D. Large step shear
Step shear is a simplest experiment to probe nonlinear flow behavior of entangled polymers. We did not think of performing a PTV observation to learn what really happens because the perception planted in our mind was so stubborn: We were told for decades that we should expect quiescent relaxation after application of a large step strain. How much more wrong could we be?
show non-quiescent relaxation in two different 10 wt % PBD solutions after a step strain of 320, 360 % and 130 % respectively. These surprising PTV observations seem to show breakup of the entanglement network due to residual elastic forces and to falsify the success of the Doi-Edwards tube theory for nonlinear flow behavior of entangled polymers. See Phys. Rev. Lett. 97, 187801 (2006) for details (PDF).