(44l) Effect of Size and Polydispersity of Polyacrylamide Solutions Rheology Under Elongational Flow

Polyacrylamide (PAM) is often used for polymer flooding application in enhanced oil recovery within the porous media of an oil reservoir. The porous media creates an extensional flow field that strongly deforms the polymer macromolecules. Extensional rheological phenomenon such as strain hardening may improve oil recovery due to an increase in capillary number. Therefore, it is important to understand how to control the extensional rheology of PAM solutions for optimum oil recovery. Here, we study the extensional flow properties of PAM solutions via the pinch-off dynamics using a dripping-onto-substrate (DoS) protocol. The DoS protocol measures the extensional properties by dispensing a fluid through a nozzle to form a pendant drop and capturing the pinch-off process as the fluid deposited to a sessile drop on a substrate with a high-speed camera. We use polyacrylamides of various molecular weight and polydispersity to investigate the role of macromolecular properties on the extensional response of polymer solutions. We characterize the extensional properties by calculating the extensional relaxation time λ_E and extensional viscosity η_E from the filament-thinning dynamics. With respect to polymer overlap concentration c^* derived from zero-shear viscosity, we observe that the shear rheology at the same c/c* is almost identical in dilute regime (c < c^*). The extensional rheology shows a stronger polymer size dependence, where larger polymer exhibits longer λ_E and a stronger strain hardening response. Concentration-dependence of λ_E follows a power-law exponent that increases with polymer size in dilute regime and transitions to linear in semi-dilute regime (c > c^*). For polydisperse polymer, we observe an earlier transition to linear scaling which corresponds to c^* derived from the weight-averaged molecular weight Mw, indicating the stronger contribution of the larger-end of the distribution. We also observe a similar trend when concentrated colloidal suspensions (volume fraction Φ ≥ 0.40) are added in the solutions. Our results suggest that we can tune the extensional response of PAM solution from the polymer size and polydispersity, which may elucidate better polymer flooding formulation for optimum oil recovery.