‘Beads on a String’ Structures and Extensional Rheometry using Jet Break-up

1. Introduction: Surface tension driven break-up of cylindrical fluid elements into droplets plays a crucial role in the use or processing of many multicomponent complex fluids like paints, inks, insecticides, cosmetics, food, etc [1, 2]. 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 capillary break-up during dripping, jetting and thinning of a stretched liquid bridge [2]. In all three scenarios (sketched in figure 1), the presence of polymers leads to a delayed pinch-off. 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' structure [2, 3]. In a capillary-thinning extensional rheometry experiment (or CABER test), the self-thinning of liquid bridge of a viscoelastic fluid follows the elasto-capillary scaling R / R 0 ~ exp  t 3