Auxetics exist in nature (classes of zeolites and crystobalites), and are also man-made (honeycombs, composites and foams). The equivalent of auxetic behaviour has been however in the first generation of Magnox reactors, with the key-brick layout designed to withstand in-plane motion during seismic loading and thermal mismatch (Figure 1).
Auxetics show significant enhancement of energy dissipation during impact and high amplitude dynamic loading, high indentation resistance and uncommon filtering/sieving properties. An auxetic filter would release particles/components with selected dimensions because of the enlargement of its unit cells when swollen by a liquid (Figure 2).
Conversely, auxetic porous filters could absorb liquids at higher volume than conventional porous materials. Trials made from Bristol-produced polyurethane auxetic foams have shown that NPR foam samples with the same dimensions of conventional ones absorb between two to four times the water volumes of the conventional base foam from which they are derived (Figure 3). The data suggest that auxetic porous materials may offer paradigmatic shifts in terms of sieving and mass transport properties, compared to the current available products.
Auxetic porous structures can be made in principle from the transformation of existing porous materials, from open to closed-cell foams, to some types of aerogels. A technological route to produce novel auxetic filters can be therefore identified and developed.
Prof. Fabrizio Scarpa
Prof. Fabrizio Scarpa has research interests in the field of auxetics - negative Poisson's ratio and negative stiffness materials. He designs, models and manufactures auxetic foams, composites and cellular structures for structural integrity, vibration damping, vibroacoustiics and multifunctional applications. He also develops predictive micromechanical and structural models for energy dissipation performance in composite and polymeric structures. Another set of research activities Prof. Scarpa carries out is about nanostructures modelling and design, with particular emphasis on graphene, CNTs, BN, ZnO and GaN nanostructures. Special focus of the nanostructures-related activities is about the mechanical characterisation of the nanomaterials for applications ranging between nanocomposites to NEMS. He is also involved in the modelling, manufacturing and characterization of natural fibre reinforcements for composites (sisal, agave, cactus), and recycled polymers (originated from rubber tyres).