Molecular-dynamics simulation of lateral friction in contact-mode atomic force microscopy of alkane films: The role of molecular flexibility

Abstract

Molecular-dynamics simulations are used to investigate lateral friction in contactmode atomic force microscopy of tetracosane (n-C24H50) films. We find larger friction coefficients on the surface of monolayer and bilayer films in which the long axis of the molecules is parallel to the interface than on a surface of molecules with the long axis perpendicular to the surface, in agreement with experimental results. A major dissipation mechanism is the molecular flexibility as manifested in the torsional motion about the molecules' C-C bonds. The generation of gauche defects as a result of this motion does not appear to be in itself a major channel of energy dissipation. As previously reported in the literature, the layer density and thereby the strength of the attractive film-tip interaction is also an important factor in energy dissipation.

Molecular Dynamics Simulations of Alkylsilane Monolayers on Silica Nanoasperities: Impact of Surface Curvature on Monolayer Structure and Pathways for Energy Dissipation in Tribological Contacts

“…Salmeron , has likewise considered extensively the pathways of load dissipation in alkane films, considering how chain tilt and the formation of both terminal and internal gauche defects play a role in this process. Finally, Soza et al demonstrate the formation of defects in tetracosane mono- and bilayers on a graphitic substrate after simulated scanning of an asperity across their surfaces and observed that friction is reduced upon artificial stiffening of the molecular torsions . This latter point implies that a more rigid film will produce lower friction by reducing energy dissipation into the surface, which is observed for more well-ordered, rigid SAMs. ,,, Defect formation, the conformational change from a trans methylene unit to a gauche methylene unit, is expected to increase on surfaces with curvature owing to the fact that, as the chains extend away from the surface, they must fill a greater volume.…”

“…It is commonly observed that disorder within the films increases as packing density decreases, and as a result, friction increases. There are two means by which increased disorder may increase friction: chemical effects, whereby the more disordered film has higher surface energy, and what may be described as kinetic effects, whereby dissipation is more favored in the less rigid, more disordered film. , Here packing is considered on flat, amorphous silica, where no underlying crystalline structure guides the packing of the film, using the same, uniform coverage assumption that was used in functionalization of the particle surfaces. It is important to note that OTS films on flat surfaces display considerable heterogeneity, with high and low density regions readily apparent by AFM imaging .…”

“…Because the silane binding locations were artificially dictated by the location of hydroxyl groups on the surface, and not by their optimal packing, this number represents an upper estimate of the extent of disorder in an optimally packed OTS film. The reduction of defects is expected to play a role in reducing the friction of these films, and molecular rigidity has also been observed to play a role, with less rigid surfaces demonstrating increased friction. , To characterize the rigidity of the monolayers, range-of-motion (ROM) calculations analogous to that used by Harrison and co-workers were employed to study the degree of molecular rigidity. In this analysis, the deviation of the vector connecting the anchor and end point of the molecules is averaged over all molecules and over the final 10 snapshots of the simulations (see Supporting Information), corresponding to the final 90 ps as with the other measurements presented.…”

“…Along with managing the heat generated in shearing contacts, minimizing friction, thereby minimizing the amount of heat the film must dissipate, is also of importance. Highly ordered films demonstrate lower shear stresses than those with greater disorder, , due to the lower surface energy of a well-ordered, methyl-terminated SAM and the reduced potential for energy dissipation into a more rigid surface layer. , To better understand how these films behave, and fail, in MEMS, it is necessary to understand how formation on rough surfaces affects the defect densities within the films as well as their ability to passivate the underlying surface, as this speaks directly to their ability to dissipate contact loads and survive repeated impact and shear.…”