Utilizing Atomistic Simulations To Map Pressure Distributions and Contact Areas in Molecular Adlayers within Nanoscale Surface-Asperity Junctions: A Demonstration with Octadecylsilane-Functionalized Silica Interfaces

Abstract

To achieve a better understanding of the mechanical effects of adsorbed films at surface contacts, methods were developed to map and examine the pressure distribution of nanoasperity contacts, modeled by molecular dynamics simulation. The methods employ smoothing functions to project the atomic forces obtained in contact simulation onto the contact plane for fitting to standard continuum contact models and subsequent analysis. Importantly, these methods allow for contact evolution between nanoscopic asperity-asperity contacts to be examined because these are the central load-bearing junctions at interfaces. To demonstrate the application and features of this approach, it was employed to examine the evolution of contact between silica nanoasperities, with an increasing density of octadecyltrichlorosilane (OTS) films employed as a model adsorbate film. Linearly increasing contact radius and linearly decreasing maximal pressure were observed as a function of the film packing density. Because contact between the underlying, high-energy silica surfaces is undesirable, the evolution of silica contact was also examined using these same methods. As more molecules were introduced into the contact, a sharp transition was observed from the narrow, high-pressure interaction between the underlying substrates, to a broad, substantially lower pressure interaction, indicating a sharp transition from the dry to lubricated condition. To study the dependence of these behaviors on contact morphology, silica nanoasperities in contact with a flat silica surface were also examined. Similar behavior, including the broadening of the contact area and the minimization of direct surface contact, were observed. The method developed herein is applicable to a variety of systems and can be employed to optimize surface protection and pressure redistribution by boundary lubricants. This method can also be extended to AFM adhesion measurements where a detailed understanding of the true contact area is critical for the quantitative determinations of molecular forces and local surface mechanics.

Structural Characterization of Alkylsilane and Fluoroalkylsilane Self-Assembled Monolayers on SiO2 by Molecular Dynamics Simulations

“…[27][28][29][30][31] Turning to SiO 2 surfaces, alkoxysilane-coated SiO 2 nanoparticles have been simulated both in vacuum 32 and in alkane solvents; 26 Ewers and Batteas also studied functionalized silica interfaces, focusing on the modification of friction and attractive forces introduced by OTS coatings. 33 and forming, to mimic the SAM formation process, 36 but obtaining tilt angles quite far from experimental estimates. Contrary to OTS, fluorinated SAMs were less studied, with a main focus on wetting and friction properties, [37][38][39][40] and their structure on SiO 2 was never investigated in detail.…”

Scale-Dependent Friction-Coverage Relations and Non-Local Dissipation in Surfactant Monolayers

“…Similar behaviour has previously been observed in MD simulations of contacting solid nanoasperities 45 as well as amorphous silica nanoasperities partially separated by low coverage OTS monolayers. 91 For both tip sizes, the maximum σ zz is not localised directly underneath the center of the sliding tip, but rather towards the leading edge. Similar observations have been made from previous NEMD simulations of close-packed alkyl monolayers indented with a spherical fullerene tip.…”

Scale-Dependent Friction-Coverage Relations and Non-Local Dissipation in Surfactant Monolayers

“…as well as amorphous silica nanoasperities protected by low coverage OTS monolayers. 44,45 Strong localisation of σ zz is indicative of larger local pressures than would be predicted using continuum contact mechanics models. 54 Since this only occurs for the sharpest tip, lowest coverage, and highest load considered, Hertz theory provides a reasonable approximation of the pressure distribution in these systems.…”

“…They observed a spatial anisotropy of pressure-induced defects within ODPA domains; molecules whose initial tilt was in the direction of the pressure gradient increased in tilt angle, while those tilted away formed gauche-defects. 43 Ewers and Batteas 44,45 investigated the compression of OTS films adsorbed on amorphous silica (a-SiO 2 ) surfaces at a range of coverages (Γ = 1.5-3.5 nm −2 ) with a hemispherical a-SiO 2 tip (r tip = 4.5 nm).…”

“…As the surface coverage was increased, a sharp transition was observed from the narrow, high-pressure interaction between the underlying substrates, to a broad, substantially lower pressure interaction. 44,45 Some NEMD simulations have also been performed to study the frictional response of SAMs penetrated by single nanoscale asperities. For example, Knippenberg et al 46 studied friction in a close-packed (Γ = 4.6 nm −2 ) alkyl monolayer (C 14 ) indented with a spherical fullerene tip (r tip = 1.3 nm) at high sliding velocity (v s = 87 m s −1 ).…”