(192bh) Screening Self-Assembled Monolayers for Lubrication Properties: Trends and Pitfalls

Self-assembled monolayers present an appealing platform for creating fine tuned lubrication schemes for micro- and nanoelectromechanical devices (MEMS and NEMS). In addition to the choice of substrate and its roughness, monomers can vary in length as well as functional group and mixtures of monomers further expand the chemical parameter space of possible materials. Using the mBuild[1] and Foyer[2] tools - developed to work together under the MoSDeF [3] project - we create a pipeline to rapidly screen self-assembled monolayers across alkane and PEG monomers with varying chain lengths and head groups on both crystalline and amorphous silica substrates.

Our results suggest that the added flexibility of PEG chains slightly decreases coefficient of friction on more realistic, amorphous substrates and even though it decreases monolayer ordering. On rougher substrates, the added flexibility provides more fluid pathways for frictional dissipation when surfaces slide past one another. Additionally, the automated procedure used to screen these monolayers revealed several quirks in the simulation setup that further strengthen the case for using realistic, amorphous silica substrates to model self-assembled monolayers. While crystalline silica substrates are frequently used in simulation studies for convenience, our results demonstrate that these idealized systems can result in anomalous behavior related to lateral vibrations during shearing.

[1] C. Klein, J. Sallai, T.J. Jones, C.R. Iacovella, C. McCabe, P.T. Cummings. "A hierarchical component based approach to screening properties of soft matter." accepted at Mol. Mod. and Sim.: Applications and Perspectives

[2]  C. Klein, A. Z. Summers, T. Ma, C. R. Iacovella, and J. Sallai, “Foyer.” [Online]. Available: https://github.com/mosdef-hub/foyer.

[3]  https://github.com/mosdef-hub.