Multivalent Adhesion and Friction Dynamics Depend on Attachment Flexibility

Self-assembled monolayers introduce chemical functionalities to material surfaces, providing a route to tune their equilibrium and dynamical properties. We report on atomic force microscopy measurements and simulations of adhesion and friction forces caused by a macromolecular host-guest system, where the host molecules are attached to silicon oxide surfaces by means of self-assembled silane layers. Different preparation routes for the silane layers lead to different flexibility of the molecular attachment. The velocity dependencies of the work of separation and of friction vary significantly for attachments with different flexibility. Stiff attachment leads to low pull-off forces at low pulling velocity and to vanishing friction forces in the limit of low sliding velocity. Flexible attachment enhances cooperative contribution of multiple molecular bonds to adhesion and friction and causes significant friction at low sliding velocity. The latter observation can be explained by the contribution of intermittent contact aging to the friction force.