We have studied both the equilibrium and the hydrodynamic lubrication forces that act in the normal direction between mica surfaces in toluene when bearing polystyrene chains anchored to each surface by a zwitterionic group at one end. The quasistatic force-distance profiles reveal the long-ranged repulsion characteristic of steric interactions between the extended brush-like layers in the good solvent and provide a measure of the brush thickness L in close agreement with earlier studies of such brushes. Dynamic measurements of the surface forces, where the surface separation D is varied sinusoidally in time, show two regimes: at D > 2.5L, the hydrodynamic forces are characteristic of Newtonian flow of liquid with the viscosity of bulk toluene, but with a shear plane shifted a distance Lh from each mica surface (qualitatively similar to earlier observations with adsorbed homopolymer layers). We find that L == Lh• When the layers are compressed {D < 2L), the hydrodynamic forces are dominated by flow of solvent through the confined polymer layers in the gap. They may be described by an "effective viscosity" which increases as D decreases, varying in fair quantitative accord with a recent model. At the highest compressions (D < L/2) the confined polymer layers behave in a solid-like manner, suggesting that the confinement between the surfaces greatly increases molecular relaxation times.
Stick-slip, or interrupted, motion rather than smooth uninterrupted motion occurs in many different phenomena such as friction, fluid flow, material fracture and wear, sound generation, and sensory "texture." During stick-slip, a system is believed to undergo transitions between a static (solid-like) state and a kinetic (liquid-like) state. The stick-slip motion between various types of pretreated surfaces was measured, and a second, much more kinetic state that exhibits ultra-low friction was found. Transitions to and from this super-kinetic state also give rise to stick-slip motion but are fundamentally different from conventional static-kinetic transitions. The results here suggest practical conditions for the control of unwanted stick-slip and the attainment of ultra-low friction.
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