Resonance in Atomic-Scale Sliding Friction

Substrate deformability and applied normal force are coupled to change nanoscale friction

Amonton's law of friction states that the friction force is proportional to the normal force in magnitude, and the slope gives a constant friction coefficient. In this work, with molecular dynamics simulation, we study how the kinetic friction at the nanoscale deviates qualitatively from the relation. Our simulation demonstrates that the friction behavior between a nanoscale AFM tip and an elastic graphene surface is regulated by the coupling of the applied normal force and the substrate deformability. First, it is found that the normal load-induced substrate deformation could lower friction at low load while increasing it at high load. In addition, when the applied force exceeds a certain threshold another abrupt change in friction behavior is observed, i.e., the stick-slip friction changes to the paired stick-slip friction. The unexpected change in friction behavior is then ascribed to the change of the microscopic contact states between the two surfaces: the increase in normal force and the substrate deformability together lead to a change in the energy landscape experienced by the tip. Finally, the Prandtl-Tomlinson model also validates that the change in friction behavior can be interpreted in terms of the energy landscape.

Reexamination of Damping in Sliding Friction

Friction is responsible for about one-third of the primary energy consumption in the world. So far, a thorough atomistic understanding of the frictional energy dissipation mechanisms is still lacking. The Amontons' law states that kinetic friction is independent of the sliding velocity while the Prandtl-Tomlinson model suggests that damping is proportional to the relative sliding velocity between two contacting objects. Through careful analysis of the energy dissipation process in atomic force microscopy measurements, here we propose that damping force is proportional to the tip oscillation speed induced by friction. It is shown that a physically well-founded damping term can better reproduce the multiple peaks in the velocity-dependent friction force observed in both experiments and molecular dynamics simulations. Importantly, the analysis gives a clear physical picture of the dynamics of energy dissipation in different friction phases, which provides insight into long-standing puzzles in sliding friction, such as velocity weakening and spring-stiffness-dependent friction.

Experimental Decoding and Tuning Electronic Friction of Si Nanotip Sliding on Graphene

Due to the coupled contributions of adhesion and carrier to friction typically found in previous research, decoupling the electron-based dissipation is a long-standing challenge in tribology. In this study, by designing and integrating a graphene/h-BN/graphene/h-BN stacking device into an atomic force microscopy, the carrier density dependent frictional behavior of a single-asperity sliding on graphene is unambiguously revealed by applying an external back-gate voltage, while maintaining the adhesion unaffected. Our experiments reveal that friction on the graphene increases monotonically with the increase of carrier density. By adjusting the back-gate voltage, the carrier density of the top graphene layer can be tuned from -3.9 × 1012 to 3.5 × 1012 cm-2, resulting in a ∼28% increase in friction. The mechanism is uncovered from the consistent dependence of the charge density redistribution and sliding barrier on the carrier density. These findings offer new perspectives on the fundamental understanding and regulation of friction at van der Waals interfaces.

Modulating Friction by the Phase of the Vertical Vibrational Excitation at Washboard Frequency

Applying external vibrations at the resonant frequencies of the frictional system has been a highly effective approach to suppress friction but usually requires additional energy consumption. In this study, we find that in addition to exerting the vibration at the resonant frequency of the frictional system, the friction force on the atomically flat silicon surface can also present a local minimum when the oscillation frequency of the vertical vibrational excitation equals the washboard frequency with respect to the sliding velocity. Moreover, compared with the additional energy consumption at the resonant frequency, applying vertical vibrational excitation at the washboard frequency requires much less energy consumption. The study further shows that the friction force under the washboard frequency can be effectively mediated depending on how the initial phase angle of the vertical vibrational excitation affects the effective substrate potential barrier at the slip moment of the tip. We have also extended the proposed friction modulation technique on atomically flat surfaces to periodic textured surfaces and confirmed its practicality and great potential for controlling friction.

Phonon mechanism of angle-dependent superlubricity between black phosphorus layers

Based on a combination of molecular dynamics simulations and quantum theories, this study discloses the phonon mechanism of angle-dependent superlubricity between black phosphorus layers. Friction exhibits 180° periodicity, i.e., the highest friction at 0° and 180° and lowest at 90°. Thermal excitation reduces friction at 0° due to thermal lubrication. However, at 90°, high temperature increases friction caused by thermal collision owing to lower interfacial constraints. Phonon spectra reveal that with 0°, energy dissipation channels can be formed at the interface, thus enhancing dissipation efficiency, while the energy dissipation channels are destroyed, thus hindering frictional dissipation at 90°. Besides, for both commensurate and incommensurate cases, more phonons are excited on atoms adjacent to the contact interface than those excited from nonadjacent interface atoms.

Resonance as a Design Strategy for AI and Social Robots

Resonance, a powerful and pervasive phenomenon, appears to play a major role in human interactions. This article investigates the relationship between the physical mechanism of resonance and the human experience of resonance, and considers possibilities for enhancing the experience of resonance within human-robot interactions. We first introduce resonance as a widespread cultural and scientific metaphor. Then, we review the nature of "sympathetic resonance" as a physical mechanism. Following this introduction, the remainder of the article is organized in two parts. In part one, we review the role of resonance (including synchronization and rhythmic entrainment) in human cognition and social interactions. Then, in part two, we review resonance-related phenomena in robotics and artificial intelligence (AI). These two reviews serve as ground for the introduction of a design strategy and combinatorial design space for shaping resonant interactions with robots and AI. We conclude by posing hypotheses and research questions for future empirical studies and discuss a range of ethical and aesthetic issues associated with resonance in human-robot interactions.

Tuning the interfacial friction force and thermal conductance by altering phonon properties at contact interface

Controlling friction force and thermal conductance at solid/solid interface is of great importance but remains a significant challenge. In this work, we propose a method to control the matching degree of phonon spectra at the interface through modifying the atomic mass of contact materials, thereby regulating the interfacial friction force and thermal conductance. Results of Debye theory and molecular dynamics simulations show that the cutoff frequency of phonon spectrum decreases with increasing atomic mass. Thus, two contact surfaces with equal atomic mass have same vibrational characteristics, so that more phonons could pass through the interface. In these regards, the coupling strength of phonon modes on contact surfaces makes it possible to gain insight into the nonmonotonic variation of interfacial friction force and thermal conductance. Our investigations suggest that the overlap of phonon modes increases energy scattering channels and therefore phonon transmission at the interface, and finally, an enhanced energy dissipation in friction and heat transfer ability at interface.

Atomistic mechanisms for frictional energy dissipation during continuous sliding

After more than a century of detailed investigations into sliding friction, we have not arrived yet at a basic understanding of energy dissipation, even for the simple geometry of a rigid slider moving over a perfectly periodic counter surface. In this article, we use a first-principles-based analysis to establish the atomistic mechanisms of frictional energy dissipation for a rigid object that moves continuously in the periodic surface potential landscape of a solid with vibrational degrees of freedom. We identify two mechanisms that can be viewed as (i) the continuous pumping of energy into the resonant modes, if these exist, and (ii) the destructive interference of the force contributions introduced by all excited phonon modes. These mechanisms act already in a purely dynamic system that includes independent, non-interacting phonon modes, and they manifest irreversibility as a kind of “dynamical stochastization”. In contrast to wide-spread views, we show that the transformation of mechanical energy into heat, that always takes place in real systems due to the coupling between phonon modes, can play only a minor role in the appearance of friction, if any. This insight into the microscopic mechanisms of energy dissipation opens a new, direct way towards true control over friction.