C. Flow birefringence
Flow birefringence can provide useful information. Broad white light-based flow birefringence is particularly convenient to provide spatial distribution of the flow condition unlike point-like laser-light-based flow birefringence measurement.
shows how the flow birefringence builds up upon application of a shear stress comparable to the plateau stress.
shows two similar observations from a home-made sliding plate rheometer in step strain: first, a finite sample sandwiched and sheared between two parallel plates; second, a different sample loading, with the side walls confining the sample in the width direction. Note the impressively uniform color. Retardation plates were necessary to bring the equilibrium sample into the region of the pink color. The bright white region is where no polarizer was placed.
looks for any complimentary information about the spatial variation of the chain deformation, the state of chain entanglement and the local shear rate. Here a white light beam was sent diametrically across an assembly of two parallel disks, with a linear polarizer and analyzer on either side of the disks. The difference colors across the gap indicate development of a gradient in the optical retardation when sheared in the stress plateau region.