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Posted: Mar 06, 2017
Phononic origins of friction revealed
(Nanowerk Spotlight) Friction is the force that opposes the relative motion or tendency of such motion of two surfaces in contact; in other words: it hampers the movement of mechanical parts. While gears, bearings, and liquid lubricants can reduce friction in the macroscopic world, the origins of friction at the nanoscale, where the phenomenological macroscopic laws of friction break down, requires other solutions.
Understanding of frictional dissipation at the interfaces of sliding nanoscale surfaces is a prime challenge for the development of nanoscale functional devices. Decreasing nanoscale friction in nanoelectromechanical (NEMS) or microelectromechanical (MEMS) system has huge implications for controlling mechanical energy losses and wear of these devices.
Take carbon nanotube (CNT) based NEMS oscillators, which have been proposed for use in ultrasensitive mass detection, nanoscale magnetometers, radio-frequency signal processing, and as a model system for exploring quantum phenomena in macroscopic systems.
"CNTs, by possessing a uniquely large disparity among its intertube and intratube interaction strengths, have been established as ultralow friction nanostructures and are serving as testbeds for tuning frictional response," Matukumilli V. D. Prasad explains to Nanowerk.
Snapshots of a typical phonon propagation where a specific phonon mode that excited on outer nanotube passes over the inner tube. The interaction process, while passing over, relates the phononic coupling to friction between nanotubes. (Image: Prasad and Bhattacharya, IIT Kharagpur)
"This work, for the first time, provides a precise connection between individual phonon mode scattering and friction force. Analogous to photon as quantized electromagnetic energy (light), phonon is a quantized vibrational energy (heat) having wave-particle nature," says Prasad.
"Looking at the friction phenomena as the manifestation of scattering events of phonon vibrational modes by studying how strongly a phonon mode couples at the interface is exciting, as this approach is fundamentally rooted in the quantum theory of friction," adds Baidurya Bhattacharya, a Professor of Civil Engineering. "And more exciting is that, this way we were able to settle some long-standing debates in nanoscale friction."
According to the team, their findings open up a new approach in designing nanomechanical devices to consider the severity of interfacial phononic scattering.
"It has implications in the development of any future nano/micro mechanical systems involving relatively sliding atomic surfaces," notes Prasad.
"Until now we have probed the frictional aspects between the solid surfaces from phononic point of view," says Bhattacharya." Further, we want to extend this to solid-liquid interfaces, which is a more difficult problem to address the interaction of phonon waves with liquids."
Earlier attempts to address nanoscale friction, especially in CNT oscillators, have been made through classical molecular dynamics, where the dissipated energy can be observed as an increased temperature of the system. The precise mechanism through which the mechanical sliding energy is converted in to heat was unknown. That was the motivation for this study.
"Imposing phonon wavepackets on the CNT model in order to observe the phonon propagation and its interaction at the interface is new and that led to solving many unclarified issues in nanofriction," concludes Prasad.