Daphne Attard

Tailoring Graphene to Achieve Negative Poisson's Ratio Properties

Graphene can be made auxetic through the introduction of vacancy defects. This results in the thinnest negative Poissons ratio material at ambient conditions known so far, an effect achieved via a nanoscale de-wrinkling mechanism that mimics the behavior at the macroscale exhibited by a crumpled sheet of paper when stretched.

Hierarchical Auxetic Mechanical Metamaterials

Auxetic mechanical metamaterials are engineered systems that exhibit the unusual macroscopic property of a negative Poissons ratio due to sub-unit structure rather than chemical composition. Although their unique behaviour makes them superior to conventional materials in many practical applications, they are limited in availability. Here, we propose a new class of hierarchical auxetics based on the rotating rigid units mechanism. These systems retain the enhanced properties from having a negative Poissons ratio with the added benefits of being a hierarchical system. Using simulations on typical hierarchical multi-level rotating squares, we show that, through design, one can control the extent of auxeticity, degree of aperture and size of the different pores in the system. This makes the system more versatile than similar non-hierarchical ones, making them promising candidates for industrial and biomedical applications, such as stents and skin grafts.

Auxetic behaviour from connected different-sized squares and rectangles

Auxetic materials exhibit the unusual property of becoming fatter when uniaxially stretched and thinner when uniaxially compressed (i.e. they exhibit a negative Poisson ratio; NPR), a property that may result in various enhanced properties. The NPR is the result of the manner in which particular geometric features in the micro- or nanostructure of the materials deform when they are subjected to uniaxial loads. Here, we propose and discuss a new model made from different-sized rigid rectangles, which rotate relative to each other. This new model has the advantage over existing models that it can be used to describe the properties of very different systems ranging from silicates and zeolites to liquid-crystalline polymers. We show that such systems can exhibit scale-independent auxetic behaviour for stretching in particular directions, with Poisson’s ratios being dependent on the shape and relative size of different rectangles in the model and the angle between them.

Three-dimensional cellular structures with negative Poisson's ratio and negative compressibility properties

A three-dimensional cellular system that may be made to exhibit some very unusual but highly useful mechanical properties, including negative Poissons ratio (auxetic), zero Poissons ratio, negative linear and negative area compressibility, is proposed and discussed. It is shown that such behaviour is scale-independent and may be obtained from particular conformations of this highly versatile system. This model may be used to explain the auxetic behaviour in auxetic foams and in other related cellular systems; such materials are widely known for their superior performance in various practical applications. It may also be used as a blueprint for the design and manufacture of new man-made multifunctional systems, including auxetic and negative compressibility systems, which can be made to have tailor-made mechanical properties.

On the auxetic properties of generic rotating rigid triangles

Materials having a negative Poissons ratio (auxetic) get fatter rather than thinner when uniaxially stretched. This phenomenon has been often explained through models that describe how particular geometric features in the micro or nanostructure of the material deform when subjected to uniaxial loads. Here, a new model based on scalene rigid triangles rotate relative to each other will be presented and analysed. It is shown that this model can afford a very wide range of Poissons ratio values, the sign and magnitude of which depends on the shape of the triangles and the angles between them. This new model has the advantage that it is very generic and may be potentially used to describe the properties in various types of materials, including auxetic foams and their relative surface density. Specific applications of this model, such as a blueprint for a system that can exhibit temperature-dependent Poissons ratios, are also discussed.

Unusual Thermoelastic Properties of Methanol Monohydrate

A simple material can shrink in one direction when heated, and expand in that direction when squeezed by hydrostatic pressure When a material is heated, we expect it to expand, and when squeezed on all sides by hydrostatic pressure, we expect it to shrink, but materials or structures can respond in unexpected ways. Some may exhibit negative thermal expansion (NTE)—shrinking when heated (1–8)—or negative compressibility (NC) (8–13)—expanding when subjected to a positive hydrostatic pressure—and some may exhibit both properties. For crystalline materials, the response can be anisotropic—observed along only some directions in a crystal (see the figure), and may only be observed in particular ranges of temperatures or pressures. On page 742 in this issue, Fortes et al. (8) report such anomalous properties from an experimental investigation on methanol monohydrate, a simple molecular crystal of a deuterated 1:1 compound of methanol and water. Such materials, used singly or in combination with conventional materials, can have useful mechanical and optical properties.

On the mechanical properties and auxetic potential of various organic networked polymers

We simulate and analyse three types of two-dimensional networked polymers which have been predicted to exhibit on-axis auxetic behaviour (negative Poissons ratio), namely (1) polyphenylacetylene networks that behave like flexing re-entrant honeycombs, commonly referred to as ‘reflexynes’, (2) polyphenylacetylene networks that mimic the behaviour of rotating triangles, commonly referred to as ‘polytriangles’ and (3) networked polymers built from calix[4]arene units. More specifically, we compute and compare their in-plane off-axis mechanical behaviour, in particular their off-axis Poissons ratios and show that in some cases, the sign and magnitude of the Poissons ratio are dependent on the direction of loading. We propose two functions that can provide a measure for the extent of auxeticity for such anisotropic materials and show that the polytriangles are predicted as the most auxetic when compared with the other networks with the reflexyne re-entrant networks being the least auxetic.

An improved analytical model for the elastic constants of auxetic and conventional hexagonal honeycombs

Cellular solids, in particular hexagonal honeycombs have been the subject of numerous studies in the last decades in view of their extensive use in many applications. In particular, there have been various studies aimed at expressing the mechanical properties of honeycombs in terms of the geometrical parameters used to describe the structure of such honeycombs. Despite improvements over the first established model, finite element simulations performed in this work on honeycombs having ribs with a realistic thickness-to-length ratio suggest that the mechanical properties for such systems differ from those predicted by current models, sometimes to a very significant extent. In view of this, we analyse in detail the deformed structures in an attempt to gain insight into how and the extent to which the shape of the ligaments, in particular its thickness and mode of connection affects deformation in conventional and re-entrant hexagonal honeycombs. Based on these observations, we propose a modified version of the previous analytical models that take into consideration the finite thickness of the ligaments.

Unimode metamaterials exhibiting negative linear compressibility and negative thermal expansion

Unimode metamaterials made from rotating rigid triangles are analysed mathematically for their mechanical and thermal expansion properties. It is shown that these unimode systems exhibit positive Poissons ratios irrespective of size, shape and angle of aperture, with the Poissons ratio exhibiting giant values for certain conformations. When the Poissons ratio in one loading direction is larger than +1, the systems were found to exhibit the anomalous property of negative linear compressibility along this direction, that is, the systems expand in this direction when hydrostatically compressed. Also discussed are the thermal expansion properties of these systems under the assumption that the units exhibit increased rotational agitation once subjected to an increase in temperature. The effect of the geometric parameters on the aforementioned thermo-mechanical properties of the system, are discussed, with the aim of identifying negative behaviour.

On the dynamics and control of mechanical properties of hierarchical rotating rigid unit auxetics

In this work, we investigate the deformation mechanism of auxetic hierarchical rotating square systems through a dynamics approach. We show how their deformation behaviour, hence their mechanical properties and final configuration for a given applied load, can be manipulated solely by altering the resistance to rotational motion of the hinges within the system. This provides enhanced tunability without necessarily changing the geometry of the system, a phenomenon which is not typically observed in other non-hierarchical unimode auxetic systems. This gives this hierarchical system increased versatility and tunability thus making it more amenable to be employed in practical application which may range from smart filtration to smart dressings.