Direct measurement of individual polymer dynamics reveals the conformation tensor in viscoelastic cross-channel flows

Much effort has been devoted to quantifying and understanding the continuum-level stresses responsible for the flow behavior of polymeric fluids. However, the underlying stretching dynamics and relaxation of individual polymers in complex, viscoelastic fluid flows remain relatively unexplored. In this study, single molecule tracking reveals the individual Lagrangian dynamics and spatially resolved conformations of dilute polymer suspensions in a microfluidic cross-slot flow. Fluorescently labeled DNA molecules are tracked for a range of Weissenberg numbers, including two different polymer contour lengths and concentrations. The Lagrangian polymer stretching is correlated with the fluid deformation history from measured flow fields that exhibit spatially non-uniform velocity gradients. A direct comparison of the measured polymer conformation tensor with a theoretical Oldroyd-B model demonstrates a strong dependence on the rheological model, while worm-like chain simulations more broadly capture features of the measured conformation. This work highlights the crucial role of complex individual molecular dynamics in refining models to predict polymeric flow behavior.