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