The manipulation of underwater bubbles on substrates has received extensive research interest from both the scientific community and industry, including the chemical industry, machinery, biology, medicine, and other fields. Recent advances in “smart” substrates have enabled the bubbles to be transported on demand. Herein, the progress in the directional transport of underwater bubbles on various types of substrates is summarized, including planes, wires, and cones. The transport mechanism can be classified as buoyancy-driven, Laplace-pressure-difference-driven, and external-force-driven according to the driven force of the bubble. Moreover, the wide applications of directional bubble transport are reported, ranging from gas collection, microbubble reaction, bubble detection and classification, bubble switch, and bubble microrobots. Lastly, the advantages and challenges of various directional bubble transportation methods are discussed, and the current challenges and future prospects in this field are also discussed. This Review outlines the fundamental mechanisms of underwater bubble transportation on solid substrates and helps to understand the methods of optimizing bubble transportation performances.

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Active mechanical metamaterials with customizable structures and deformations, active reversible deformation, dynamically controllable shape-locking performance and stretchability are highly suitable for applications in soft robotics and flexible electronics, yet challenging in integrating them due to their mutual conflicts. Here, we introduce a class of phase-transforming mechanical metamaterials (PMMs) that integrate the above properties. Periodically arranging basic actuating units according to the designed pattern configuration and positional relationship, PMMs can customize complex and diverse structures and deformations. Liquid-vapor phase transform provides active reversible large deformation while silicone matrix offers stretchability. The contained carbonyl iron powder endows PMMs with dynamically controllable shape-locking performance, thereby achieving magnetically assisted shape-locking and energy-storing in different working modes. We build theoretical model and finite element simulation to guide the design process of PMMs, so as to develop a variety of PMMs with different functions and suitable for different applications, such as programmed PMM, reconfigurable antenna, soft lens, soft mechanical memory, biomimetic hand, biomimetic flytrap and self-contained soft gripper. PMMs are applicable to achieve various 2D deformations and 2D to 3D deformations and integrate multiple properties, including customizable structures and deformations, active reversible deformation, rapid reversible shape-locking, adjustable energy-storing and stretchability, which could open a new application avenue in soft robotics and flexible electronics.