The adhesion and friction between pairs of ordered and disordered self-assembled monolayers on ${\mathrm{S}\mathrm{i}\mathrm{O}}_2$ are studied using molecular dynamics. The disorder is introduced by randomly removing chains from a well ordered crystalline substrate and by attaching chains to an amorphous substrate. The adhesion force between monolayers at a given separation increases monotonically with chain length at full coverage and with coverage for fixed chain length. Friction simulations are performed at shear velocities between $0.02–2\mathrm{m}/\mathrm{s}$ at constant applied pressures between 200 and 600 MPa. Stick-slip motion is observed at full coverage but disappears with disorder. With random defects, the friction becomes insensitive to chain length, defect density, and substrate.

• Received 3 May 2004

DOI:https://doi.org/10.1103/PhysRevLett.93.166103