OBSERVATIONS OF POLYMER CONFORMATION DURING FLOW-THROUGH A FIXED FIBER BED

Linear birefringence measurements of dilute and semi-dilute polyisobut ylene solutions following flow through a disordered fixed fibre bed of 2.47% solids volume fraction provide both transient and steady measur ements of chain deformation. Our results indicate that the flexible po lyisobutylene polymers undergo a large conformation change, stretching in the direction of the average flow. This occurs even though the ave rage flow in the bed is a plug flow which would not cause any polymer stretch by itself. The polymer stretch or conformation change increase s with the number of chain interactions with bed fibres and ultimately reaches a steady-state value that can be correlated with the pore-siz e Deborah number (i.e. a characteristic polymer relaxation time divide d by a characteristic flow time in the bed pore). Large changes in the polymer conformation are noted for values of the Deborah number, De > 5. In addition, the time to steady state scales with the characterist ic flow time within a pore over a large range of Deborah numbers. The pressure drop across the fibre bed was also measured simultaneously wi th the birefringence measurement and was found to be directly proporti onal to the birefringence throughout the range of De investigated. Thu s, we show empirically, for the first time, that chain elongation, whi ch produces normal stress anisotropy within the fluid, is directly res ponsible for the increased flow resistance. These findings are then an alysed in the light of recent theories for the response of polymer mol ecules in fixed bed flow fields (Shaqfeh and Koch 1992). It is shown t hat our results are consistent with the interpretation that these flow s are stochastic strong flows, which create an apparent 'coil-stretch' transition. After extending the theory of Shaqfeh and Koch to account for the specifics in the experiments, including the bed geometry and statistics as well as the polydispersity of the polymer solutions, it is shown that the theory can predict most of the experimental results both qualitatively and quantitatively.