The optical brirefringence pattern obtained from high density polyethylene flowing in the steady state through a contraction-expansion slit at 155°C. The flow direction is from bottom to top at an apparent wall shear rate of 12 s-1. The slit dimensions are: length = 1.5mm, width = 1.4mm depth = 10mm. The slit width contracts from 10mm to 1.4mm. Picture courtesy of Lino Scelsi.