Inspired by the need to develop materials capable of targeted and extreme volume changes during operation, numerical simulations and experiments are combined to design a new class of soft architected materials that achieve a reduction of projected surface-area coverage during swelling.

Most of the auxetic materials that have been characterized experimentally or studied analytically are anisotropic and this limits their possible applications, as they need to be carefully oriented during operation. Here, through a combined numerical and experimental approach, we demonstrate that 2D auxetic materials with isotropic response can be easily realized by perforating a sheet with elongated cuts arranged to form a periodic pattern with either six-fold or three-fold symmetry. Moreover, we also show that the auxetic behavior can be easily tuned by varying the length of the cuts and that it is retained even under large levels of applied deformation beyond the limit of small strains. This novel, simple and scalable design can serve as an important guideline for designing and fabricating isotropic auxetic materials that can have a significant impact on a wide range of applications.

3D printing and numerical analysis are combined to design a new class of architected materials that contain bistable beam elements and exhibit controlled trapping of elastic energy. The proposed energy-absorbing structures are reusable. Moreover, the mechanism of energy absorption stems solely from the structural geometry of the printed beam elements, and is therefore both material- and loading-rate independent.

Submitted for the MAR14 Meeting of The American Physical Society Complex ordered patterns in mechanical instability induced geometrically frustrated triangular cellular structures SUNG KANG, SICONG SHAN, ANDREJ KOSMRLJ, WIM NOORDUIN, SAMUEL SHIAN, JAMES WEAVER, DAVID CLARKE, KATIA BERTOLDI, Harvard University — Geometrical frustration arises when a local order cannot propagate throughout the space due to geometrical constraints. It plays a major role in many natural and synthetic systems including water ice, spin ice, and metallic glasses. All of these geometrically frustrated systems are degenerate and tend to form disordered ground-state configurations. Here, we report a theoretical and experimental study on the behavior of buckling-induced geometrically frustrated triangular cellular structures. To our surprise, we find that mechanical instabilities induce complex ordered patterns with tunability. For structures with low porosity, an ordered symmetric pattern emerges, which shows striking correlations with the ideal spin solid. In contrast, for high porosity systems, an ordered chiral pattern forms with a new spin configuration. Our analysis using a spin-like model reveals that the connected geometry of the cellular structure plays a crucial role in the formation of ordered states in this system. Since in our study geometrical frustration is induced by a mechanical instability that is scale-independent, our findings can be extended to different materials, stimuli, and length scales, providing a general strategy to study and visualize the physics of frustration. Sung Kang Harvard University Date submitted: 07 Nov 2013 Electronic form version 1.4

Mechanical instabilities in periodic porous elastic structures may lead to the formation of homogeneous patterns, opening avenues for a wide range of applications that are related to the geometry of the system. This study focuses on an elastomeric porous structure comprising a triangular array of circular holes, and shows that by controlling the loading direction, multiple pattern transformations can be induced by buckling. Interestingly, these different pattern transformations can be exploited to design materials with highly tunable properties. In particular, these results indicate that they can be effectively used to tune the propagation of elastic waves in phononic crystals, enhancing the tunability of the dynamic response of the system. Using a combination of finite element simulations and experiments, a proof‐of‐concept of the novel material is demonstrated. Since the proposed mechanism is induced by elastic instability, it is reversible, repeatable, and scale‐independent, opening avenues for the design of highly tunable materials and devices over a wide range of length scales.

We investigate numerically and experimentally the effects of geometric and material nonlinearities introduced by deformation on the linear dynamic response of two-dimensional phononic crystals. Our results not only show that deformation can be effectively used to tune the band gaps and the directionality of the propagating waves, but also reveal how geometric and material nonlinearities contribute to the tunable response of phononic crystals. Our study provides a better understanding of the tunable response of phononic crystals and opens avenues for the design of systems with optimized properties and enhanced tunability.

Geometrical frustration arises when a local order cannot propagate throughout the space because of geometrical constraints. This phenomenon plays a major role in many systems leading to disordered ground-state configurations. Here, we report a theoretical and experimental study on the behavior of buckling-induced geometrically frustrated triangular cellular structures. To our surprise, we find that buckling induces complex ordered patterns which can be tuned by controlling the porosity of the structures. Our analysis reveals that the connected geometry of the cellular structure plays a crucial role in the generation of ordered states in this frustrated system.

We report a new class of tunable and switchable acoustic metamaterials comprising resonating units dispersed into an elastic matrix. Each resonator consists of a metallic core connected to the elastomeric matrix through elastic beams, whose buckling is intentionally exploited as a novel and effective approach to control the propagation of elastic waves. We first use numerical analysis to show the evolution of the locally resonant band gap, fully accounting for the effect of nonlinear pre-deformation. Then, we experimentally measure the transmission of vibrations as a function of the applied loading in a finite-size sample and find excellent agreement with our numerical predictions. The proposed concept expands the ability of existing acoustic metamaterials by enabling tunability over a wide range of frequencies. Furthermore, we demonstrate that in our system the deformation can be exploited to turn on or off the band gap, opening avenues for the design of adaptive switches.

