Recent understanding of solid-liquid friction in ionic liquids

Abstracts: Ionic liquids (ILs)-solid interfacial phenomenon is undoubtedly one of the topic issues in energy related fields, especially bringing great challenges in the development of chemical engineering. Solid-ILs friction under microscale shows outstanding properties such as widely studied electro-controllability by surface potential or electric field, and promising performance of superlubricity. Vast applications, such as boundary lubrication and supercapacitors, have been benefiting from these studies. In this review, we summarized the recent advances of the measurements of solid-ILs friction, and focused on the microscale and fascinating findings. A brief tutorial for this topic, containing both experimental and theoretical methods, was also provided. It was anticipated that this review would help researchers grasp the emerging field and stimulate the advance of new ideas and methods in ILs on solids

Load-induced dynamical transitions at graphene interfaces

The structural superlubricity (SSL), a state of near-zero friction between two contacted solid surfaces, has been attracting rapidly increasing research interest since it was realized in microscale graphite in 2012. An obvious question concerns the implications of SSL for micro- and nanoscale devices such as actuators. The simplest actuators are based on the application of a normal load; here we show that this leads to remarkable dynamical phenomena in microscale graphite mesas. Under an increasing normal load, we observe mechanical instabilities leading to dynamical states, the first where the loaded mesa suddenly ejects a thin flake and the second characterized by peculiar oscillations, during which a flake repeatedly pops out of the mesa and retracts back. The measured ejection speeds are extraordinarily high (maximum of 294 m/s), and correspond to ultrahigh accelerations (maximum of 1.1×1010 m/s2). These observations are rationalized using a simple model, which takes into account SSL of graphite contacts and sample microstructure and considers a competition between the elastic and interfacial energies that defines the dynamical phase diagram of the system. Analyzing the observed flake ejection and oscillations, we conclude that our system exhibits a high speed in SSL, a low friction coefficient of 3.6×10−6, and a high quality factor of 3×107 compared with what has been reported in literature. Our experimental discoveries and theoretical findings suggest a route for development of SSL-based devices such as high-frequency oscillators with ultrahigh quality factors and optomechanical switches, where retractable or oscillating mirrors are requiredISSN:0027-8424ISSN:1091-649