Traditionally, friction force has been the benchmark for quantifying energy dissipation in frictional phenomena. In this study, we introduce an atomic chain friction model that illuminates the conversion of kinetic energy into potential energy through interfacial forces. The energy dissipation process is characterized by the release of partial potential energy in the form of phonons, quantifiable by a frictional damping coefficient. We have determined that this damping coefficient is significantly influenced by the intrinsic dynamic properties of the friction system. Expanding on this foundation, we have formulated an advanced phononic friction model that accurately predicts the friction forces measured using an atomic force microscope. Our model reveals that energy dissipation is caused by vibrations occurring both parallel and perpendicular to the sliding motion. These findings profoundly enhance our understanding of the basic mechanics of friction and open new avenues for innovative strategies in the active management and reduction of energy dissipation in diverse mechanical systems.

Four-dimensional printing allows for the transformation capabilities of 3D-printed architectures over time, altering their shape, properties, or function when exposed to external stimuli. This interdisciplinary technology endows the 3D architectures with unique functionalities, which has generated excitement in diverse research fields, such as soft robotics, biomimetics, biomedical devices, and sensors. Understanding the selection of the material, architectural designs, and employed stimuli is crucial to unlocking the potential of smart customization with 4D printing. This review summarizes recent significant developments in 4D printing and establishes links between smart materials, 3D printing techniques, programmable structures, diversiform stimulus, and new functionalities for multidisciplinary applications. We start by introducing the advanced features of 4D printing and the key technological roadmap for its implementation. We then place considerable emphasis on printable smart materials and structural designs, as well as general approaches to designing programmable structures. We also review stimulus designs in smart materials and their associated stimulus-responsive mechanisms. Finally, we discuss new functionalities of 4D printing for potential applications and further development directions.

Atomistic mechanisms of frictional energy dissipation have attracted significant attention. However, the dynamics of phonon excitation and dissipation remain elusive for many friction processes. Through systematic fast Fourier transform (FFT) analyses of the frictional signals as a silicon tip sliding over a graphite surface at different angles and velocities, we experimentally demonstrate that friction mainly excites non-equilibrium phonons at the washboard frequency and its harmonics. Using molecular dynamics (MD) simulations, we further disclose the phononic origin of structural lubrication, i.e., the drastic reduction of friction force as the contact angle between two commensurate surfaces changes. In commensurate contacting states, friction excites a large amount of phonons at the washboard frequency and many orders of its harmonics that perfectly match each other in the sliding tip and substrate, while for incommensurate cases, only limited phonons are generated at mismatched washboard frequencies and few low order harmonics in the tip and substrate.