MACROMOLECULAR STRAIN IN PERIODIC MODELS OF POROUS-MEDIA FLOWS

This paper is concerned with the deformation of macromolecules in flow s which model porous media and its influence on macrorheological param eters. Two flow cells (''oscillatory convergent-divergent'' and ''cyli nder array'') have been constructed. Adding an extra cylinder to the s ymmetry axis of the cylinder array cell creates a stagnation point. Th ese cells simulate important extensional components of the flows occur ring in porous media, yet they possess well-defined geometries and bet ter characterised flow fields. A numerical simulation of viscous flow in the cylinder array cell applicable to the flow regimes used in expe riments has been performed. Solutions of high molecular weight polysty rene (with concentrations well below the conventional critical concent ration) were used in the experiments. Birefringence measurements revea led that molecules can achieve large strains, even in extensional flow s with short residence times. This observation is discussed using a du mb-bell model with conformation dependent elastic and friction coeffic ients. Simultaneous pressure drop measurements showed the onset of vis coelasticity coupled with an increase in birefringence intensity. Conc entration effects, the influence of the stagnation point on the observ ed onset of viscoelasticity and a possible mechanism of thickening are discussed.