(509h) Characterizing Tip-Surface Contact Between Monolayer Functionalized Silica Surfaces through Molecular Dynamics Simulation

Chemisorbed alkylsilane monolayer coatings have been shown to reduce friction, adhesion, and wear of surfaces under shearing contact.  Exploration of monolayer properties at the nanoscale is commonly performed through atomic force microscopy (AFM).  However, inconsistencies in values and trends of friction properties may arise between separate studies as a result of variance in probe tip size. Flater et al. proposed that for tip contact of a monolayer-functionalized surface, plowing of monolayer chains provides an additional contribution to friction force.[1] Molecular dynamics simulations of Harrison and coworkers reveal an increased coefficient of friction during shearing of a buckyball tip compared to shearing of an infinitely flat amorphous surface, supporting the idea of molecular plowing presence and the dependence of shear behavior on tip size.[2] However, it remains unclear under which contact dimensions tip-surface contact behavior begins to deviate from that of infinitely flat (surface-surface) contact.  Furthermore, although simulations of surface-surface contact may provide a good approximation to the contact behavior of many AFM tips, system size effects have yet to be fully explored.  Using molecular dynamics simulations, we examine system size dependence during surface-surface contact for alkylsilane monolayers and study the influence of substrate size for both flat and curved tip-like contacting agents to help better define the tip-surface contact regime.

For systems of surface-surface contact, we find contact behavior to have a minimal dependence on system size; however, clear trends arise for systems of tip-surface contact.  We observe a distinct transition in friction force as a function of substrate size for flat tip contacting agents.  We attribute this to size influence on monolayer disorder arising from chain tilting at tip edges.  A similar transition is found for contacting agents displaying curvature, where shear behavior is shown to transition from sliding to a plowing-like mechanism.  This behavior is investigated through examination of penetration depth and local disorder of the countersurface monolayer, which reveals that less highly curved tips are unable to exhibit sufficient monolayer penetration and may be effectively modeled through surface-surface simulations. 

[1]      Flater EE, Ashurst WR, Carpick RW, Nanotribology of octadecyltrichlorosilane monolayers and silicon: self-mated versus unmated interfaces and local packing density effects, Langmuir, 23, (2008).

[2]      Knippenberg MT, Mikulski PT, Harrison JA, Effects of tip geometry on interfacial contact forces, Modelling and Simulation in Materials Science and Engineering, 18, (2010).