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P1.3.1

Thermally activated phenomena observed by atomic force microscopy Enrico Gnecco1*, Elisa Riedo2**, Roland Bennewitz1, Ernst Meyer1, and Harald Brune3 1 Institute of Physics, University of Basel, Klingelbergstrasse 82, CH-4056 Basel, Switzerland 2 Georgia Institute of Technology, School of Physics, Atlanta, GA 30332 USA 3 Institut de Physique des Nanostructures, EPFL, CH-1015 Lausanne, Switzerland ABSTRACT Thermal effects may affect the velocity dependence of friction on the nanoscale in different ways. In a dry environment the stick-slip motion of a nanotip sliding across a crystalline surface is modified by thermal vibrations, which leads to a logarithmic increase of friction with the sliding velocity at very low speeds (v < 10 µm/s). At higher speeds the role of thermal activation is negligible, and friction becomes velocity-independent. An analytical expression, which explains both regimes of friction vs. velocity, is introduced. In a humid environment the situation is complicated by water capillaries formed between tip and surface, which act as obstacles for thermally activated jumps. Depending on the wettability of the surface, different tendencies in the velocity dependence are observed. INTRODUCTION The study of friction on the nanometer scale is strictly related with the advancement of atomic force microscopy (AFM). The first studies performed by M. Mate et al. in 1987 revealed that the lateral force FL acting on the AFM tip scanning a crystal surface has the same periodicity of the surface lattice, due to the stick-slip effect [1]. Friction on the nanometer scale is rarely proportional to the normal force (load) FN applied on the tip. Assuming that the tip-surface contact is an ideally smooth sphere-flat surface contact, a relationship like FL ∝ FN2 / 3 is expected, with variations due to adhesion between tip and surface [2]. Also the traditional Coulomb’s law, stating that friction does not depend on the sliding velocity, is not always verified in small contacts [3, 4]. Deviations from the Coulomb’s law are well known since a long time even on the macroscopic scale [2, 5]. These deviations have been related to the increase of adhesion with the contact time, to plastic deformations, creep, etc. The single point of contact formed by an AFM tip is probably the best environment to understand the microscopic origin of the velocity dependence of nanoscopic friction. The vibrations occurring in the area of contact lead to a logarithmic velocity dependence of friction. At very low speeds (< 10 µm/s) the contact vibrations can easily induce a tip jump, which reduces the mean value of the friction force. Capillary condensation leads to the opposite effect. If sliding is slow enough, strong water bridges between tip and surface can be formed, which increases the friction force [6]. With the present contribution we discuss these basic mechanisms of the velocity dependence of friction, and extend the analysis given in Ref. [4] introducing a recent analytical expression, which joins together the low-speed regime