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Introduction

My research revolves around the exploration and design of tunable artificial materials, with a focus on the control of motion, stress, and elastic energy. Guided by theoretical and numerical analysis, I exploit transformative microstructure (realized through the coupling between mechanics and temperature, liquid, and magnetism) to develop novel tunable metamaterials with superior and unusual properties. The research review is as follows.

  • Soft magnetoactive metamaterials for remote tunability of elastic waves

Soft magnetoactive materials consist of magnetizable particles embedded in a soft matrix. Their ability to rapidly and reversibly change the shape and properties under remote magnetic stimuli, makes them an attractive material platform for human-technology interfaces, soft robotics, actuators and sensors, and biomedical devices. I propose to exploit the unique transformative ability of soft magnetoactive materials integrated into the neat metamaterial design to develop novel tunable and multifunctional soft magnetoactive metamaterials with superior elastic wave properties. The recent research advances include:

(1) I derived the exact solutions for periodic hard-magnetic soft multilayers (with intrinsic magnetization), and showed that the transverse wave Bragg scattering band gap can be tuned by magnetic fields (Mech. Mater. 169, 104325, 2022); (2) I proposed a hard magnetic material-based asymmetric mechanical metamaterial design that allows remote and reversible control of the local resonant band gap (Extreme Mech. Lett. 59, 101957, 2023);

(3) I investigated the conditions for the existence of solitary waves enabled by a magnetic field in hard-magnetic soft mechanical metamaterial systems consisting of a periodic array of units of hard-magnetic inclusion embedded in a soft matrix (Int. J. Solids Struct. 280, 112396, 2023).

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  • Mechanical metamaterials with tailored nonlinear elastic responses

Quasi-zero-stiffness (QZS) isolators of high-static-low-dynamic stiffness play an important role in ultra-low frequency vibration mitigation. While the current designs of QZS mainly exploit the combination of negative-stiffness corrector and positive-stiffness element, and only have a single QZS working range, here a class of tailored mechanical metamaterials with programmable QZS features is proposed (Adv. Funct. Mater. 31, 2101428, 2021). These programmed structures contain curved beams with geometries that are specifically designed to enable the prescribed QZS characteristics. When these metamaterials are compressed, the curved beams reach the prescribed QZS working range in sequence, thus enabling tailored stair-stepping force-displacement curves with multiple QZS working ranges.

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  • Subwavelength Dirac degeneracy and elastic topological valley modes

Dirac degeneracy plays a key role in achieving topological protections. It has been shown that the material with C3v lattice symmetry has deterministic Dirac-like degeneracies at the high-symmetry points in the Brillouin zone. However, these Dirac degeneracies are constructed by the Bragg scattering eigenstates, and their frequencies are determined by the lattice constant. This means lowering the operating frequency generally demands a large lattice size due to the Bragg condition. To this end, I design and experimentally realized subwavelength elastic topological valley transportation with negative effective mass density for the first time (Phys. Rev. B 101, 014101, 2020).

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  • Digital elastic metamaterials for low-frequency wave manipulations

To broaden the band gaps at low frequencies in a tunable way, we proposed a tunable digital elastic metamaterial, which consists of a primary frame and auxiliary beams with embedded electromagnets. Switching electromagnets between the attaching (1 bit) and detaching (0 bit) modes activates different waveguides in the metamaterial (Adv. Mater. 28, 9857, 2016). Furthermore, by incorporating local resonant modes with different polarizations in the digital unit, we proposed a 3D tunable, polarizer-like elastic metamaterial that contains a 3D-printed frame and eight built-in electromagnets. By controlling the current to change the position of the electromagnets, each unit exhibits three modes, and the expected polarization of elastic waves can be obtained from hybrid waves in a tunable manner (Adv. Sci. 6, 1900401, 2019).

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  • Fluid-solid metamterials with tunable mass distribution

Most strategies for designing tunable elastic metamaterials are based on tuning the stiffness of the microstructure. To address this limitation, a novel fluid-solid metamaterial with tunable mass distribution is developed by introducing liquid into the solid substrate. By controlling the liquid distribution in the unit cell, I designed metamaterials with tunable local resonant band gap and effective mass density (Appl. Phys. Lett. 112, 221906, 2018), and the programmable elastic valley Hall insulator with reconfigurable topological interface propagating route (Extreme Mech. Lett. 28, 76, 2019). Both are validated by experiments.

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  • 4D printing and temperature-controlled morphing structures and metamaterials

I reported for the first time that the extruded polylactic acid (PLA) filaments exhibited significant heat-induced shrinkage. The underlying mechanism was revealed that a uniform internal strain related to phase transition is stored in the polymer during the printing (Sci. Rep. 5, 8936, 2015). This internal strain will be released during the subsequent heating process, leading to abnormal heat-shrinkage properties. Furthermore, I exploited this mechanism to design shape reconfigurable network materials and morphing structures (Sci. Rep. 6, 22431, 2016).

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