Effects of Polymer Concentration and Molecular
Weight on the Dynamics of Visco-Elasto-Capillary Breakup
by Matthieu Verani
Capillary-break-up measurements of viscoelastic polymer solutions are performed using a Capillary Breakup Extensional Rheometer (CABER). The device consists of two coaxial plates which are used to form and hold a liquid bridge of the test fluid. An axial step strain is applied to the fluid by raising the top plate and the elongated fluid thread then evolves towards breakup under the combined action of viscous, elastic and capillary forces. The test fluids used in the present study are a series of diluted polystyrene Boger fluids (‘PS025’ and its dilutions) and a new polystyrene Boger fluid (labelled ‘MV1’) comprising of a lower molecular weight solute. This fluid is less susceptible to gravity, and allows us to observe the coil-stretch transition of high molecular weight polymers. The persistent dependence of the measured relaxation time on concentration, even in the dilute regime predicted from theory, is demonstrated both experimentally and numerically. Indeed, numerical simulations of the evolution of the stress contributions and of the radius using a single mode FENE-P model based on the one-dimensional analysis of Entov and Hinch for transient extensional flows are compared to the experimental observations of the radius of the liquid filament. Below a critical dilution, the stress in the necking thread is carried solely by the solvent with no appreciable contribution from the polymer chains, and the dynamics of the necking process change appreciably. A sensitive force transducer is also added to the CABER. This allows us to measure the initial tensile stress resulting from the axial step strain. Existing one dimensioned models for analyzing capillary breakup measurements have assumed this to be identically zero. However recent similarity solutions for viscoelastic models have shown that in fact the force is not zero but monotonically decays towards zero at the same necking rate as the filament radius. We present the first experimental measurements of this small but finite (~O(1e-4 N)) tensile force and show that indeed it decays with similar dynamics to the measured radius.