Elasticity and Extensional Rheology of PEO-PEG-Based Aqueous Boger Fluids

Elasticity influences flow behavior, processability, and applications of complex fluids. Elasticity can be characterized using torsional rheometers by measuring normal stresses in steady shear, elastic modulus in oscillatory shear, and stress relaxation after step-strain. Complex fluids display flow kinematics, as well as instabilities that are quite distinct from Newtonian fluids. However, determining the effects of elasticity are not straightforward, as most complex fluids that show measurable elasticity, also display rate-dependent viscosity. Simultaneous manifestation of these complex fluid phenomena leads to their conflation! To resolve this problem, David Boger developed a ‘purely elastic’ fluid that displays a constant viscosity. Such fluids, known as Boger fluids, are typically formulated by dissolving high molecular weight polymers in relatively high-viscosity solvents, to facilitate measurements with torsional rheometry. However, most Boger fluids are too viscous to emulate fluids used in coating flows. Though aqueous PEO-PEG Boger fluids allow access to these germane viscosities, but characterization of elasticity using state-of-the-art rheometers suffers due to the resolution limit of torque and normal force transducers, or due to elastic instabilities. Furthermore, many coating flows are susceptible to stream-wise velocity gradients associated with extensional flows, and hence characterization of extensional rheology response is necessary. In this contribution, we investigate the shear and extensional rheology of PEO/PEG Boger fluids. We show that dipping-onto-substrate (DoS) protocols that rely on characterization of pinching dynamic allow measurement of both relaxation time and extensional viscosity. Even though most studies treat Boger fluids as experimental equivalents of Oldroyd-B fluids, the pinching dynamics displays terminal viscoelastocapillary regime, illustrating finite extensibility effects. The extensional relaxation time for aqueous PEO/PEG Boger fluids shows the concentration-dependent scaling also observed for aqueous PEO solutions, where measurements on torsional rheometers show no evidence of elasticity. Thus we show the possibility of utilizing aqueous polymer solutions as model Boger fluids, with elasticity quantified using DoS rheometry, and illustrate the extensional viscosity response exhibits strain-hardening, that could impact the coating flows, in addition to elastic effects.