Drop pinch-off and filament dynamics of wormlike micellar fluids

Observations are presented of several novel phenomena involved in the dynamics of a pendant drop of viscoelastic micellar fluid falling through air. Generally, when a drop falls a filament forms connecting it to the orifice; the filament eventually breaks due to an instability. The filament dynamics and instabilities reported here are very different from the standard Newtonian and non-Newtonian cases. At low surfactant concentration, the cylindrical filament necks down and pinches off rapidly (similar to10 ms) at one location along the filament. After pinch-off, the free filament ends retract and no satellite drops are produced. At higher concentrations, the pinch-off also occurs along the filament, but in a more gradual process (similar to1 s). Furthermore, the free filament ends do not fully retract, instead retaining some of their deformation. The falling drop is also observed to slow or even stop (stall) before pinch-off, indicating that sufficient elastic stress has built up to balance its weight. We investigate this stall by generalizing Keiller's simple model for filament motion [J. Non-Newtonian Fluid Mech. 42 (1992) 37], using instead the FENE-CR constitutive equation. Numerical simulations of this model indicate that stall occurs in the range of low solvent viscosity, high elasticity, and high molecular weight. At the highest concentrations, we observe a surface "blistering" instability along the filament long before pinch-off occurs. (C) 2003 Elsevier B.V. All rights reserved.