Boundary Lubrication and Surface Mobility of Mixed Alkylsilane Self-Assembled Monolayers

Abstract

We use a combination of experiment and theory to study the boundary lubrication characteristics of mixed self-assembled monolayers (SAMs) formed by tethering alkylsilanes with different chain lengths to planar substrates. The structure of mixed SAMs is characterized using atomic force microscopy, infrared spectroscopy, and hexadecane contact angle measurements. Lateral force microscopy is used to quantify the load and velocity dependence of friction force for one-component (pure) and two-component (mixed) SAMs. We find that if two-component SAMs and one-component SAMs are created with comparable packing density, the two-component systems possess lower friction coefficients. The friction force measured on silicon surfaces grafted with mixed SAMs increases linearly with the logarithm of sliding velocity over a broad range, whereas for pure SAMs, the friction force first increases linearly with the logarithm of velocity and then reaches a plateau. This plateau is believed to arise from the viscoelasticity of...