This study examines the surface defects known as sharkskin and the phy
sical mechanisms that cause its appearance. Authors of previous papers
who studied different polyethylenes attribute the occurrence of shark
skin, either to the initiation of slip at the fluid-wall interface or
to the existence of local tensile stresses at the die exit. To test th
e slip hypothesis, the present authors studied the flow of a linear lo
w density polyethylene (LLDPE) through capillaries of different geomet
ries. The results obtained show clearly that experimental methods for
determining slip velocities do not give conclusive evidence of the exi
stence of slip at the wall for the flow of the LLDPE used, under condi
tions that would normally give rise to sharkskin. The observations obt
ained using silicone fluids of different molecular weights and an LLDP
E suggest that this phenomenon results from the cracking of the fluid
at the die exit, due to the high tensile stresses in that region. Such
an explanation is supported by flow birefringence measurements and by
numerical modeling results published in the literature.