(128a) Polyelectrolytes Dynamics and Rheology, in a Pinch

Stringiness, stickiness and dispensing behavior of complex fluids are often assessed qualitatively by stretching a liquid bridge between two surfaces (say thumb and forefinger), and observing the capillarity-driven pinching of fluid neck and assessing filament lifespan. Dispensing and drop formation/deposition processes associated with printing, spraying, atomization and coating flows involve similar filament pinching dynamics. Stream-wise velocity gradients that spontaneously arise within the pinching filament create an extensional flow field. However, the influence of macromolecular parameters and interactions on the response to extensional flows remains relatively less well-understood, partially due to the well-known challenges in the characterization of extensional rheology response. In this talk, we show that dripping-onto-substrate (DoS) rheometry protocols that involve visualization and analysis of pinching of a columnar neck formed between a nozzle and a sessile drop can be used for measuring extensional viscosity and extensional relaxation time of polymeric complex fluids. The DoS rheometry protocols that we developed and popularized allow characterization of nonlinear viscoelasticity and extensional rheology of printing inks and polymer solutions that are beyond the measurable range of commercially-available capillary break-up extensional rheometer (CaBER).

Biological macromolecules like proteins, DNA, polysaccharides, and many industrial polymers, are classified together as polyelectrolytes. In solution, the repeat units in their backbone are decorated with dissociated, charge bearing ionic groups, surrounded by a cloud of counter-ions. Even though a large number of polyelectrolytes are processed or used as rheology modifiers in paints, pharmaceuticals, fertilizers, pesticides, and cosmetics, the shear and extensional rheology response of the charged macromolecular solutions is not as well understood as for their uncharged counterparts, and motivate this study. We characterize the pinching dynamics as well as shear and extensional rheology of solutions of three model polyelectrolytes poly(sodium 4-styrenesulfonate) (NaPSS), poly(acrylic acid) (PAA) and sodium carboxymethylcellulose (NaCMC) as a function of solvent and salt concentration. We identify universalities in the shear and extensional rheology response of salt-added unentangled semi-dilute solutions, and examine the influence of solvent properties on polyelectrolyte dynamics. Unlike shear relaxation time that decreases with increase in polymer concentration in the unentangled, semidilute salt-free solutions, the measured extensional relaxation time always shows an increase with polymer concentration for the unentangled systems. We elucidate the influence of both electrostatic and hydrodynamic interactions and stretching of macromolecules on stickiness, printability, jettability and overall processability.