Elena Pierro

Sticky Bio-inspired Micropillars: Finding the Best Shape

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.

Adhesion tilt-tolerance in bio-inspired mushroom-shaped adhesive microstructure

We studied experimentally and theoretically the effect of different tilt angles on the adhesion of mushroom-shaped adhesive microstructures. The marginal measured influence of tilting on pull-off forces is quantitatively well confirmed by numerical and theoretical calculations and was shown to be a direct consequence of an optimized stress distribution. In addition, the presence of a joint-like narrowing under the contact elements, as found in some biological attachment systems, was shown to further contribute to the tilt-tolerance. The results obtained allow us to explain the advantage of the widely observed mushroom-shaped contact geometry in nature for long-term and permanent adhesion.

Effect of interfacial air entrapment on the adhesion of bio-inspired mushroom-shaped micro-pillars

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.

Loading-unloading hysteresis loop of randomly rough adhesive contacts

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.

The Influence of the Fractal Dimension of Rough Surfaces on the Adhesion of Elastic Materials

Abstract 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 Df 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 Df is negligible. However, the scenario strongly changes at higher loads as Df 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.

Contact versus noncontact measurement of a large composite fuselage panel

This paper presents a comprehensive study between accelerometer, laser vibrometer and microflown probe measurements aimed at comparison of modal model quality assessment. Object of an investigation was a large composite fuselage panel. An extensive test campaign was performed with application of SIMO, MIMO, random and harmonic excitation, velocity and acceleration sensors, contact and non-contact measurement techniques. Advantages and disadvantages of applied instrumentation are discussed taking into account test data variability and the trade-off between workload and test data quality. Presented are real-life measurement problems related to the specific set up conditions. Finally a statistical analysis of estimated models is evaluated to bring to light general assessment of test campaign. Such assessment has a vital importance of successful fault detection based on modal parameters observation as well as in uncertain non-deterministic numerical model updating.

Contact Mechanics of Mushroom-Shaped Adhesive Structures

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 surfaces covered with cylindrical micropillars 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 chapter 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. Comparisons with experiments are shown, to assess the theoretical findings, and the influence of the effect of tilted pull-off on the adhesion of individual mushroom shaped pillar is evaluated.

Advanced Vibro-Acoustic Techniques for Noise Control in Helicopters

Originally, aircraft noise was not much of an issue because of the overarching requirement of improving vehicle performance in critical operational conditions. For many years, the design of aircraft and helicopter has been challenged by the need to improve aeromechanical performance, leaving little rooms for considering additional design parameters such as noise, vibrations and passenger comfort. More recently, a paradigm shift is being recorded that includes vibro-acoustics comfort in the set of critical design parameters. In the particular case of helicopters, this is mostly a consequence of a shift in the definition of the typical helicopter mission: in the past helicopters were mainly used in military context where flight mechanics, maneuverability and avionics are the key performance indicators; todays a trend is visible in the related industry that targets more and more civil applications, where the payload and environmental impact assume a much more significant role. In this respect, vibroacoustic comfort and more in general noise performance are becoming one of the key market differentiators: passenger comfort is included in the key performance indicators (ref. the US101 Marine One case). In order to cope with this new and more stringent set of performance requirements, specific tools and techniques are required that allow better identifying the noise sources, the mechanics of transfer from source to target (pilot and/or passenger area) and defining an effective noise control strategy. In this scenario, the authors want to present a modus operandi to tackle these issues by several advanced experimental methodologies, since the requirement of a quieter helicopter needs a systematic study of its Noise Vibration and Harshness (NVH) behaviour. This chapter collects the results of several investigations carried out on two different helicopters: EUROCOPTER EC-135 and Agusta Westland AW-109. The proposed methodology faces these issues considering that the noise is a result of the contribution of source(s) reaching the receiver via various transmission paths. Therefore, in a noise control strategy , the designer should consider both the excitation phenomena (i.e. sources) as well as the transfer functions between the source(s) and receiver(s) (i.e. transmission paths). The sections of this chapter discuss both sources and transmission paths. In particular, Sections 2 addresses the deep analysis of transmission paths, taking into account the vibro-acoustic interaction that in cumbersome systems as helicopters is of pivotal importance; Sections 2 and 3 deal with noise and vibration sources, 6

Experimental Guidelines for NVH Improvements in Helicopter Vibro-Acoustic Comfort

The noise of helicopters has started to become an important issue and the next level technical challenge is to include new design parameters such as vibro-acoustic comfort in the design process of a modern rotorcraft. In this scenario a wide experimental campaign has been performed on a EC-135 helicopter cabin in order to assess the effectiveness of several techniques for NVH (Noise Vibration and Harshness) improvements. Vibro-acoustical modal analyses of the cabin internal and external surface as well as a pure acoustical modal analysis of the cabin enclosure have been performed in order to verify the vibro-acoustical coupling between the cabin cavity and the cabin mechanical structure. Secondly a detailed leakage point detection on the cabin walls has been carried out by a few techniques finding out possible noise sources. Through such experimental analyses, some guidelines are suggested as well as particular techniques/instrumentations in order to improve human comfort on the helicopter cabin.Copyright