Capillary breakup extensional rheometry (CaBER) on semi-dilute and concentrated polyethyleneoxide (PEO) solutions

Semi-dilute (\(c^\ast c e) solutions of PEO yield uniformly thinning, cylindrical filaments in capillary breakup extensional rheometry (CaBER) experiments. Up to c ≈ c e thinning can be characterized by a single elongational relaxation time λ E. Comparison with the longest shear relaxation time, λ S reveals that λ E/λ S decreases with increasing concentration or molecular weight according to (c[η]) − 4/3. This is attributed to the large deformation the solutions experience during filament thinning. A factorable integral model including a single relaxation time and a Soskey or Wagner damping function accounting for the large deformation in CaBER experiments is used to calculate λ E/λ S and provides good agreement with experimental results. Irrespective of concentration or molecular weight a beads-on-a-string structure occurs prior to filament breakup at a diameter ratio D/D 0 ≈ 0.01. This instability is supposed to be closely related to a flow-induced phase separation.