Hardening phenomenon in wormlike micellar solutions

Surfactant solutions containing Cetylpyridinium Chloride (CPyCl), a classical ionic surfactant, and Sodium Diclofenac (Diclo), a non-steroidal anti-inflammatory drug used as a binding salt¹, were prepared in a wide range of concentrations. More specifically, solutions were made at different concentrations of CPyCl, i.e. 5.0, 16.7, 33.0, 100 and 200 mM, and various Diclo contents. Linear and nonlinear rheological experiments were performed at 25°C with a strain-controlled ARES rheometer (TA instruments, USA), using a 50 mm - 0.017rad cone-plate geometry.

The frequency sweep tests revealed different viscoelasticity levels and morphologies, depending on surfactant and salt levels. For the more entangled systems, the transient behavior in shear flow was investigated through startup measurements, performed at different shear rates, covering a wide range of Weissenberg (Wi) numbers. During the transient regime, some samples show flow instabilities and strain hardening phenomena at Wi>1, other solutions do not, behaving like ordinary linear polymers in fast shear flows^2,3.

We tried to rationalize this behavior, by identifying the most important parameters, i.e. number of entanglements, characteristic micellar lengths, relaxation times etc., and by clarifying their role in the transient dynamics. A possible mechanism for the onset of flow singularities, related to the scission energy of the wormlike micelles, is suggested and discussed⁴.



¹ T. H. Ito et al, Langmuir 30, 11535 (2014).

² Inoue et al, Langmuir 21, 1201 (2005).

³ S. Costanzo et al, Macromolecules 49, 392 (2016).

⁴ R. Pasquino et al, J. Rheol. 67, 353 (2023).