publicationtype: Journal Article10 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , On adhesive theories in multilayered interfaces, with particular regard to "surface force apparatus" geometryAdhesion is a key factor in many tribological processes, especially wear. We generalize a recent formulation for the indentation of a multilayered material using an efficient integral transform method, to the case of adhesion, using a simple energetic transformation in the JKR regime. Then, we specialize the study for the geometry of the Surface Force Apparatus, which consists of two thin layers on a substrate, where the intermediate layer is softer than the other two. We find the pull-off force under "force control" (i.e. for "soft" loading systems), as well as under "displacement control" (i.e. for "rigid" systems), as a function of the geometrical thicknesses and material properties ratios, and the method is fully implemented in a fast Mathematica code, available to the public (see Appendix).Publicationtype: Journal ArticleScopus© Citations 2 471 346