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Posted: July 17, 2006
Control over friction
(Nanowerk News) Scientists of the National Center of Compentence in Research (NCCR) Nanoscale Science have demonstrated how friction can be switched on and off in atomic-scale contacts. This realization is of enormous importance for the development of novel mini-machines. The results of the research group from the University of Basel are published today in Science.
Friction is the force that impedes smooth sliding between bodies in contact. In everyday life, we fight friction: in machines we use lubricants to reduce friction and with it abrasion and the waste of energy. We also use positive aspects of friction: on icy roads sand increases friction and enables us to drive safely in winter conditions. It would be ideal to control friction according to our desires. The research group of Professor Ernst Meyer from the Institute of Physics at the University of Basel (Switzerland) has contributed an important step towards this goal.
The researchers employed an extremely sensitive instrument called Friction Force Microscope. As in a miniature record player, a sharp needle is brought into contact with a surface. The needle is moved over the surface and the friction force is recorded. On the nanometer scale, the tip of the needle does not slide smoothly over the surface but jumps from atom to atom. If these jumps can be avoided frictionless sliding becomes possible. To reach this the team of Prof. Meyer has applied a fast oscillating voltage between the needle and several different samples. The needle moves up and down at a high frequency without loosing contact. When the frequency of excitation matches resonances of the mechanical system friction can be reduced below a measurable level. Computer simulations explain this remarkable effect and reveal the conditions under which it occurs.
This state of suppressed friction is often referred to as “superlubricity”. The achievement of superlubricity without specific materials or lubricants but through dynamic actuation ensures a wide applicability of the method. The new results are quite promising for the development of novel nano-electro-mechanical systems (NEMS). Like the more extensively investigated micro-electro-mechanical systems (MEMS), NEMS are tiny machines with moving parts, where traditional lubrication is not applicable, but which are often plagued by problems related to friction, namely sticking parts. The new results could significantly improve the functionality of NEMS. Contacting parts in NEMS are usually small enough to be compared to the situation in the Friction Force Microscope. In the case of macroscopic bodies, the wide distribution of contact sizes and mechanical resonances makes the technique difficult to apply. Nevertheless it is not excluded that resonance-induced reduction of friction occurs in many natural phenomena, for example in biological systems or, on much
larger scales, in the motion of tectonic plates.
The National Center of Competence in Research (NCCR) “Nanoscale Science” is a long-term interdisciplinary research program focusing on nanoscale structures which aims to provide new impact and ideas to the life sciences, to the sustainable use of resources, and to information and communication technologies (www.nccr-nano.ch). Within the NCCR, the University of Basel has the role of a leading house, coordinating a network of universities, federal research institutes and industrial partners in which scientists from a wide variety of disciplines work closely together. The NCCR is a research instrument of the Swiss National Science Foundation.