Additionally, considering that our microchannels have widths of similar order to its height, the flow and the subsequent birefringence along a vertical slice of the microchannel is not uniform and so the retardance is also an average over each vertical position in the microchannel. For these reasons, we do not convert retardance to birefringence. However, since the light source and the microchannel height is kept constant for all experiments we can still accurately compare the measured retardance values. To accurately measure the retardance, we modeled the optical system and its output polarization state with imperfect linear polarizers and an arbitrarily polarized light source using Mueller matrices. We then developed a procedure where multiple measurements can be combined to cancel out the contributions arising from the imperfections in the optical setup. For more information on the retardance measurement theory and procedure see supplementary section S2. For shear flow experiments, the polymer solutions were flowed at volumetric flow rates of 1 to 260 µL/min, corresponding to shear strain rates of 5,950 to 1,550,000 s -1 . For extensional flow experiments, the polymer solutions were flowed at volumetric flow rates of 1 to 140 µL/min, corresponding to extensional strain rates of 655 to 91,700 s -1 . At flowrates above 140 μL/min the presence of birefringent "pipes" can occur where the centerline birefringence reduces as a result of flow modification from the polymer which lowers the extensional strain rate making it unreliable to quantify at these conditions .