Friction anisotropy in confined alkanes: Linear and branched molecules

• Confined hexadecane (linear); and squalane (branched) films were simulated.
• Friction anisotropy was significant for hexadecane and was negligible for squalane.
• Friction anisotropy for hexadecane depends on the film thickness and sliding velocity.
• Mosaic like structures formed for the hexadecane.
• Such structures and film rigidity are sources of friction anisotropy for hexadecane.

Recent experiments with molecularly thin films have suggested that the existence of friction force anisotropy and non-zero transverse forces depend strongly on the structure of the lubricant molecule being linear or branched. Here results of molecular dynamics simulations of short alkanes (CnH2n+2) confined between model mica; support some of these findings. The results however suggest that the observed transverse force is thickness dependant and only persists for very thin films (less than 3 molecular layers). Simulations of a linear (hexadecane) and a branched (squalane) alkane systems show that such transverse frictional forces are smaller with the branched lubricant. Through comprehensive simulations it is shown that for hexadecane structural transition of the linear alkane films into tetratic order with the formation of crystalline bridges across the film is a contributing factor to existence of these anisotropic forces. By creating a disordered film it is also shown that the anisotropy increases with pressure, suggesting the rigidity of the film is also a contributing factor to the observed phenomenon.