  An experimental study of the physical origin and the mechanisms of the
sharkskin instability is presented. Extrusion flows through a slit die are
studied for two materials: a linear low density polyethylene (LLDPE) and a low
density polyethylene (LDPE). By combining laser-Doppler velocimetry (LDV) with
rheological measurements in both uniaxial extension and shear, the
distributions of tensile and shear stresses in extrusion flows are measured for
both materials.
