Lateral Force Microscopy Study of the Frictional Behavior of Self-Assembled Monolayers of Octadecyltrichlorosilane on Silicon/Silicon Dioxide Immersed in n-Alcohols

Lateral force microscopy (LFM) was used to evaluate the frictional behavior of surfaces modified with self-assembled monolayers (SAMs) and immersed in n -alcohols (CH 3 (CH 2 ) x OH where x = 0−8,11) as a function of applied normal load, sliding velocity, and solvent chain length. SAMs were formed from octadecyltrichlorosilane (OTS) on silicon/silicon dioxide substrates. The objective was to investigate how the solvent environment affected the frictional behavior of OTS and to characterize the effectiveness of OTS as a boundary lubricant in liquid environments for applications such as fluidic self-assembly and microelectromechanical devices. Three characteristic frictional regimes were observed at low, intermediate, and high loads. Maxima as a function of the sliding velocity appeared in the frictional forces for intermediate applied normal loads of ∼20−40 nN for x = 1−8. These maxima shifted to lower sliding velocities with increases in the applied normal load and with increases in the chain length of the solvent. The frictional maxima were interpreted by adapting concepts of viscoelasticity for bulk polymer systems to the two-dimensional systems of SAMs. Maxima were interpreted to result from localized relaxation processes in the SAMs that depend on the extent of solvent partitioning in the compressed region under the tip. The characteristic relaxation times of the alkyl chains increased with increased applied normal load and with increased solvent chain length. The behavior as a function of x was consistent with both a mechanism of solvent partitioning controlled by the free volume distribution in the SAM and a mechanism of insertion into defects. The relaxation times of the alkyl chains were related to a molecular model of energy dissipation based on the adsorption and desorption of the chain tails from the surface of the atomic force microscopy tip. The total frictional forces was consistent with superposition of relaxation processes and viscous drag on the tip and plowing effects that become dominant at high applied normal loads.