Macromolecular strain in periodic models of porous media flows

This paper is concerned with the deformation of macromolecules in flows which model porous media and its influence on macrorheological parameters. Two flow cells ("oscillatory convergent-divergent" and "cylinder array") have been constructed. Adding an extra cylinder to the symmetry axis of the cylinder array cell creates a stagnation point. These cells simulate important extensional components of the flows occurring in porous media, yet they possess well-defined geometries and better characterised flow fields. A numerical simulation of viscous flow in the cylinder array cell applicable to the flow regimes used in experiments has been performed. Solutions of high molecular weight polystyrene (with concentrations well below the conventional critical concentration) were used in the experiments. Birefringence measurements revealed that molecules can achieve large strains, even in extensional flows with short residence times. This observation is discussed using a dumb-bell model with conformation dependent elastic and friction coefficients. Simultaneous pressure drop measurements showed the onset of viscoelasticity coupled with an increase in birefringence intensity. Concentration effects, the influence of the stagnation point on the observed onset of viscoelasticity and a possible mechanism of thickening are discussed.