Filament stretching and capillary breakup extensional rheometry measurements of viscoelastic wormlike micelle solutions

A filament stretching extensional rheometer and capillary breakup extensional rheometer are used to measure the extensional rheology of a series of wormlike micelle solutions experiencing a uniaxial elongational flow. The experiments are performed using a series of wormlike micelle solutions of both cetylpyridinium chloride and sodium salicylate (NaSal) in an aqueous sodium chloride solution and cetyltrimethylammonium bromide and NaSal in de-ionized water. The linear viscoelasticity of all the wormlike micelle solutions is well described by a Maxwell model with just one or two relaxation times while the steady shear measurements all demonstrate characteristics of shear banding at large shear rates. In transient homogeneous uniaxial extension imposed by a filament stretching rheometer, each of the wormlike micelle solutions demonstrate significant strain hardening. At large extension rates, the wormlike micelle solution filaments are all found to fail through a dramatic rupture near the axial midplane at a constant stress independent of imposed extension rate. The result is an extensional viscosity that decays linearly with increasing extension rate. This filament failure likely stems from the local scission of individual wormlike micelle chains. For the more concentrated solutions, as the imposed extension rate is reduced, a critical extension rate is found below which the filament does not rupture, but instead elastocapillary pinch off is recovered and the elastic tensile stresses achieved in the fluid filament grow far beyond the value observed at rupture. This dramatic upturn in the elastic tensile stress and the extensional viscosity at low extension rates is not intuitively expected and is most likely a result of structural changes to the entangled wormlike micelle solution. Strain hardening is also observed in capillary breakup rheometry experiments, however, when the results of filament stretching and capillary breakup rheometry measurements at nominally the same extension rate are superimposed, the results do not agree; the extensional viscosity measurements from filament stretching are in some instances more than an order of magnitude larger. This result calls into question the use of capillary breakup rheometry for quantitatively measuring the extensional viscosity of wormlike micelle solutions