The shape of a falling jet formed by concentrated polymer solutions

Abstract

We studied the shape of free-falling stable jets created by viscoelastic concentrated poly(acrylonitrile) solutions that were pressed out of a capillary at different outputs. The viscosity of the solutions varied by more than 1000 times, and the elasticity increased along with growing concentration. The main goal of the study was to compare theoretical predictions with experimental data. The theoretical argumentations were based on the momentum and rheological equations. We analyzed the superposition of viscoelastic, capillary, and inertial forces for fluids with different rheological properties flowing with different velocities changing more than 100 times. Although elasticity is definitely present, the Weissenberg number in all cases under study appeared less than one and, respectively, played a secondary role except for the most concentrated solution. Then we discussed the applicability of two main models based on the combination of visco-inertial and capillary-inertial forces. The best fitting and dominant input of different forces appeared dependent on the properties of the fluids and conditions of flow. At low polymer concentration, the jet profile corresponds better to the capillary-inertial model, while the visco-inertial regime of flow becomes dominant at higher velocities and highly viscous solutions. At very high concentrations (25% in our case), both of the considered models do not allow describing the complete experimental data due to the increasing role of elasticity.