Roberto Caruana-Gauci

Smart metamaterials with tunable auxetic and other properties

Auxetic and other mechanical metamaterials are typically studied in situations where they are subjected solely to mechanical forces or displacements even though they may be designed to exhibit additional anomalous behaviour or tunability when subjected to other disturbances such as changes in temperature or magnetic fields. It is shown that externally applied magnetic fields can tune the geometry and macroscopic properties of known auxetics that incorporate magnetic component/s, thus resulting in a change of their macroscopic properties. Anomalous properties which are observed in such novel magneto-mechanical systems include tunable Poisson’s ratios, bi-stability or multi-stability, depending on the applied magnetic fields, and other electromagnetic‐mechanical effects such as strain dependent induced electric currents and magnetic fields. The properties exhibited depend, amongst other things, on the relative position and orientation of the magnetic insertion/s within the structure, the geometry of the system and the magnetic strength of the magnetic components, including that of the external magnetic field.

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.

Mechanical metamaterials: Materials that push back

Proposed mechanical metamaterials that contract under tension and expand on compression represent a new approach to realize mechanical properties yet unknown in nature that could lead to applications in microelectromechanical systems.

Smart hexagonal truss systems exhibiting negative compressibility through constrained angle stretching

Negative compressibility is the ability to expand in at least one dimension rather than shrinking upon the application of an externally applied hydrostatic pressure. It is shown that, contrary to current perception, negative linear compressibility may be obtained from re-entrant hexagonal truss systems of specific geometric features which deform through non-equal changes in the lengths of the cell walls when deforming through a constrained angle stretching rather than other modes of deformation (such as flexure or hinging, modes of deformation that also lead to auxetic behaviour in honeycombs). Negative compressibility is predicted in the vertical direction for particular re-entrant geometries of this smart hexagonal truss system when the vertical ribs are much stiffer than the inclined ribs.

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.

Colossal magnetocaloric effect in magneto-auxetic systems

We show that a mechanically driven magnetocaloric effect (MCE) in magneto-auxetic systems (MASs) in the vicinity of room temperature is possible and the effect can be colossal. Even at zero external magnetic field, the magnetic entropy change in this reversible process can be a few times larger in magnitude than in the case of the giant MCE discovered by Pecharsky and Gschneidner in Gd5(Si2Ge2). MAS represent a novel class of metamaterials having magnetic insertions embedded within a non-magnetic matrix which exhibits a negative Poissons ratio. The auxetic behaviour of the non-magnetic matrix may either enhance the magnetic ordering process or it may result in a transition to the disordered phase. In the MAS under consideration, a spin 1/2 system is chosen for the magnetic component and the well-known Onsager solution for the two-dimensional square lattice Ising model at zero external magnetic field is used to show that the isothermal change in magnetic entropy accompanying the auxetic behaviour can take a large value at room temperature. The practical importance of our findings is that MCE materials used in present engineering applications may be further enhanced by changing their geometry such that they exhibit auxetic behaviour.

Negative linear compressibility: Giant response

Materials displaying negative linear compressibility are, at present, the exception rather than the rule. An unusually large and persistent example of this phenomenon in the molecular framework material zinc dicyanoaurate dramatically expands the range of mechanical responses conceivable in other materials.

Auxetic metamaterials inspired from wine-racks

Abstract‘Wine-rack’ motifs are formally shown to exhibit the unexpected property of a negative Poisson’s ratio (auxetic behaviour) for loading in particular directions. This property is confirmed through the analysis of analytical expressions for the in-plane off-axis mechanical properties derived for an idealised hinging wine-rack model as well as through molecular simulations of nanoscale molecular systems. It is also shown that auxeticity for loading off-axis complements the more well-known property of negative compressibility demonstrated in other directions which results from the very high positive Poisson’s ratio exhibited on-axis.