Control of adhesion and friction between surfaces in liquid with self-assembled monolayers of alkylsiloxanes

Control of friction and wear is critical to the design and effectiveness of micro- and nanoscale processes in liquid environments, e.g. fluidic self-assembly (FSA) and microelectromechanical systems (MEMS). We used several scanning probe microscopy techniques to investigate the use of self-assembled monolayers (SAMS) formed from alkylsilanes (SiClm(CH2) n-X, m = 0--3, n = 0--3), on silicon/silicon dioxide substrates as molecularly thin lubricants in liquid environments over a wide range of operating conditions. We identified contacts between methyl-methyl and methyl-carboxyl terminated surfaces in weakly polar solvents such as ethanol as potentially useful combinations of surface and solvent for use in FSA and MEMS that resulted in both low adhesive forces and low friction coefficients.;We investigated the effect of the solvent environment on the friction behavior of methyl-terminated SAMs of octadecyltrichlorosilane (OTS) in n-alcohols, n = 1--12, as a function of sliding velocity and applied normal load. This system displayed three load-dependent characteristic friction behaviors, as well as maxima in the friction force as a function of sliding velocity at intermediate loads of 20--40 nN. By analogy with bulk viscoelastic systems, we interpreted these maxima in terms of characteristic time scales and activation energies and proposed a molecular model for the energy dissipation between the SAMs and the SPM tip. The maxima result from localized relaxation processes that occur in the compressed region under the tip and depend on the nature and extent of solvent partitioning in the SAM.;We tested the validity of our model by investigating the effects of changes in the chain density and temperature on the appearance of the maxima in the friction force as a function of sliding velocity and applied normal load. We found that the maxima in the friction force shifted to higher sliding velocities with increases in temperature from 25--75°C and decreases in chain density, consistent with the predictions of our model.