NUMERICAL-SIMULATION OF EXTENSIONAL DEFORMATIONS OF VISCOELASTIC LIQUID BRIDGES IN FILAMENT STRETCHING DEVICES

Large extensional deformations of viscoelastic fluid columns in filame nt stretching rheometers are studied through numerical simulations up to Hencky strains of greater than epsilon = 4. The time-dependent axis ymmetric calculations incorporate the effects of viscoelasticity, surf ace tension, fluid inertia, plus a deformable foe surface and provide quantitative descriptions of the evolution in the filament profile, th e kinematics in the liquid column and the resulting dynamic evolution in the viscous and elastic contributions to the total stress. In addit ion to investigating the variation in the apparent Trouton ratio expec ted in experimental measurements using this new type of extensional rh eometer, we also investigate the generic differences between the respo nse of Newtonian and viscoelastic fluid filaments described by the Old royd-B model. For small strains, the fluid deformation is governed by the Newtonian solvent contribution to the stress and the filament evol ution is very similar in both the Newtonian and viscoelastic cases. Ho wever, in the latter case at large strains and moderate Deborah number s, elastic stresses dominate leading to strain-hardening in the axial mid-regions of the column and subsequent drainage of the quasi-static liquid reservoir that forms near both end-plates. These observations a re in good qualitative agreement with experimental observations. For s mall initial aspect ratios and low strains, the non-homogeneous deform ation predicted by numerical simulations is well described by a lubric ation theory solution. At larger strains, the initial flow non-homogen eity leads to the growth of viscoelastic stress boundary layers near t he free surface which can significantly affect the transient Trouton r atio measured in the device. Exploratory design calculations suggest t hat mechanical methods for modifying the boundary conditions at the ri gid end-plates can reduce this non-homogeneity and lead to almost idea l uniaxial elongational Bow kinematics. (C) 1998 Elsevier Science B.V.