Unsteady-state flow of flexible polymers in porous media

In this paper we report an investigation of the unsteady-state flow of poly mer solutions through granular porous media. The experiments were performed using high-molecular-weight nonionic and anionic polyacrylamides dissolved in water containing NaCl and model porous media obtained by packing silico n carbide (SiC) grains having a narrow grain size distribution. Before inje ction in porous media, the polymer solutions were carefully filtered accord ing to a method that was proved to be efficient in removing any possibly re maining microgels, The SIC grain surface was passively oxidized by a contro lled thermal treatment in order to obtain a surface partially covered by a thin silica layer having adsorption properties similar to those of quartzit ic sand. By packing SiC grains of different sizes, porous media having iden tical adsorption properties and well-known pore throats sizes can be obtain ed with a good reproducibility. Parameters investigated include pore size, velocity gradient, polymer concentration, and adsorption energy. A striking unsteady-state flow behavior (pressure build-up at constant Row rate) is o bserved when three conditions are fulfilled: (a) the velocity gradient is l arger than that known to be able to induce a coil-stretch transition, (b) t he polymer adsorbs on the pore surfaces, and (c) the length of stretched ma cromolecules is larger than the effective pore throat diameter. When one of these conditions is not satisfied the Row remains steady. These observatio ns are interpreted by a mechanism involving the adsorption and bridging acr oss pore restrictions of elongated chains. We propose to refer to this pecu liar mode of polymer adsorption as bridging adsorption, (C) 2001 Academic P ress.