The superlative adhesive properties of some biological attachment systems, such as those of geckos, spiders, and insects, have inspired researchers from different fields (e.g. biology, physics and engineering) to conceive and design man-made microstructured surfaces that might mimic their performance. Among the several proposed designs, very recently mushroom-shaped adhesive microstructures have drawn the interest of scientists and engineers, because experiments have proved their superiority compared to other micro- and nano-structures. In this article, we explain theoretically the physical mechanism behind the enhanced adhesion of such microstructures, and provide for the first time a useful tool to predict adhesive performance depending on the geometry, mechanical properties of the material, and energy of adhesion. Our theoretical predictions are strongly supported by the available experimental data. The present study can streamline the optimisation of adhesive microstructures for industrial applications

Very recently, both experimental and theoretical investigations have shown that microstructured surfaces covered with mushroom-shaped micropillars present strongly enhanced adhesive properties if compared to flat surfaces made of the same material. However, different geometries lead to different adhesive performance, and finding the optimal solution has become of utmost importance. This paper presents on which physical basis the optimal mushroom pillar shape should be sought, and it provides a relatively simple methodology to achieve the result. Calculations demonstrate that the adhesive performance of the pillar strongly depends on the geometry of the terminal plate. The best performance is achieved when the ratio s/R(i) between the plate thickness (s) and the pillar internal radius (R(i)) is close to 0.2-0.3, and the ratio R(e)/R(i) is larger than 2, where R(e) is the external radius of the plate.

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In this paper we investigate the loading and unloading behavior of soft solids in adhesive contact with randomly rough profiles. The roughness is assumed to be described by a self-affine fractal on a limited range of wave-vectors. A spectral method is exploited to generate such randomly rough surfaces. The results are statistically averaged, and the calculated contact area and applied load are shown as a function of the penetration, for loading and unloading conditions. We found that the combination of adhesion forces and roughness leads to a hysteresis loading-unloading loop. This shows that energy can be lost simply as a consequence of roughness and van der Waals forces, as in this case a large number of local energy minima exist and the system may be trapped in metastable states. We numerically quantify the hysteretic loss and assess the influence of the surface statistical properties and the energy of adhesion on the hysteresis process.

Recent theoretical and experimental studies have shown that mushroom shaped micro-pillars exhibit strongly enhanced adhesive performance in comparison to other pillar shapes. However, in the presence of interfacial impurities (e.g. solid particles or air bubbles) the adhesive strength could drastically drop. In this paper we theoretically investigate the effect of the entrapment of micro-bubbles of air at the interface between the mushroom shaped micro-pillar and a rigid substrate on the adhesive performance. We calculate the critical pull-off stress as a function of the initial volume of the entrapped air, and compare these results with those obtained when, instead of air, small external solid particles are entrapped at the interface. Our results show that the presence of entrapped air is more critical since it strongly reduces the suction effect. The critical stress, indeed, is about 35-40% smaller than the value observed in the case of solid particles, thus resulting in a considerable reduction of the adhesive performance of the mushroom shaped pillar

Very recently, both experimental and theoretical investigations have shown that micro-structured surfaces covered with mushroom shaped micropillars present strongly enhanced adhesive properties if compared to standard flat surfaces made of the same material. However, different geometries lead to different adhesive performance, and finding the optimal solution has become of utmost importance. In this review we summarize the main detachment mechanisms of flat-topped and mushroom-topped soft micro pillars and show how the geometry of the pillars should be designed in order to obtain the best adhesive performances. We also discuss the effect of air entrapment at the interface between the pillar and the substrate and investigate the influence of the non uniform pillar height and thermal fluctuations on pull-off force.

Viscoelasticity plays a key role in many practical applications and in different reasearch fields, such as in seals, sliding-rolling contacts and crack propagation. In all these contexts, a proper knowledge of the viscoelastic modulus is very important. However, the experimental characterization of the frequency dependent modulus, carried out through different standard procedures, still presents some complexities, then possible alternative approaches are desirable. For example, the experimental investigation of viscoelastic beam dynamics would be challenging, especially for the intrinsic simplicity of this kind of test. This is why, a deep understanding of damping mechanisms in viscoelastic beams results to be a quite important task to better predict their dynamics. With the aim to enlighten damping properties in such structures, an analytical study of the transversal vibrations of a viscoelastic beam is presented in this paper. Some dimensionless parameters are defined, depending on the material properties and the beam geometry, which enable to shrewdly design the beam dynamics. In this way, by properly tuning such disclosed parameters, for example the dimensionless beam length or a chosen material, it is possible to enhance or suppress some resonant peaks, one at a time or more simultaneously. This is a remarkable possibility to efficiently control damping in these structures, and the results presented in this paper may help in elucidating experimental procedures for the characterization of viscoelastic materials.

In this paper we analyse the adhesion between a rubber block and a rigid randomly rough profile. The focus of the investigation is on the influence of the work of adhesion and of the fractal dimension D f of the rough profile on the contact behaviour. In particular, we analyse how the contact area and the power spectral density of the deformed profile are affected by the two aforementioned quantities. We find that at sufficiently small loads the influence of D f is negligible. However, the scenario strongly changes at higher loads as D f strongly affects the number of contact spots. Calculations show that the contact area depends linearly on the work of adhesion, whereas only a negligible influence of the work of adhesion is found on the power spectral density (PSD) of the deformed profile.