An ultrathin underwater metasurface with low frequency, broadband, high-efficiency absorption, and high-hydrostatic-pressure-resistance performance is proposed based on the integrated principle of local resonance and cavity resonance. The design-parameter complexity is overcome by the global-optimization capability of the genetic algorithm. As a demonstration, an average sound-absorption coefficient of 92.3% at 500–10 000 Hz with an ultrathin thickness of 32 mm is achieved, including more than 99% sound-absorption coefficient at 4670–8630 Hz. In the proposed metasurface, the complex surface impedances of the three subsurfaces provide unique views for the high-efficiency sound-absorption coupling mechanism by the equivalent-circuit model. Further investigation demonstrates that the excellent property of high-efficiency sound absorption is maintained under 4.5-MPa hydrostatic pressure. The proposed metasurface provides more possibilities for underwater noise suppression.

Low-frequency noise control has aroused continuous interests owing to the intolerant structural thickness of traditional sound absorbing materials comparable to the working wavelength. The low-frequency sound absorbing materials remain challenging in scientific and engineering communities until now. Herein, a controllable broadband sound absorbing metasurface is designed and fabricated successfully by combining micro-perforated panel, coiled channel and embedded partition boards with the thickness of only 7 cm. The proposed metasurface possesses an average absorption coefficient over 0.8 within 0°–75° wide incident angle. Most importantly, the frequency of the absorption peak could be tuned flexibly by changing the number of partition boards in the cavity, providing an effective way for sound absorption on-demand design. The proposed acoustic metasurface opens the possibility of real-life application in low-frequency noise control engineering field.

Herein, a new method for steering liquid metals (LMs) using only a magnetic field in open 3D space is proposed. The magnetic LM is composed of the alloy Galinstan and iron particles. The 3D horizontal and vertical manipulation of a magnetic LM can be realized via an external magnetic field. The magnetically actuated LM is not only manipulated on various complex pathways in the horizontal plane, but also vertically in 3D space without the use of electrolytes and electrodes. As a proof‐of‐principle, an intelligent delivery vehicle that can avoid obstacles and traps horizontally and overcome gravity vertically to offload a cargo is designed and implemented successfully. Furthermore, a biomimetic soft robotics that can realize both in‐plane and out‐of‐plane locomotion is demonstrated using only magnetic field. The novel 3D motion of the demonstrated system facilitates the development of practical LM‐based smart structures and devices.