[Paper Review] Simulations of the kinetic friction due to adsorbed surface layers

Gang He, Mark O. Robbins|arXiv (Cornell University)|Aug 14, 2000

Lubricants and Their Additives4 citations

TL;DR

This study uses molecular dynamics simulations to show that adsorbed molecular layers on crystalline surfaces naturally produce kinetic friction that obeys Amontons’ laws and exhibits a logarithmic velocity dependence, consistent with experimental observations. The friction arises from thermally activated hopping of adsorbed molecules between metastable potential wells, with the force decreasing at lower velocities due to reduced thermal activation of high-force states.

ABSTRACT

Simulations of the kinetic friction due to a layer of adsorbed molecules between two crystalline surfaces are presented. The adsorbed layer naturally produces friction that is consistent with Amontons' laws and insensitive to parameters that are not controlled in experiments. The kinetic friction rises logarithmically with velocity as in many experimental systems. Variations with potential parameters and temperature follow variations in the static friction. This correlation is understood through analogy with the Tomlinson model and the trends are explained with a hard-sphere picture.

Motivation & Objective

• To investigate the molecular origins of kinetic friction in systems with adsorbed surface layers.
• To determine whether adsorbed layers can explain the experimentally observed logarithmic velocity dependence of kinetic friction.
• To explore the connection between kinetic and static friction in the presence of adsorbed layers.
• To assess the insensitivity of friction to experimentally uncontrolled parameters such as surface orientation and molecular size.
• To provide a molecular-scale explanation for Bowden and Tabor’s phenomenological model of Amontons’ laws.

Proposed method

• Molecular dynamics simulations with rigid fcc (111) crystal surfaces and a monolayer of adsorbed molecules.
• Use of simplified Lennard-Jones potentials for interatomic interactions to enable large-scale, long-timescale simulations.
• Application of periodic boundary conditions in the lateral directions to minimize edge effects and simulate bulk behavior.
• Systematic variation of sliding velocity, temperature, potential parameters (ε_wf, σ_wf), and crystallographic misorientation (θ) to probe friction dependence.
• Analysis of individual monomer dynamics to identify the role of thermally activated hopping between potential minima in determining kinetic friction.
• Use of a hard-sphere model to interpret the linear relationship between shear stress and normal pressure (τ = τ₀ + αP), linking it to the geometry of the surface of closest approach.

Experimental results

Research questions

• RQ1 Does the presence of an adsorbed molecular layer lead to kinetic friction that satisfies Amontons’ laws?
• RQ2 How does kinetic friction depend on sliding velocity, and does this dependence match the experimentally observed logarithmic trend?
• RQ3 To what extent are frictional forces insensitive to experimentally uncontrolled parameters like surface orientation and molecular size?
• RQ4 What is the molecular mechanism underlying the logarithmic velocity dependence of kinetic friction?
• RQ5 Why is there a strong correlation between kinetic and static friction in systems with adsorbed layers?

Key findings

• Kinetic friction increases logarithmically with sliding velocity over more than two decades, matching experimental observations and rate-state models.
• The intercept τ₀ in the linear relation τ = τ₀ + αP is relatively independent of velocity below 1 m/s, indicating a velocity-independent baseline friction.
• The slope α of the linear shear stress–pressure relation is insensitive to the number of adsorbed molecules per unit area and to variations in adsorption strength ε_wf, due to the hard-sphere geometry of the surface of closest approach.
• The kinetic friction is strongly correlated with static friction, and both vary in response to the same parameters (e.g., potential parameters, temperature), suggesting a common molecular origin.
• The logarithmic velocity dependence arises from thermally activated hopping of adsorbed molecules: at lower velocities, more molecules remain in high-force states before thermal activation, reducing the average force.
• The hard-sphere model explains the insensitivity of α to molecular density and adsorption strength, as α depends only on the geometric slope of the surface of closest approach, not on the number of molecules.