Quantifying lubricant droplet spreading on a flat substrate using molecular dynamics

Understanding the physical behavior of polymer-based lubricants on the nanoscale is of critical importance to a myriad of engineering applications and devices. We have used molecular dynamics simulations to quantitatively evaluate the physical mechanisms underlying perfluoropolyether lubricant spreading on a solid substrate. We quantify the effect of molecular mass, molecule length, and lubricant and substrate functional end groups on lubricant spreading. The results show that lubricant functional end groups play a critical role in lubricant spreading on the nanoscale. Lubricant spreading increases with increasing molecule length for lubricant with functional end groups, but decreases with the increase in molecule length for lubricant without functional end groups. In the former case, the fraction of the lubricant chain that is functional is the primary driving factor for lubricant spreading, while in the latter case, the molecular mass is most important. For both lubricants with and without functional end groups, spreading is inhibited by molecule entanglement beyond a critical molecule length, and spreading becomes independent of lubricant functional end groups and molecular mass.