Vibration with different frequencies widely distributed in natural environments, living creatures, and artificial machines could be a nearly bottomless, locally obtained, and green energy source. By collecting ubiquitous vibrations through micro-transducers, it becomes possible to power wearable consumer electronics integrated in clothes, implantable devices in human bodies, portable terminals in the Internet of Things and even unsupervised vehicles in harsh environments. However, it’s a long-standing challenge to efficiently utilize the dispersed vibration energy especially within the high frequency range, since the amplitudes of the high frequency vibrations at local parts of objects are usually relatively weak.
Elastic artificial materials including phononic crystals (PCs) with point defects, graded refractive index materials, mechanical metamaterials and triboelectric nanogenerators with structures bring opportunities to concentrate vibration energy, while the concentration efficiency, working bandwidth, and deformability of current methods limit their further application in real life.
Among these methods, the acoustic black hole (ABH) approach is an appealing way to dramatically concentrate vibrational energy. An ABH design is a wedge-shaped structure with a decrease in thickness obeying the power-law profile. Theoretically, the group velocity of incident elastic waves will be reduced to zero when waves propagate to the tip of the ABH. At the same time, amplitude of vertical displacement will become infinite, resulting in non-reflection and extreme concentration of wave energy at the tip. However, a truncated tip of ABH in practice does exist due to limitations in fabrication, thus greatly weakening the ABH performance and resulting in large reflecting coefficients (as large as 50%–70%). In addition, the rigid, irregular structure of traditional ABHs reduces the strength of the whole device and requires extra protection of the wedge tip.
In a new article published in the National Science Review, scientists at the Huazhong University of Sci. & Tech. in Wuhan, China present the latest advances in concentrating vibrations at broad frequencies. Co-authors Hanchuan Tang, Zhuoqun Hao, Ying Liu, Ye Tian, Hao Niu, and Jianfeng Zang report a soft and disordered hyperuniform elastic metamaterials (DHEM). Their strategy has achieved a remarkably high efficiency vibration concentration in broad frequency band. The maximum enhancement factor could reach to ~4000 at 1930 Hz, which is almost two orders higher than the reported values. The DHEM design using soft material with rational sizes from ~1 cm to ~100 cm covers a broad range of frequencies from ~100 Hz to ~10 kHz, which are emitted by many vibration sources including domestic appliances, factories and transportation systems, for example. Moreover, the performance of DHEM under bending and compressing is validated using soft materials, enabling the conformal attachment on uneven objects.
“Our study lays the groundwork for fully utilizing the vibration energy that is widely distributed in nature and provides a promising way to enhance vibration sensing in industry, reduce traditional energy consumption, charge long-term working devices in the Internet of Things or unmanned detection vehicles, and, as a result, build a greener world,” they state in an article titled “Soft and disordered hyperuniform elastic metamaterials for highly efficient vibration concentration.”
Soft DHEM is designed by arranging steel rods into soft polymer matrix according to a special disorder distribution called disordered hyperuniformity, constructing an internal reflection interface to approximate the theoretical perfect acoustic black hole.
“The essence of DHEMs is to replace the wedge structure of ABHs with DHPSs in order to reduce the difficulty of fabrication and to provide a realization of the theoretical ABH. The simulation results show that the DHEM allows some extent manufacturing deviation. Moreover, the DHEM makes the power-law profile an internal instead of an external border in the ABH case, enabling the DHEM to have an arbitrary external shape (e.g. a rectangle with better structural stability instead of a wedge with a fragile tip). Furthermore, our method provides a platform to integrate more functional structures like waveguides and graded metamaterials with DH patterns.”
“It should be noted that the bandwidth of the DHEM can be further controlled by changing the filling ratio of the phononic structure. This allows tailoring the dispersion relationship in a DHPS so that a narrower or wider bandgap can be achieved. In addition, the working frequency of a DHEM can be shifted to lower or higher bands by changing the sample sizes or material parameters of the matrix and fillers. Thus, the working frequencies of a DHEM extend from ∼10 Hz to ∼10 kHz with sizes in the range of ∼1 cm to ∼1000 cm; this covers the noise spectrum for most domestic appliances, factories and transport vehicles.”
This research received funding from he Ministry of Science and Technology of China, and the National Natural Science Foundation of China.
See the article:
Hanchuan Tang, Zhuoqun Hao, Ying Liu, Ye Tian, Hao Niu, Jianfeng Zang
Soft and Disordered Hyperuniform Elastic Metamaterials for Highly
Efficient Vibration Concentration
National Science Review, nwab133, https://doi.org/10.1093/nsr/nwab133
National Science Review