Abstract
We study the elasto-capillary self-thinning and ultimate breakup of three polystyrene-based ideal elastic fluids by measuring the evolution in the filament diameter as slender viscoelastic threads neck and eventually break. We examine the dependence of the transient diameter profile and the time to breakup on the molecular weight, and compare the observations with simple theories for breakup of slender viscoelastic filaments. The evolution of the transient diameter profile predicted by a multimode FENE-P model quantitatively matches the data provided the initial stresses in the filament are taken into account. Finally, we show how the transient uniaxial extensional viscosity of a dilute polymer solution can be estimated from the evolution in the diameter of the necking filament. The resulting “apparent extensional viscosity” profiles are compared with similar results obtained from a filament 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 explained by examining the effective stretch rate in each configuration. Filament thinning studies and filament stretching experiments thus form complementary experiments that lead to consistent measures of the transient extensional viscosity of a given test fluid.
Highlights
Small quantities of polymeric additives often have very pronounced effects on fluid motion, and one particular type of flow that has been studied extensively in this regard is the necking and breakup of polymeric liquid jets
We have examined the dynamics of elasto-capillary thinning for three different model elastic liquids
The evolution in the midpoint diameter profile of necking and breaking fluid filaments was measured in a filament stretching device by elongating an initially cylindrical fluid filament to a predetermined Hencky strain and monitoring the evolution in Dmid(t) using a laser micrometer
Summary
ANNA AND MCKINLEY results, numerical simulations, stability analyses, and similarity solutions for the approach to breakup, is provided by Eggers1997͒. Chang and co-workers have presented an asymptotic analysis and numerical simulations of the elasto-capillary necking dynamics of viscoelastic jets described by the Oldroyd-B modelChang et al ͑1999͔͒. These stability analyses have shown that the extensional rheological response of a fluid dictates whether or not it will form stable jets and filaments. Schummer and Tebel1983͒ described a ‘‘free jet elongational rheometer,’’ in which the diameter of a periodically forced fluid jet undergoing capillary thinning is monitored as a function of time Another filament breakup device, referred to as the ‘‘microfilament rheometer’’ was introduced in 1990 ͓Bazilevsky et al ͑1990͔͒, and is similar in operation to the filament stretching device introduced at about the same timeMatta and Tytus1990͒; Sridhar et al ͑1991͒; Tirtaatmadja and Sridhar1993͔͒. We compare the molecular weight dependence of the steady-state extensional viscosity obtained from the asymptotic behavior of filament breakup experiments to the behavior predicted from kinetic theory for bead-spring chains
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