SPATIALLY-RESOLVED BIREFRINGENCE STUDIES OF PLANAR ENTRY FLOW

Stress distributions near the re-entrant corners in two-dimensional, 4:1 contraction flows of weakly elastic and more highly elastic poly(dimethylsiloxane) (PDMS) melts were investigated with a high-resolution, polarization modulated flow birefringence method. Measurements were taken at the limiting spatial resolution of 100 mum length scales, and Weissenberg numbers We of the flows ranged from 0.12 to 0.91. For the weakly elastic liquid, measured profiles of the stress tau(rtheta) were in agreement with predictions of Second Order Fluid (SOF) theory. However, less favorable agreement was seen between the experimental and theoretical profiles of (tau(rr) - tau(thetatheta)); the overall levels and spatial gradients in the measured profiles were lower than SOF or Newtonian predictions. After consideration of many possible causes for these deviations, they were imputed at least in part to corner rounding. While the theory treats the re-entrant corner as perfectly sharp, the corners in the experiment possessed a slight, unavoidable rounding. Stress profiles for the more highly elastic fluid were qualitatively similar to those for the weakly elastic liquid, but stress gradients were more pronounced near the re-entrant corners. Nonetheless, the stress gradients were not as steep as would be expected from published numerical simulations. No instabilities, oscillations or time-dependent irregularities were observed.