STICK TO SLIP TRANSITION AND ADHESION OF LUBRICATED SURFACES IN MOVING CONTACT

The friction of dry self-assembled monolayers, chemically attached to a solid surface and comprising a well-defined interface for sliding, i s compared to the case of two solids separated by an ultrathin confine d liquid. The monolayers were condensed octadecyltriethoxysilane (OTE) . The liquid was squalane (C30H62), a film 2.0 nm thick confined betwe en parallel plates of mica. The method of measurement was a surface fo rces apparatus, modified for oscillatory shear. The principal observat ions were the same in both cases: (1) Predominantly elastic behavior i n the linear response state was followed by a discontinuous transition to a mostly dissipative state at larger deformations. The elastic ene rgy stored at the transition was low, of the order of 0.1 kT per molec ule. This transition was exactly repeatable in repetitive cycles of os cillation and reversible with pronounced hysteresis. (2) The dissipati ve stress in the sliding state was almost independent of peak sliding velocity when this was changed over several decades. Significant (alth ough smaller) elastic stress also persisted, which decreased with incr easing deflection amplitude but was almost independent of oscillation frequency. (3) The adhesive energy in the sliding state was significan tly reduced from that measured at rest. This similarity of friction in the two systems, dry and wet sliding, leads us to speculate that, sim ilar to plastic deformation of solids, sliding in the confined liquid films is the result of slippage along an interface.