Apparent Slip of Newtonian Fluids Past Adsorbed Polymer Layers

Abstract

We show unexpected drag reduction as solvent flows past polymer-coated surfaces in solution. Adsorbed polymer layers of two types, poly(vinyl alcohol) (PVA) homopolymers in deionized water and poly(vinylpyridine)−polybutadiene diblock copolymers (PVP−PB) in tetradecane, were placed at variable spacings larger than twice the layer thickness within a modified surface forces apparatus, and the hydrodynamic forces owing to flow of solvent past these layers were measured as a function of surface spacing, pumping frequency, and pumping velocity (product of frequency and amplitude). When the flow rate was below a critical level, which depended on the system, the findings agreed with a simple hydrodynamic picture in which the solvent appeared to flow past surfaces of defined spacing, the solid−solid spacing less twice the “hydrodyamic radius” (RH). The value of RH was not always constant, however. In the PVA system it tripled as frequency was raised from 1 to 50 Hz though it was frequency-independent in the PVP−P...