Atomic Force Microscopy (AFM) is a powerful and convenient experimental measurement device in the field of nano-scale tribology. It was successfully applied to explore superlubricity in a graphene-gold interface and superlubricity due to repulsive van der Waals forces, to grow tribofilms and to address numerous other problems. At the same time, many phenomena cannot be explored experimentally and theoretical calculations must be performed. Molecular dynamic simulation method (MD) is a useful computational tool that is also frequently employed along with AFM experimentation. In principle, it is capable of solving any type of tribological problem without introducing additional assumptions beyond the accuracy of the interatomic potentials. This comes for the price of high computational costs and the conditions employed in the MD and AFM experiments are frequently different, making the quantitative comparison less reliable.
This is particularly true for the study of a stick-slip friction, a fundamental phenomenon in nanotribology, rising for example between AFM tip and clean crystalline substrate. The mechanical constraints and data acquisition systems allow to slide the AFM tip with the speeds m/s at fastest. The same stick-slip behavior can be explored by means of MD simulations. However, the typical sliding speed has to be higher than m/s in order to consider realistic sliding time. This comes due to the limitation in the integration time step employed in MD, which is typically in the range of femtoseconds.
An international group of researchers from University of Pennsylvania, California Merced, Calgary and University of Akron performed AFM and MD friction experiments with Au (111) and silicon oxide tips with matched conditions and at overlapping speeds. To achieve this, both the AFM and MD set ups were improved. Instead of moving the AFM tip, the samples were moved. The samples were glued on a high-frequency shear piezo to increase the scanning speed up to 580µm/s. Parallel replica dynamics method (PRD) was employed to decrease the sliding speed during the MD simulations down to as low as 25µm/s. Algorithmically simple (yet having a solid theoretical background), RPD allows to increase the rate of time accumulation in the simulation almost times faster compared to conventional MD with the aid of parallel processors.
Using this sophisticated method, the gap between AFM and MD was closed and a good agreement was documented. The research team validated MD simulations to be applicable for the interpretation of AFM data.
Further details can be found in the original article Dynamics of Atomic Stick-Slip Friction Examined with Atomic Force Microscopy and Atomistic Simulations at Overlapping Speeds by Xin-Z. Liu, Zhijiang Ye, Yalin Dong, Philip Egberts, Robert W. Carpick, and Ashlie Martini
Credit for image: Dynamics of Atomic Stick-Slip Friction Examined with Atomic Force Microscopy and Atomistic Simulations at Overlapping Speeds by Xin-Z. Liu, Zhijiang Ye, Yalin Dong, Philip Egberts, Robert W. Carpick, and Ashlie Martini