Bi-mode artificial metamaterials have anisotropic mechanical properties, with the ratio of bulk modulus and shear modulus approaching an infinite value in ideal conditions. As a type of 'quasi-fluid' material composed of solid material, it has potential applications in the design of functional devices with desired acoustic, thermal, and mechanical performance. However, the microstructures of such metamaterials are currently mostly determined by parameter synthesis on the basis of existing heuristic configuration designs, which may considerably restrict their topologies and shapes. In this study, new octagon and hexagonal honeycomb bi-mode metamaterials (two-dimensional) are designed with a more systematic approach based on the independent point-wise interpolation method of topology optimization. The objective function is defined as a weighted combination of the bulk and shear moduli.By tuning the values of different weighted coefficients, the transition mechanism can be acquired from the regular microstructure to the bi-mode metamaterial with needle-like or double-cone rods. It is also found that simply increasing the volume fraction in the single material design cannot further improve the target performance, but introducing a small amount of hard material into the design domain can noticeably enhance the bulk modulus. One representative optimized microstructure is fabricated by 3D printing with stainless steel and polymer materials. Uniaxial quasi-static compression tests and finite element simulations reveal the layer-wise deformation modes of the bi-mode 'quasi-fluid' metamaterial and its capacity to absorb external energy.
Active topological phase transitions widely occur in active matters and biological systems, such as developing embryos. Since the discovery of the intriguing bulk-boundary effects of topological insulators in Hermitian and non-Hermitian systems, various electric, optical, acoustic, and mechanical topological metamaterials with efficient energy transmission and robust defect-immunization have been designed. To date, however, it remains a challenge to precisely and fast manipulate the topological phase transitions in elastic topological insulators. In this paper, on the basis of theoretical analysis and numerical simulations, we propose an active strategy to achieve this aim through a combination of pneumatic actuation and liquid metals. The proposed method can precisely tune the connecting stiffness and vertex mass in the tight Su–Schrieffer–Heeger model. Thus, we realize the effective and fast control of topological phase transitions and elastic wave bandgap switching. We also uncover the active spinning bulk-boundary effects and higher-order topological states in the elastic topological insulators, demonstrating the high effectiveness and practicability of the proposed method. In addition, the differences between the 1D edge and 0D corner higher-order states are specified by information entropy theory. This work not only gains insights into the active manipulation of topological phase transitions but also inspires novel strategies to design active topological materials through untethered methods, e.g., magnetism or biological cells.
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