THE ROLE OF END-EFFECTS ON MEASUREMENTS OF EXTENSIONAL VISCOSITY IN FILAMENT STRETCHING RHEOMETERS

The transient extensional viscosity function of two semi-dilute polyis obutylene polymer solutions is investigated in a filament stretching r heometer of the type developed by Tirtaatmadja and Sridhar (J. Rheol., 37 (1993) 1081). A velocity compensation algorithm which yields a con stant deformation rate at a single point in both Newtonian and non-New tonian fluid samples is detailed. Good experimental reproducibility is obtained in the device and measurements with a viscous Newtonian oil yield steady-state Trouton ratios of <(eta)over bar>/mu = 3 +/- 0.5. B oth viscoelastic fluids show the onset of significant strain-hardening for Hencky strains greater than two, and transient extensional viscos ities that increase by three orders of magnitude. Good agreement betwe en the results for the two different fluids is obtained when tests are performed at identical values of the Deborah number. The maximum Henc ky strain achievable in the device is epsilon = 5 and steady-state val ues of the extensional viscosity are never achieved over the range of strain rates attainable. Measurements show that the evolution of the t ensile force exerted by the deforming filament is a strong function of the initial aspect ratio of the cylindrical test sample. A lubricatio n analysis for small sample aspect ratios demonstrates that this varia tion results from large radial pressure gradients arising from the non -homogeneous shear flow near the rigid disks. This simplified analysis provides a good description of the experimental observations, and tes ts conducted with the Newtonian oil suggest that only at Hencky strain s epsilon > 2 does the extensional deformation of the filament dominat e the shearing flow. In the case of viscoelastic fluid filaments, the initial shearing motion near the fixed endplates significantly affects the measured tensile stress in the filament at all later times in the extension. Careful attention is focused on the non-homogeneous deform ation induced at the endplates that constrain the fluid sample at each end of the test apparatus. Measurements of the filament profile and s urface curvature with a video-imaging system show distinct differences in the evolution of Newtonian and non-Newtonian samples. In both case s, different strain histories are experienced by fluid elements at dif ferent axial positions in the filament, and a spatially homogenous def ormation is not achieved in the viscoelastic filaments until Hencky st rains epsilon > 4 are attained. Observations at larger strains and hig h Deborah numbers, De > 3, indicate the onset of an elastic instabilit y near the stationary endplate which results in the deforming fluid co lumn partially decohering from the endplate.