Surfactant and viscoelastic effects on drop deformation in 2‐D extensional flow

The deformation and breakup of polymeric drops suspended in an immiscible Newtonian fluid are examined for planar extensional flow in a computer controlled four roll mill are examined. Previous work from this lab with aqueous polymeric drops showed an increase in the degree of deformation compared to Newtonian drops, and two distinct breakup mechanisms, tip streaming, and tip streaming with stretch. This led to the conclusion that breakup for non-Newtonian drops occurred at a lower capillary number and by a distinctly different mechanism than for a Newtonian drop with a similar ratio of internal to external viscosity. This conclusion was puzzling, because numerical computations with the Chilcott-Rallison model showed that the influence of viscoelasticity for that model was to decrease the degree of deformation rather than increasing it. In this work, the results for aqueous polymer solutions are compared with results for ethylene glycol polymer solutions. The results suggest that tip streaming and tip streaming with stretch in the original experiments resulted from an undetected surfactant effect, which acts to produce a more pointed drop shape that destabilizes the drop to tip streaming. Viscoelasticity, on the other hand, acts to decrease the deformation of polymeric drops at a given capillary number and increases the critical capillary number, as predicted numerically, when compared to Newtonian drops with an equivalent viscosity ratio.