The origin of "melt fracture" in polymer extrusion: a subcritical flow instability?

Prof.dr. Wim van Saarloos, Lorentz Institute - University Leiden

Extrusion of a polymer  refers to the situation when a polymer melt or solution flows out of a tube so that it
forms a nice smooth thread upon cooling. A longstanding problem of great  industrial relevance is the fact that above some critical
extrusion velocity, the surface of the extrudate starts to become irregular. This limits the possibility to obtain a smooth thread; at
large enough flow rates the corrugations can  become so large that the thread breaks - hence the name "melt fracture". In this talk I will
discuss how various seemingly unrelated recent developments in the field of nonequilibrium  pattern formation  lead one to suspect that a
weakly nonlinear ("subcritical") instability of Poiseuille flow of a polymer solution lies at the origin of melt fracture. I will then
present  theoretical and experimental evidence in support of this scenario.

This work is done in collaboration with D. Bonn (ENS, Paris)