A comprehensive study of the extensional rheology of complex fluids

Non-Newtonian fluids play an important role in our daily life. The visco-elastic nature of these fluids comprises a class of materials found in variety of items ranging from food, to plastic products, to the cosmetic products. The design and tunability of non-Newtonian fluids is only possible through an understanding of their complex dynamics and rheology. In this thesis, a filament stretching rheometer is used to investigate the extensional rheology of three important classes of complex visco-elastic fluids namely surfactants, suspensions and polymers. Surfactants with their unique molecular amphiphilic chemistry allow them to form long wormlike micellar structures, which behave like a “living polymer”. In our experiments we have chosen to focus on the mixed anionic (NaOA) and cationic (C8TAB) surfactants to obtain both linear and branched wormlike micelles. Our measurements demonstrate that branched micelles do not strain harden as much as linear micelles. We have proposed that results are likely due to the new stress relief mechanisms available to branched micelles which appear to be extremely efficient in extensional flows. We have performed experiments to study the extensional properties of shear-thickening colloidal suspensions of silica in polypropylene glycol as a function of concentration and extension rate. Our results demonstrate that at a critical extension rate, there is a dramatic increase in both the rate and magnitude of the strain hardening with increasing extension rate. The observed results are due to the formation of strings aligned in the flow direction, similar to the mechanism postulated to explain the shear thickening of these fluids. This hypothesis is confirmed by small angle light scattering measurements. Finally, we have investigated the extensional flow-induced crystallization of commercial grade poly 1-butene polymer melts. We quantified the degree of crystallinty of the stretched polymers obtained from differential scanning calorimetry measurements to help interpret the role of homogeneous extensional flows in crystallization dynamics. Our results showed a dramatic 70% increase in crystallinity with increasing extension rate compared to quiescent case. These observations clearly demonstrate the ability of extensional flows to enhance the nucleation rate and crystallization kinetics of the poly 1-butene samples

Extensional rheology of shear-thickening nanoparticle suspensions

A filament-stretching rheometer is used to measure the extensional properties of shear-thickening nanoparticle suspensions as a function of concentration and extension rate. The experiments are performed using a series of colloidal suspensions consisting of concentrations of 17.5 wt%, 25 wt% and 30 wt% of fumed silica nanoparticles in polypropylene glycol. The shear rheology of these suspensions was found to demonstrate dynamic shear-thickening behavior owing to the formation of large hydrodynamic clusters. The critical value of angular frequency for the onset of shear-thickening was found to increase monotonically with decreased strain amplitude. The extensional rheology of all the tested suspensions demonstrated modest strain-hardening at low strain rates. At a critical extension rate, a dramatic increase in both the speed and magnitude of the strain-hardening is observed for both the 25 wt% and 30 wt% suspensions with increasing extensional rate. The steady state extensional viscosity as a function of extension rate shows sharp extensional thickening transition very similar to shear flows. The increase in strain-hardening is likely due to the formation of strings and clusters ordered in the flow direction. This hypothesis is confirmed by small-angle light scattering measurements of the flow of the nanoparticle suspension through a microfluidic hyperbolic contraction. The degree of alignment of nanoparticles is quantified from the analysis of the scattering patterns and found to increase significantly with increasing extension rate

The effect of preshear on the extensional rheology of wormlike micelle solutions

Abstract The effect of initial microstructural deformation, alignment, and morphology on the response of wormlike micelle solutions in transient uniaxial extensional flows is investigated using a pre-shear device attached to a filament stretching rheometer. In filament stretching experiments, increasing the strength and the duration of the pre-shear just before stretch is found to delay the onset of strain hardening. In these experiments, the wormlike micelle solution filaments fail through a rupture near the axial midplane. The value of the elastic tensile stress at rupture is found to decrease with increasing pre-shear rate and duration. The most dramatic effects are observed at shear rates for which shear banding has been independently observed. The reduction in the strain hardening suggests that pre-shear before filament stretching might break down the wormlike micelles reducing their size before stretch. Strain hardening is also observed in capillary breakup rheometry experiments; however, the pre-sheared wormlike micelle solutions strain harden faster, achieve larger steadystate extensional viscosities and an increase in the extensional relaxation time with increasing shear rate and duration. The difference between the response of the wormlike micelles in filament stretching and capillary breakup experiments demonstrates the sensitivity of these self-assembling micelle networks to pre-conditioning

The effect of preshear on the extensional rheology of wormlike micelle solutions

Abstract The effect of initial microstructural deformation, alignment, and morphology on the response of wormlike micelle solutions in transient uniaxial extensional flows is investigated using a pre-shear device attached to a filament stretching rheometer. In filament stretching experiments, increasing the strength and the duration of the pre-shear just before stretch is found to delay the onset of strain hardening. In these experiments, the wormlike micelle solution filaments fail through a rupture near the axial midplane. The value of the elastic tensile stress at rupture is found to decrease with increasing pre-shear rate and duration. The most dramatic effects are observed at shear rates for which shear banding has been independently observed. The reduction in the strain hardening suggests that pre-shear before filament stretching might break down the wormlike micelles reducing their size before stretch. Strain hardening is also observed in capillary breakup rheometry experiments; however, the pre-sheared wormlike micelle solutions strain harden faster, achieve larger steadystate extensional viscosities and an increase in the extensional relaxation time with increasing shear rate and duration. The difference between the response of the wormlike micelles in filament stretching and capillary breakup experiments demonstrates the sensitivity of these self-assembling micelle networks to pre-conditioning