Explain the enhanced scission in biaxial extensional flow

Explain why the normalized average lifetime of wormlike micelles decreases more rapidly with deformation rate in biaxial extensional flow than in uniaxial and planar extensional flows, despite the identical largest strain-rate eigenvalue for these extensional flows.

Background

The paper compares flow-induced scission of wormlike micelles under shear, uniaxial extensional, planar extensional, and biaxial extensional flows. The normalized average micellar lifetime decreases under all sufficiently strong flows, but the decrease is slightly more pronounced in biaxial extensional flow than in uniaxial and planar extensional flows.

The authors test whether this difference can be explained by the largest eigenvalue of the strain-rate tensor, a purely kinematic measure. Because that eigenvalue is identical for the extensional flows considered, the observed difference remains unexplained at this stage. The later introduction of an effective extension rate incorporating micellar orientation accounts for the difference empirically, but the quoted passage identifies the underlying reason as unclear.