(237a) Capillary Break-up During Jetting of Weakly Viscoelastic Fluids: Fluids Mechanics and Rheometry

Surface tension driven break-up of cylindrical fluid elements into droplets plays a crucial role in the use or processing of many multicomponent, microstructured complex fluids like paints, inks, insecticides and pesticides, cosmetics, food, etc. These industrial fluids are typically formulated using dilute polymer solutions, and are exposed to a wide range of shear and extension rates. Since the polymer solutions and the resulting dispersions have low viscosity and short relaxation times, their non-Newtonian behavior is not apparent in the conventional rheometric measurements. However, the presence of even a dilute amount of polymer alters the character of instability growth and capillary break-up during jetting. The interplay of capillary, inertial, elastic and viscous effects on small length and time scales typically leads to complex dynamics in a necking fluid thread and in some cases, the extensional stresses generated in the neck lead to formation of very thin and stable filaments between drops, or to ?beads-on-a-string' structures. We use experiments and simulations to study the influence of both elasticity and extensibility on the growth of instability and capillary break-up of harmonically perturbed jets of the viscoelastic fluids. We show how and when capillary thinning analysis can be applied to capillary break-up during jetting to measure rheological response of fluids. While bead formation and extension rates are self-determined in a Capillary Breakup Extensional Rheometer (CABER) experiment, we show that it is possible to influence the dynamics of the capillary break-up during jetting by controlling the amplitude and frequency of the imposed disturbances.