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CNR: Alamanacco della Scienza

Archivio

N. 9 - 11 ago 2010
ISSN 2037-4801

International info   a cura di Cecilia Migali

Tecnologia

A new study on ballistic nanofriction  

Friction is a very hot topic, for both scientific and technological reasons, and a lot of effort is currently devoted to understand the underlying physics, in various directions. One direction which is currently neglected experimentally and  theoretically is high speed friction, as most  fundamental physical tribology studies focus on exceedingly low speeds. This is understandable, because low speeds are easier to access in  laboratory devices, and easier to understand  theoretically because all excitations have time to dissipate. Yet, high speed sliding (10 m/s or  more) is not at all uncommon in the real world: but the physics friction literature offers very little to address this situation, even in the simplest case of a crystal island of tip sliding on a crystal surface. This is therefore an area  where some pioneering work is called for. The simplest way for an initial exploration approach  is no doubt molecular dynamics simulation, where frictional data of some realism can be generated and directly submitted to physical questioning. Researchers found in fact that direct simulation of a simple but realistic case, that of gold clusters on a graphite surface, throws important newand general light on high speed friction. At high speed, they discovered and characterized a novel ballistic friction regime, with totally different characteristics from conventional diffusive friction at low speed. For a cluster or a molecule, they found for the first time how an understanding of the mutual role of rotations and translations is essential to high speed friction, no less than at low speed. A broad crossover regime of intermediate speeds between ballistic and diffusive friction is identified and described. By addressing the simulation results in the light of simple theoretical  considerations, they drawed what appear to be pretty general, relatively system unspecific conclusions. One conclusion is that the temperature dependence of ballistic friction - which increases with increasing T - is predicted and indeed found to be opposite to that of smooth  low speed friction, which drops with temperature. Another is that rotations and translations, which are correlated (they occur together) at low speeds, are instead anti-correlated at high speeds, where kinetic energy is bouncing back and  forth between the two. Yet another one is that, despite their deeply mechanisms, the high speed frictional force grows roughly linearly with speed, similar to low speed diffusive friction. The realism of the simulations suggests some direct experimental tests, on sliding clusters and molecules. At a more general level, the present characterization of ballistic friction should provide a good starting ground for future experimental and technological advances.

 

Fonte: Andrea Vanossi, Cnr-Iom Democritos Simulation Center & Sissa, tel. 040/3787448, email vanossi@sissa.it