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

BW Ewers and JD Batteas, LANGMUIR, 30, 11897-11905 (2014).

DOI: 10.1021/la500032f

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.

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