Geometric reconfigurations in cellular structures have recently been exploited to realize adaptive materials with applications in mechanics, optics, and electronics. However, the achievable symmetry breakings and corresponding types of deformation and related functionalities have remained rather limited, mostly due to the fact that the macroscopic geometry of the structures is generally co‐aligned with the molecular anisotropy of the constituent material. To address this limitation, cellular microstructures are fabricated out of liquid crystalline elastomers (LCEs) with an arbitrary, user‐defined liquid crystal (LC) mesogen orientation encrypted by a weak magnetic field. This platform enables anisotropy to be programmed independently at the molecular and structural levels and the realization of unprecedented director‐determined symmetry breakings in cellular materials, which are demonstrated by both finite element analyses and experiments. It is illustrated that the resulting mechanical reconfigurations can be harnessed to program microcellular materials with switchable and direction‐dependent frictional properties and further exploit ”area‐specific” deformation patterns to locally modulate transmitted light and precisely guide object movement. As such, the work provides a clear route to decouple anisotropy at the materials level from the directionality of the macroscopic cellular structure, which may lead to a new generation of smart and adaptive materials and devices.

Problems of flexible mechanical metamaterials, and highly deformable porous solids in general, are rich and complex due to nonlinear mechanics and nontrivial geometrical effects. While numeric approaches are successful, analytic tools and conceptual frameworks are largely lacking. Using an analogy with electrostatics, and building on recent developments in a nonlinear geometric formulation of elasticity, we develop a formalism that maps the elastic problem into that of nonlinear interaction of elastic charges. This approach offers an intuitive conceptual framework, qualitatively explaining the linear response, the onset of mechanical instability and aspects of the post-instability state. Apart from intuition, the formalism also quantitatively reproduces full numeric simulations of several prototypical structures. Possible applications of the tools developed in this work for the study of ordered and disordered porous mechanical metamaterials are discussed. arXiv:1910.01953v1 [cond-mat.soft]

Nonreciprocity can be passively achieved by harnessing material nonlinearities. In particular, networks of nonlinear bistable elements with asymmetric energy landscapes have recently been shown to support unidirectional transition waves. However, in these systems energy can be transferred only when the elements switch from the higher to the lower energy well, allowing for a one-time signal transmission. Here, we show that in a mechanical metamaterial comprising a 1D array of bistable arches nonreciprocity and reversibility can be independently programmed and are not mutually exclusive. By connecting shallow arches with symmetric energy wells and decreasing energy barriers, we design a reversible mechanical diode that can sustain multiple signal transmissions. Further, by alternating arches with symmetric and asymmetric energy landscapes we realize a nonreciprocal chain that enables propagation of different transition waves in opposite directions.

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Programmable Anisotropic Transformations In article number 2105024, Joanna Aizenberg, Katia Bertoldi, and co‐workers report that by programming independently anisotropy at the molecular and structural levels, unprecedented director‐determined symmetry breaking can be realized in microcellular structures made of liquid crystalline elastomers (LCEs). These can be exploited to achieve switchable and direction‐dependent frictional properties as well as to modulate light.