FLOW PAST A SPHERE IN POLYSTYRENE-BASED BOGER FLUIDS - THE EFFECT ON THE DRAG COEFFICIENT OF FINITE EXTENSIBILITY, SOLVENT QUALITY AND POLYMER MOLECULAR-WEIGHT

This study experimentally investigates how the drag behavior of a sphe re falling in a Boger fluid is affected by the fluid's extensibility, solvent quality and chain molecular weight. By means of terminal veloc ity measurements under conditions of creeping flow, deviations from Ne wtonian drag behavior in a homologous series of three polystyrene-base d Boger fluids were measured as a function of Weissenberg number (We). The polymer-solvent quality and polymer molecular weight of each of t he Boger fluids were manipulated in a manner that produced a systemati c variation in the degree of extensibility of the fluids. Here, we def ine extensibility in terms of the FENE model parameter L, the ratio of a fully extended coil length to an equilibrium coil length. It is exp ected that L varies by nearly a factor of three over this series of Bo ger fluids. The compositions and ranking of the degree of extensibilit y of the three Boger fluids are the following: a high extensibility fl uid of 2.0 x 10(7) g mol(-1) polystyrene in a poor, dioctyl phthalate (DOP)-based solvent, a medium extensibility fluid of 2.0 x 10(7) g mol (-1) polystyrene in a good, tricresyl phosphate (TCP)-based solvent an d a low extensibility fluid of 2.0 x 10(6) g mol(-1) polystyrene in a poor DOP-based solvent. The principal result of the falling sphere exp eriments is that while the low extensibility fluid shows only small de viations from Newtonian behavior at all tested We, the two fluids of g reater extensibility show drag enhancement of more than 300% for We > 1. These experiments are in agreement with the earlier work of Tirtaat madja et al. and Chmielewski et al., indicating that the drag does not correlate with We alone. Here, by using monodisperse polymers and sys tems in which the polymer-solvent interactions have been characterized , we are also able to determine the sensitivity of the dimensionless d rag to polymer molecular weight and polymer-solvent quality.