(291d) Macromolecular Engineering of Formulations: Rheology, Stringiness, Spinnability, and Printability

Rheology, stringiness, spinnability, gloopiness and printability of formulations is often assessed qualitatively in daily life by dripping from a nozzle or spoon or by stretching a liquid bridge between two surfaces (say thumb and forefinger) and observing the pinching time. The handy tests involve complex free-surface flows including the formation of columnar neck that undergoes spontaneous capillarity-driven pinching. For simple (Newtonian and inelastic) fluids, a complex interplay of capillary, inertial, and viscous stress determines the pinching dynamics. In rheologically complex fluids, extra elastic stresses as well as non-Newtonian shear and extensional viscosities dramatically alter the pinch-off dynamics. Stream-wise velocity gradients that arise within the thinning columnar neck create an extensional flow field, and many complex fluids exhibit a much larger resistance to elongational flows than Newtonian fluids with similar shear viscosity. Characterization of extensional viscosity, extensional relaxation time and finite extensibility effects, as well as macromolecular properties that determine pinch-off dynamics are beyond capabilities of conventional shear and extensional rheology techniques in which free surface flows are absent. Here we show that dripping-onto-substrate (DoS) rheometry protocols we developed recently can be used for measuring extensional rheology response of polymeric complex fluids, including low viscosity printing inks and polymer solutions that are beyond the measurable range of commercially-available capillary break-up extensional rheometer (CaBER).

Using DoS rheometry protocols that involve visualization and analysis of capillary-driven thinning and pinch-off dynamics of a columnar neck formed between a nozzle and a sessile drop, we elucidate the stretched polymer hydrodynamics underlying observed rheological response and on stringiness, printability, gloopiness, spinnability and processing behavior of polymeric complex fluids. We find that the extensional relaxation times exhibit concentration-dependent variation distinct from shear rheology response measured using shear rheometry or anticipated by blob models developed for relaxation of weakly perturbed chains. We show that the influence of molecular weight and chemistry can be evaluated a priori, using three macromolecular parameters: flexibility, extensibility and segmental dissymmetry for neutral polymers, and discuss additional influence of charge for polyelectrolytes. Finally, we characterize and analyze the pinch-off dynamics of multicomponent formulations to elucidate the influence of macromolecules on extensional rheology and processability of model food, cosmetics, coating, adhesive, and pharmaceutical formulations.