A. Yielding in startup shear
Startup shear, commonly applied to examine the flow response of polymer solutions and melts, is often characterized by the stress overshoot. Conventional knowledge expected homogeneous shear in various shearing apparatuses including cone-plate and parallel plate planar Couette cells. The origin of the well-known stress overshoot was elusive. Today, we regard it as a signature of yielding. Under publications, see “Universal scaling characteristics of stress overshoot in startup shear of entangled polymer solutions” in J. Rheol. 52, 681 (2008) (PDF); "Universal scaling behavior in startup shear of entangled linear polymer melts" in J. Rheol. 53, 617 (2009) (PDF). See also “Yielding during startup deformation of entangled linear polymeric liquids”, J. Rheol., to be published (PDF).
Movie 1 reveals development of inhomogeneous flow field across the gap at shear rates of
2.5 s-1 . See Phys. Rev. Lett. 96, 016001 (2006) for details (PDF).
Movie 2 captures elastic breakup of entanglement network in a monodisperse more entangled
polybutadiene solution (15 wt. %) at a shear rate of 1 s-1. See Macromolecues 41, 2663