Dear Colleagues

This Wednesday, at 17:15, the Kolloquium series of SS05 will continue with the talk outlined below. From 16:45, there will be tea and coffee in front of the lecture hall 16 G 05 at the Irchel Campus of the University (the usual place!). For directions or further information, go to

http://www.physik.unizh.ch/teaching/kolloquium05/kolloquium.html

You will notice that there is still one slot of the Kolloquium program open: June 29, the last date before SS05 ends. Suggestions for a speaker are welcome.

With best regards,

Juerg Osterwalder

Atomic-Scale Friction and How to Avoid it: Superlubricity and Thermolubricity

Joost W.M. Frenken Kamerlingh Onnes Laboratory, Leiden University, The Netherlands

Despite its apparent simplicity, our understanding of the phenomenon of friction is still far from complete. This talk is devoted to interfaces that slide with nearly zero friction. Using a special friction force microscope [1-3], we quantitatively track the forces between a sharp tip and a flat surface, with a friction force resolution as low as 15 pN. To our surprise we observe a transition from atomic-scale "stick-slip" motion to a situation of (near) frictionless, continuous sliding, for tungsten tips in contact with graphite surfaces [4-6]. Our observations strongly suggest that a graphite nano-flake is attached to the tungsten tip, so that we actually record the forces between two parallel graphite lattices. By rotating the substrate with respect to the tip, we periodically go through fully aligned and completely misoriented configurations. When the misalignment is sufficiently severe, the lateral forces on the atoms in the flake cancel, thereby dramatically reducing the total friction force. This phenomenon is referred to as 'superlubricity' and is proposed to play a key role in the lubricating properties of graphite. We have developed a theory that also accounts for thermal excitations in the contact and find that temperature can act as a surprisingly efficient lubricant. Our experimental results exhibit clear signs of this 'thermolubricity', even at room temperature [7].

[1] M. Dienwiebel et al., Rev.Sci.Instrum., in press [2] M. Dienwiebel et al., in "Nanotribology. Critical assessment and Research Needs", Ed. S.M. Shu, Z.C. Ying, Kluwer, Dordrecht, (2002) p. 115. [3] T. Zijlstra et al., Sensors and Actuators A: Physical 84 (2000) 18. [4] M. Dienwiebel et al., Surf. Sci. 576 (2005) 197. [5] G.S. Verhoeven et al., Phys. Rev. B 70 (2004) 165418. [6] M. Dienwiebel et al., Phys. Rev. Lett. 92 (2004) 126101. [7] S.Yu. Krylov et al., to be published.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Prof. Dr. Jürg Osterwalder Physik-Institut, Universität Zürich Winterthurerstr. 190 CH-8057 Zürich, Switzerland phone: +41 44 635 5827 <<< NOTE NEW AREA CODE ! >>> lab: +41 44 635 5745 sekr: +41 44 635 5781 fax: +41 44 635 5704 e-mail: osterwal@physik.unizh.ch web: http://www.physik.unizh.ch/groups/grouposterwalder/ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~