Friction of mesoscopic contacts - Analysis by manipulation of nanoparticles using atomic force microscopy

• Applicant: Dr. Dirk Dietzel
• Subject Area: Experimental Condensed Matter Physics ; Statistical Physics, Nonlinear Dynamics, Complex Systems, Soft and Fluid Matter, Biological Physics
• Term: from 2014 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 261462831

Although friction is an everyday phenomenon, there has been little success so far in finding a consistent physical model to derive it from fundamental principles. On the macroscale things are surprisingly well described by Amontons law of friction, which states that friction is proportional to the load but is independent of the apparent contact area. This law is rationalized by the assumption that friction is directly proportional to the 'true' contact area, i.e. the number of interface atoms in contact. However, current findings challenge the general validity of this fundamental assumption. Unfortunately, established techniques to measure nanoscale friction, like conventional friction force microscopy, are unsuited to address questions related to extended nanocontacts. Therefore experimental studies to understand the fundamentals of friction have mostly focused on contact areas of only a few nanometers square. To measure friction on larger scales we will thus apply a new approach, using an atomic force microscope to push nanoparticles on a surface while the pushing force is simultaneously recorded. With this approach we can measure friction of mesoscopic contacts with atomically defined interfaces. Moreover, it is possible to tune crucial experimental parameters of the interface, like crystallinity, orientation and shape. To identify the key parameters governing the scaling of interfacial friction, we will perform a systematic comparison of the friction of nanoparticles with different sizes and structures to the predictions from current theoretical models. Depending on the specific interface structure those models anticipate distinct sublinear friction-area scaling laws, a behavior often referred to as 'structural lubricity' or 'superlubricity'. By applying those scaling laws, it should then be even possible to control friction. More precisely, we intend to utilize a thermally induced structural phase transition to change friction of Sb-nanoparticles by at least an order of magnitude. Another part of the project is assigned to the analysis of dynamic processes at the interface between particle and substrate. We will focus on the analysis of the fundamental particle jumps between adjacent minima on the substrate's potential energy landscape. Especially the temperature and velocity dependence of the resulting stick-slip motion can reveal unique insight into dynamic interface processes. It is known, that even a slight, sub-monolayer contamination of the interface can lead to significant changes of interfacial friction. Experiments will thus be done under ultra-high vacuum conditions. In addition, it is planned to analyze the influence of controlled interface contamination. This topic is especially relevant from a technological point of view, since minimal contamination is unavoidable in any real world interface. Those investigations should help to assess how vanishing friction due to superlubricity can be utilized in technological devices.

• DFG Programme: Research Grants
• International Connection: Slovakia, Spain, USA
• Participating Persons: Professor Enrico Gnecco, Ph.D.; Professor Dr. Udo Schwarz; Professor Dr. Ivan Stich