Shear-banding in polyacrylamide solutions revealed via optical coherence tomography velocimetry

We used optical coherence tomography velocimetry inside a fluids rheometer to study the rheology of a family of polyacrylamide (PAM) solutions that contain different polymer molecular weights and concentrations, with picolitre probing volumes. The linear velocity profiles obtained from low molecular weight samples, characteristic of Newtonian fluids under shear, become shear-banded when longer polymer chains (molecular weights 5 000 000 and above) are used at sufficiently high concentrations. Upon increasing the concentration further, shear-banding becomes less dominant and significant wall-slip takes place on the two plates of the rheometer. We describe the shear-banding and wall-slip phenomena in our samples in terms of parameters calculated from the velocity profiles, and use our data to suggest a dynamic phase diagram indicating the linear , shear-banding , and wall-slip regimes as functions of PAM molecular weight and concentration. Polyacrylamide (PAM) solutions are Newtonian at low concentrations ( c ) and molecular weight ( M w ), but show marked shear-banding or dominant wall-slip in certain regions of the c - M w dynamic phase diagram. The dynamic phase diagram is based on velocity profiles obtained using optical coherence tomography velocimetry.