Emergence of the interplay between hierarchy and contact splitting in biological adhesion highlighted through a hierarchical shear lag model

Contact unit size reduction is a widely studied mechanism as a means to improve adhesion in natural fibrillar systems, such as those observed in beetles or geckos. However, these animals also display complex structural features in the way the contact is subdivided in a hierarchical manner. Here, we study the influence of hierarchical fibrillar architectures on the load distribution over the contact elements of the adhesive system, and the corresponding delamination behaviour. We present an analytical model to derive the load distribution in a fibrillar system, including hierarchical splitting of contacts, i.e. a "hierarchical shear-lag" model that generalizes the well-known shear-lag model used in mechanics. The influence on the detachment process is investigated introducing a numerical procedure that allows the derivation of the maximum delamination force as a function of the considered geometry, including statistical variability of local adhesive energy. Our study suggests that contact splitting generates improved adhesion only in the ideal case of infinitely compliant contacts. In real cases, to produce efficient adhesive performance, contact splitting needs to be coupled with hierarchical architectures to counterbalance high load concentrations resulting from contact unit size reduction, generating multiple delamination fronts and helping to avoid detrimental non-uniform load distributions. We show that these results can be summarized in a generalized adhesion scaling scheme for hierarchical structures, proving the beneficial effect of multiple hierarchical levels. The model can thus be used to predict the adhesive performance of hierarchical adhesive structures, as well as the mechanical behaviour of composite materials with hierarchical reinforcements.Comment: 33 pages, 7 figures, 1 table in pres

A hierarchical lattice spring model to simulate the mechanics of 2-D materials-based composites

It is known that structural biological materials such as bone or dentin show unprecedented damage tolerance, toughness, and strength. The common feature of these materials is their hierarchical heterogeneous structure, which contributes to increased energy dissipation before failure occurring at different scale levels. These structural properties are the key to achieve superior nanocomposites. Here, we develop a numerical model in order to simulate the mechanisms involved in damage progression and energy dissipation at different size scales in composites, which depend both on the heterogeneity of the material (defects or reinforcements) and on the type of hierarchical structure. Both these aspects have been incorporated into a 2-D model based on a lattice spring model approach, accounting for geometrical non-linearities and including statistically based fracture phenomena. The model has been validated by comparing numerical results to linear elastic fracture mechanics results as well as to finite elements simulations, and then employed to study how hierarchical structural aspects impact on composite material properties, which is the main novel feature of the approach. Results obtained with the numerical code highlight the dependence of stress distributions (and therefore crack propagation) on matrix properties and reinforcement dispersion, geometry, and properties, and how the redistribution of stresses after the failure of sacrificial elements is directly involved in the damage tolerance of the materia

Estrategias de desarrollo económico que generan dinámicas de bien común: conformación de las agendas de desarrollo del Centro Bajío Occidente y Sur-Sureste

Este breve ensayo recoge algunas de las prácticas de inteligencia colectiva que se están realizando para la conformación de las agendas de desarrollo del Centro Bajío Occidente y el Sur Sureste. Las prácticas de inteligencia colectiva que buscan identificar problemas compartidos y proponer soluciones comunes pueden  ser vistas como aproximaciones pragmáticas para traducir el bien común a políticas públicas

Estrategias de desarrollo económico que generan dinámicas de bien común: conformación de las agendas de desarrollo del Centro Bajío Occidente y Sur-Sureste

Este breve ensayo recoge algunas de las prácticas de inteligencia colectiva que se están realizando para la conformación de las agendas de desarrollo del Centro Bajío Occidente y el Sur Sureste. Las prácticas de inteligencia colectiva que buscan identificar problemas compartidos y proponer soluciones comunes pueden  ser vistas como aproximaciones pragmáticas para traducir el bien común a políticas públicas

The influence of substrate roughness, patterning, curvature, and compliance in peeling problems

NMP is supported by the European Commission under the Graphene FET Flagship (WP14 'Polymer composites' No. 604391) and FET Proactive 'Neurofibres' grant No. 732344. FB is supported by 'Neurofibres' grant No. 732344

An experimental and numerical study on the mechanical properties of carbon nanotube-latex thin films

This research was supported by the U.S. National Science Foundation (NSF) under grant number CMMI-CAREER 1253564 and supplement CMMI-CAREER 1542532. In addition, N.M.P. acknowledges support by the European Research Council (ERC StG Ideas 2011 BIHSNAM no. 279985, ERC PoC 2013-2 KNOTOUGH no. 632277, ERC PoC 2015 SILKENE no. 693670), by the European Commission under the Graphene Flagship (WP10 ‘Nanocomposites’, no. 604391) and by the Provincia Autonoma di Trento (‘Graphene nanocomposites’, no. S116/2012-242637 and reg. delib. no. 2266). L.B. and F.B. are supported by BIHSNAM. Computational resources were provided by HPC@POLITO (http://www.hpc.polito.it

Optimal adhesion control via cooperative hierarchy, grading, geometries and non-linearity of anchorages

Optimization of dry adhesion in biological organisms is achieved using various strategies at different scale levels. In the past, studies have shown how contact splitting is used effectively by animals such as geckos and insects to increase the total peeling line of contacts and therefore the adhesion force. Also, tapering of contacts or grading of their mechanical properties has been shown to be instrumental in the achievement of improved adhesion efficiency. On a more macroscopic scale, structures such as spider web anchorages exploit hierarchical structure or nonlinear constitutive material properties to improve resilience and to achieve tunability in adhesion/detachment characteristics. Here, we analyse some of these properties and propose some mechanisms for the optimization of adhesion that have thus far been neglected in modelling approaches, and could be potentially exploited for the design of bioinspired adhesives. We consider hierarchical structure, contact tapering, grading of mechanical properties, and their interaction. It emerges that these mechanisms contribute on various size scales to the achievement of optimal adhesive properties through structural complexity and hierarchical organization

An experimental-numerical study of the adhesive static and dynamic friction of micro-patterned soft polymer surfaces

New possibilities have emerged in recent years, with the development of high-precision fabrication techniques, to exploit microscale surface patterning to modify tribological properties of polymeric materials. However, the effect of surface topography, together with material mechanical parameters, needs to be fully understood to allow the design of surfaces with the desired characteristics. In this paper, we experimentally assess the effect of various types of micropatterned Polydimethylsiloxane surfaces, including anisotropic ones, on macroscopic substrate friction properties. We find that it is possible, through surface patterning, to modify both static and dynamic friction coefficients of the surfaces, demonstrating the possibility of achieving tunability. Additionally, we compare experimental observations with the numerical predictions of a 2D Spring-Block model, deriving the material parameters from tests on the corresponding flat surfaces. We find a good quantitative agreement between calculated and measured trends for various micropattern geometries, demonstrating that the proposed numerical approach can reliably describe patterned surfaces when appropriate material parameters are used. The presented results can further contribute to the description and understanding of the frictional effects of surface patterning, with the aim of achieving surfaces with extreme tunability of tribological propertie