Elasto-capillary thinning and breakup of model elastic liquids

We study the elasto-capillary self-thinning and ultimate breakup of three p olystyrene-based ideal elastic fluids by measuring the evolution in the fil ament diameter as slender viscoelastic threads neck and eventually break. W e examine the dependence of the transient diameter profile and the time to breakup on the molecular weight, and compare the observations with simple t heories for breakup of slender viscoelastic filaments. The evolution of the transient diameter profile predicted by a multimode FENE-P model quantitat ively matches the data provided the initial stresses in the filament are ta ken into account. Finally, we show how the transient uniaxial extensional v iscosity of a dilute polymer solution can be estimated from the evolution i n the diameter of the necking filament. The resulting "apparent extensional viscosity" profiles are compared with similar results obtained from a fila ment stretching rheometer. Both transient profiles approach the same value for the steady state extensional viscosity, which increases with molecular weight in agreement with the Rouse-Zimm theory. The apparent discrepancy in the growth rate of the two transient curves can be quantitatively explaine d by examining the effective stretch rate in each configuration. Filament t hinning studies and filament stretching experiments thus form complementary experiments that lead to consistent measures of the transient extensional viscosity of a given test fluid. (C) 2001 The Society of Rheology.