Interfacial Slip Violations in Polymer Solutions: Role of Microscale Surface Roughness

Abstract

Violations of the no-slip hydrodynamic boundary condition are investigated in polymer solutions using gap-dependent shear flow measurements between parallel plates. Solutions of high molar mass, 3.8 × 106 ≤ Mw ≤ 20.06 × 106, narrow molecular weight distribution polystyrene, PS, in diethyl phthalate, DEP (a good solvent of low volatility), are the main focus of the study. Violations of the no-slip condition in these systems are evident from an apparent reduction in solution viscosity as the plate separation is reduced. Viscosity measurements at multiple plate separations allow the slip velocity Vs and slip extrapolation length b to be quantified. This article reports b and Vs for polymer solutions over a wide range of shear stress and solution concentrations, 0.025 ≤ φ ≤ 0.079, that is, for polymer solutions spanning the range from marginally entangled to highly entangled liquids. For the most entangled solutions studied, extremely large extrapolation lengths, b > 200 μm, are observed. Extrapolation lengths b ranging from 2 to 30 μm are observed for less entangled PS/DEP solutions, indicating that significant levels of slip are possible even in moderately entangled polymer liquids. The extrapolation length is also reported to manifest a nonmonotonic but universal dependence on shear stress for the solutions studied. These observations are compared with expectations for slip by polymer depletion, adhesive failure, and polymer disentanglement processes. The effect of surface roughness on slip violations is also investigated using parallel plate fixtures roughened by attaching a single layer of micro- or nanosized silica glass spheres. Surfaces with root-mean-square (rms) roughness, Rq, ranging from molecularly smooth to macroscopically rough, 9 nm ≤ Rq ≤ 1.4 × 104 nm, are created using this procedure. We find that rms surface roughness Rq ≥ 0.65Rg effectively eliminates interfacial slip violations in entangled polymer solutions. This finding is discussed in terms of the mechanism of slip in entangled polymer systems.