Non‐Equilibrium Phase Behavior of Confined Molecular Films at Low Shear Rates

• Published in: physica status solidi (b) 2017-12, Vol.254 (12), p.n/a
• Main Authors: Maćkowiak, Szymon, Heyes, David M., Pieprzyk, Slawomir, Dini, Daniele, Brańka, Arkadiusz C.
• Format: Article
• Language: English
• Subjects: anomalous friction; confined systems; Lennard‐Jones films; molecular dynamics; Prandtl‐Tomlinson model; shear
• ISSN: 0370-1972; 1521-3951
• DOI: 10.1002/pssb.201600862

In a recent publication [Maćkowiak et al., J. Chem. Phys. 145, 164704 (2016)] the results of Non‐Equilibrium Molecular Dynamics (NEMD) simulations of confined sheared Lennard‐Jones molecular films have been presented. The present work builds on that study by focusing on the low wall speed (shear rate) regime. Maps are given of the steady‐state structures and corresponding friction coefficients in the region where a transition from static to kinetic friction is observed. The boundary between static and kinetic friction regions is determined as a function of wall speed and applied pressure, which is located for wall speeds up to about 0.8 m s−1. It was found that stick‐slip behavior extends to pressures as high as 1000 MPa. The NEMD equations of motion are shown to be consistent with the Prandtl–Tomlinson model in the ‘soft spring’ limit, which leads to a new expression for the friction coefficient. This study provides new details and insights into the nature of anomalous friction behavior in the so‐called Plug‐Slip part of the nonquilibrium phase diagram regime. Non‐equilibrium molecular dynamics simulations of confined sheared molecular films in the low wall speed regime are used to explore the static–kinetic friction boundary as a function of applied pressure and wall sliding speed. A new expression for the friction coefficient is derived based on the Prandtl–Tomlinson model in the “soft spring” limit to which the equations of motion correspond to.