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Abstract The design of devices based on acoustic or optical fields requires the fabrication of cavities and structures capable of efficiently trapping these waves. A special type of cavity can be designed to support resonances with a theoretically infinite quality factor, named bound states in the continuum or BICs. The experimental measurement of such modes is still a challenging problem, as they are, by definition, not accessible from external perturbations. Here we report on the theoretical design and experimental realization of a two-dimensional, fully open acoustic resonator supporting BICs. This accidental BIC, whose symmetry is chosen during design by properly tailoring the geometrical properties of the system, is completely accessible and allows for the direct measurement of the whole pressure field and properties. We experimentally demonstrate its existence with high quality factor and field enhancement properties. 

Abstract Acoustic tweezers use ultrasound for contact-free manipulation of particles from millimeter to sub-micrometer scale. Particle trapping is usually associated with either radiation forces or acoustic streaming fields. Acoustic tweezers based on single-beam focused acoustic vortices have attracted considerable attention due to their selective trapping capability, but have proven difficult to use for three-dimensional (3D) trapping without a complex transducer array and significant constraints on the trapped particle properties. Here we demonstrate a 3D acoustic tweezer in fluids that uses a single transducer and combines the radiation force for trapping in two dimensions with the streaming force to provide levitation in the third dimension. The idea is demonstrated in both simulation and experiments operating at 500 kHz, and the achieved levitation force reaches three orders of magnitude larger than for previous 3D trapping. This hybrid acoustic tweezer that integrates acoustic streaming adds an additional twist to the approach and expands the range of particles that can be manipulated. 

Acoustic energy harvesters (AEHs) open up opportunities to recycle noise waste and generate electricity. They provide potential power solutions to a wide range of sensors. However, the practicality of AEHs has long been limited by their narrow bandwidths and low efficiencies. In this study, we present an ultra-broadband AEH and a highly efficient AEH that transforms sound energy into usable electrical power. Our broadband device comprises an electrodynamic loudspeaker driver and an optimized acoustic metamaterial matching layer and is capable of converting 7.6% to 15.1% of total incident sound energy from 50 to 228 Hz. Moreover, we demonstrate that by replacing the loudspeaker surround with a lower-loss material such as PDMS, the energy conversion rate can be significantly increased to 67%. The proposed broadband AEH has a fractional bandwidth eight times the state-of-the-art, while the proposed highly efficient AEH has a peak efficiency three times the state-of-the-art. The outstanding performance makes our designs cost-effective and scalable solutions for noise reduction and power generation. 

There is a trade-off between the sparseness of an absorber array and its sound absorption imposed by wave physics. Here, near-perfect absorption (99% absorption) is demonstrated when the spatial period of monopole-dipole resonators is close to one working wavelength (95% of the wavelength). The condition for perfect absorption is to render degenerate monopole-dipole resonators critically coupled. Frequency domain simulations, eigenfrequency simulations, and the coupled mode theory are utilized to demonstrate the acoustic performances and the underlying physics. The sparse-resonator-based sound absorber could greatly benefit noise control with air flow and this study could also have implications for electromagnetic wave absorbers. 

This paper investigates the mechanism of self-stabilizing, three-dimensional Mie particle manipulation in water via an acoustic tweezer with a single transducer. A carefully designed acoustic lens is attached to the transducer to form an acoustic vortex, which provides angular momentum on the trapped polymer sphere and leads to a fast-spinning motion. The sphere can find equilibrium positions spontaneously during the manipulation by slightly adjusting its relative position, angular velocity, and spinning axis. The spinning motion greatly enhances the low-pressure recirculation region around the sphere, resulting in a larger pressure induced drag. Simultaneously, the Magnus effect is induced to generate an additional lateral force. The spinning motion of the trapped sphere links the acoustic radiation force and hydrodynamic forces together, so that the sphere can spontaneously achieve new force balance and follow the translational motion of the acoustic tweezer. Non-spherical objects can also be manipulated by this acoustic tweezer. 

Abstract This broad review summarizes recent advances and “hot” research topics in nanophononics and elastic, acoustic, and mechanical metamaterials based on results presented by the authors at the EUROMECH 610 Colloquium held on April 25–27, 2022 in Benicássim, Spain. The key goal of the colloquium was to highlight important developments in these areas, particularly new results that emerged during the last two years. This work thus presents a “snapshot” of the state-of-the-art of different nanophononics- and metamaterial-related topics rather than a historical view on these subjects, in contrast to a conventional review article. The introduction of basic definitions for each topic is followed by an outline of design strategies for the media under consideration, recently developed analysis and implementation techniques, and discussions of current challenges and promising applications. This review, while not comprehensive, will be helpful especially for early-career researchers, among others, as it offers a broad view of the current state-of-the-art and highlights some unique and flourishing research in the mentioned fields, providing insight into multiple exciting research directions. 

Metamaterials have offered unprecedented potentials for wave manipulations. However, their applications in underwater acoustic wave control have remained largely unexplored. This is because of the limited material choices and the lack of reliable fabrication techniques for the complicated structures. Herein, a metamaterial with microlattice structures as the building blocks is proposed for underwater operations. By designing the building blocks of the metamaterial and assembling them in a layered fashion, anisotropy is embedded in the structure, which results along different effective sound speeds in orthogonal directions. The designed metamaterial is fabricated by metal additive manufacturing using aluminum and steel. Experiments are performed using a resonator tube to evaluate its performance in water. An anisotropy ratio of around 2 is achieved, which is in good agreement with numerical simulations. The proposed metamaterial provides an effective means for underwater sound control with reduced fabrication difficulties and increased service life. 

Acoustic tweezers use ultrasound for contact-free, bio-compatible, and precise manipulation of particles from millimeter to submicrometer scale. In microfluidics, acoustic tweezers typically use an array of sources to create standing wave patterns that can trap and move objects in ways constrained by the limited complexity of the acoustic wave field. Here, we demonstrate spatially complex particle trapping and manipulation inside a boundary-free chamber using a single pair of sources and an engineered structure outside the chamber that we call a shadow waveguide. The shadow waveguide creates a tightly confined, spatially complex acoustic field inside the chamber without requiring any interior structure that would interfere with net flow or transport. Altering the input signals to the two sources creates trapped particle motion along an arbitrary path defined by the shadow waveguide. Particle trapping, particle manipulation and transport, and Thouless pumping are experimentally demonstrated.