Shape programmable structures and devices are typically fabricated using shape memory materials and are attracting increasing interest because they can be designed to have multiple properties and functions. However, shape memory materials, such as NiTi-based metal alloys and block copolymers, suffer from severe structural fatigue due to the microstructural changes occurring during each thermal cycle. Moreover, there is only a limited set of shape memory material systems, limiting the possible number of applications for reconfi gurable structures and devices. To overcome all these issues and expand the range of applications of shape programmable structures, we designed a library of 2D elastic structures that can be programmed to retain multiple shapes simply by applying a force. The building block of our structure consists of a unit comprising interconnected elastic arches and we demonstrate both numerically and experimentally that geometric nonlinearity and snap-through instabilities can be effectively exploited to reconfi gure the system into multiple shapes. Because our system exploits mechanical instability, our fi ndings can be extended to different materials and length scales, outlining a general strategy to effectively design a new generation of shape programmable structures.

Buckling-induced reversible symmetry breaking and amplification of chirality using macro- and microscale supported cellular structures is described. Guided by extensive theoretical analysis, cellular structures are rationally designed, in which buckling induces a reversible switching between achiral and chiral configurations. Additionally, it is demonstrated that the proposed mechanism can be generalized over a wide range of length scales, geometries, materials, and stimuli.

Soft cellular structures that comprise a solid matrix with a square array of holes open avenues for the design of novel soft and foldable structures. Our results demonstrate that by simply changing the shape of the holes the response of porous structure can be easily tuned and soft structures with optimal compaction can be designed.

Most materials have a unique form optimized for a specific property and function. However, the ability to reconfigure material structures depending on stimuli opens exciting opportunities. Although mechanical instabilities have been traditionally viewed as a failure mode, here we exploit them to design a class of 2D soft materials whose architecture can be dramatically changed in response to an external stimulus. By considering geometric constraints on the tessellations of the 2D Euclidean plane, we have identified four possible periodic distributions of uniform circular holes where mechanical instability can be exploited to reversibly switch between expanded (i.e. with circular holes) and compact (i.e. with elongated, almost closed elliptical holes) periodic configurations. Interestingly, in all these structures buckling is found to induce large negative values of incremental Poisson's ratio and in two of them also the formation of chiral patterns. Using a combination of finite element simulations and experiments at the centimeter scale we demonstrate a proof-of-concept of the proposed materials. Since the proposed mechanism for reconfigurable materials is induced by elastic instability, it is reversible, repeatable and scale-independent.

The attention here is focused on the magnetoelectric (ME) coupling in Terfenol-D/P(VDF-TrFE)/Terfenol-D laminated composites. The electroactive P(VDF-TrFE) 75/25mol% copolymer films were prepared using three different annealing conditions: 135°C for 2h , 6h and 140°C for 2h. The results show that the laminate with the copolymer film annealed at 135°C for 2h exhibits the highest ME coupling, while the one with the copolymer film annealed at 140°C for 2h shows the lowest. Microstructure examination demonstrates that the crystallites grow significantly in size with either the annealing time or the temperature although the crystallinity increases slightly. It is suggested that the copolymer film with smaller crystallite size, or higher volume fraction of the amorphous/crystallite interfacial layers, is beneficial to the piezoelectric effect and consequently the ME coupling of the laminated composites.

Mitochondria are highly-dynamic organelles, but it is challenging to monitor quantitatively their dynamics in a living cell. Here we developed a novel approach to determine the global occurrence of mitochondrial fission and fusion events in living human epithelial cells (Hela) and mouse embryonic fibroblast cells (MEF). Distinct patterns of sequential events including fusion followed by fission (Fu-Fi), the so-called "kiss and run" model previously described, fission followed by fusion (Fi-Fu), fusion followed by fusion (Fu-Fu), and fission followed by fission (Fi-Fi) were observed concurrently. The paired events appeared in high frequencies with short lifetimes and large sizes of individual mitochondria, as compared to those for unpaired events. The high frequencies of paired events were found to be biologically significant. The presence of membrane uncoupler CCCP enhanced the frequency of paired events (from both Fu-Fi and Fi-Fu patterns) with a reduced mitochondrial size. Knock-out of mitofusin protein Mfn1 increased the frequency of fission with increased lifetime of unpaired events whereas deletion of both Mfn1 and Mfn2 resulted in an instable dynamics. These results indicated that the paired events were dominant but unpaired events were not negligible, which provided a new insight into mitochondrial dynamics. In addition to kiss and run model of action, our data suggest that, from a global visualization over an entire cell, multiple patterns of action appeared in mitochondrial fusion and fission.