General shear-thinning dynamics of confined fluids

The shear properties for a number of thin fluid films under high pressure were investigated as a function of sliding velocity (shear rate) using the surface forces apparatus. It was found that the relationship between the effective viscosity η_eff and shear rate γ̇ in the shear-thinning regime could be expressed by a simple equation, log₁₀ η_eff =C - nlog₁₀γ̇, where C ≈ 4.7 ± 0.2 and n ≈ 0.9 ± 0.1. This equation can be applied to a variety of fluid systems from simple liquids to polymer melts, which transition to glasslike phases in confined geometries. Since the effect of confinement on the "slowing down" of molecular motions is equivalent to that of decreasing temperature, this universally can be explained using conventional glass-transition theories for bulk fluids. Assuming the confined fluid to be in a state where dynamics are dominated by excluded volume effects, its η_eff should correspond to that of the bulk at or near the glass-transition temperature. Thus, characteristic relaxation times in the system should correlate with the time scales of the primary relaxation processes associated with submolecular rearrangements, which are an essential feature of the glass transition and not very different for various fluid materials.