THE CONFORMATION OF SEMIRIGID POLYMERS DURING FLOW-THROUGH A FIXED FIBER BED

Linear conservative birefringence measurements of dilute and semi-dilu te solutions of the semi-rigid polymer xanthan gum in a glycerine and water solvent are conducted during flow through a disordered fiber bed . Even though the plug flow through the bed contains no average veloci ty gradient our results show that the xanthan gum molecules stretch, o n average, in the direction of flow. The polymer stretch or birefringe nce increases with the number of fiber interactions within the bed unt il a steady-state conformation is attained. The degree of stretch incr eases monotonically with increasing De(pore) (the ratio of a character istic polymer relaxation time and the characteristic flow time in a be d pore). The results show a number of qualitative differences when com pared to those for the flow of flexible polyisobutylene (PIB) (Evans e t al., J. Fluid Mech., 28 (1994) 319) through the same fiber bed. In p articular, xanthan gum attains a steady-state conformation after conve cting 12 pore-lengths in the bed (compared to approximately 25 for the PIB solution) and this value is independent of De(pore) whereas it in creased with increasing De(pore) for the flexible polymer. Additionall y the steady-state birefringence of the xanthan gum solutions displays no critical De(pore) at which the birefringence increases dramaticall y (as does the flexible polymer system) and instead exhibits a slow gr owth with increasing De(pore). These unique results are interpreted an d understood using novel Brownian dynamics simulations of a semi-rigid bead-rod polymer model flowing through a simulated stochastic porous bed flow field created via a spectral expansion. Our results show that the simulations can predict most of the experimental results, at leas t qualitatively, and in some instances quantitatively, and therefore p rovide an important tool for understanding these complicated phenomena .