Effect of structural transitions of n-hexadecane in nanoscale confinement on atomic friction

Atomic-scale friction of graphene supported on silica as a function of immersion time in non-polar n-hexadecane is investigated using atomic force microscope (AFM) and molecular dynamics (MD) simulations. With an increase in immersion time, n-hexadecane molecules diffuse and intercalate at the graphene-silica confinement initially, decreasing friction forces until a transition time, and beyond which the forces increase till the end of the measurement (∼70 h). This non-monotonic change in friction is explained by the structural transformations of the intercalated n-hexadecanes at graphene-silica nanoconfinement. A disorder-to-order transition of n-hexadecane molecules in the intercalated layer is established by measuring the structure of solvation layers formed above the graphene layer and the stick-slip friction loops. The intercalated layer structure affects the flexibility of graphene, determining the quality of the contact between the AFM tip and substrate, which dominates the atomic friction. Our combined experiments and simulations have enabled a fundamental understanding of dissipation mechanisms for supported-graphene in non-polar solvents, critical for employing two-dimensional materials as additives in friction-reducing or rheological oils.