When materials are compressed along a particular axis they are most commonly observed to expand in directions orthogonal to the applied load. The property that characterizes this behavior is the Poisson's ratio which is defined as the ratio between the negative transverse and longitudinal strains. The majority of materials are characterized by a positive Poisson's ratio which is approximately 0.5 for rubber and 0.3 for glass and steel. Materials with a negative Poisson's ratio will contract (expand) in the transverse direction when compressed (stretched) and, although they can exist in principle, demonstration of practical examples is relatively recent. Discovery and development of materials with negative Poisson's ratio, also called auxetics, was first reported in the seminal work of Lakes in 1987. [1] There is significant interest in the development of auxetic materials because of tremendous potential in applications in areas such the design of novel fasteners [2] , prostheses [3] , piezocomposites with optimal performance [4] and foams with superior damping and acoustic properties [5] . The results of many investigations [6][7] suggest that the auxetic behavior involves an interplay between the microstructure of the material and its deformation. Examples of this are provided by the discovery that metals with a cubic lattice [8] , natural layered ceramics [9] , ferroelectric polycrystalline ceramics [10] and zeolites [11] may all exhibit negative Poisson's ratio behavior. Moreover, several geometries and mechanisms have been proposed to achieve negative values for the Poisson's ratio, including foams with reentrant structures [1] , hierarchical laminates [12] , polymeric and metallic foams [13] , microporous polymers [14] , molecular networks [15] and manybody systems with isotropic pair interactions [16] . Negative Poisson's ratio effects have also been demonstrated at the micron scale using complex materials which were fabricated using soft lithography [17] and at the nanoscale with sheets assemblies of carbon nanotubes [18] .A significant challenge in the fabrication of materials with auxetic properties is that it usually involves embedding structures with intricate geometries within a host matrix. As such, the manufacturing process has

It is becoming increasingly recognized that nonlinear phenomena give an opportunity to provide robust control of the properties of soft metamaterials. A class of elastic instabilities are discussed which arise when a soft cellular material is compressed. The global nature of the induced pattern switch makes it a prime candidate for controlling macroscopic photonic and auxetic properties of the material. We demonstrate the robustness of the phenomena using a range of soft materials and show how the shape of the repeat unit of the periodic pattern can be used to influence the global characteristics of the soft solid.

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We report the results of an experimental and numerical investigation into a novel pattern transformation induced in a regular array of particles with contrasting dimensions and softness. The results indicate new directions for the creation of soft solids with tunable acoustic and optical properties.

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