Drag reduction in polymer mixtures

Abstract

Drag reduction was studied in dilute toluene solutions of a mixture of two polymers: polyisobutylene (of three different molecular weights) and 1,4-cis-isoprene rubber in the turbulent region at low (up to 5000) Reynolds numbers. Experiments were carried out with mixed solutions at a concentration equal to optimum concentration of polyisobutylene or higher than it. Drag reduction of the polymer mixtures depending on the ratio of the two polymers showed a positive deviation from the additive straight line at all concentrations investigated. To evaluate the degree of deviation from additivity, the excess drag reduction, was introduced which represents the difference between the actually measured drag reduction and that read from the additive straight line. The excess drag reduction showed almost no dependence on the molecular weight of polyisobutylene in the investigated range of this magnitude. Deviation from additivity depending on the ratio of the two polymers in the mixture growed higher with increasing the flow rate at a given molecular weight of polyisobutylene. The highest excess drag reduction was observed in solutions containing a larger amount of the lower molecular isoprene rubber polymer. The effect of polymer coils on drag reduction in binary polymer solutions was studied. An assumption was made that higher drag reduction in the polymer mixtures as compared to the additive was due to the change of polymer coil dimensions caused by the copresence of the macromolecules of both polymers in the solution. It was further supposed that low shear stresses at which the experiments were carried out caused sufficient orientation and deformation of isoprene rubber enlarged molecules and the contribution of the latter in increasing drag reduction of the mixture was higher.