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

filament stretching extensional rheometer and capillary breakup extensional rheometer are used o measure the extensional rheology of a series of wormlike micelle solutions experiencing a niaxial elongational flow. The experiments are performed using a series of wormlike micelle olutions of both cetylpyridinium chloride and sodium salicylate NaSal in an aqueous sodium hloride solution and cetyltrimethylammonium bromide and NaSal in de-ionized water. The linear iscoelasticity of all the wormlike micelle solutions is well described by a Maxwell model with ust one or two relaxation times while the steady shear measurements all demonstrate haracteristics of shear banding at large shear rates. In transient homogeneous uniaxial extension mposed by a filament stretching rheometer, each of the wormlike micelle solutions demonstrate ignificant strain hardening. At large extension rates, the wormlike micelle solution filaments are ll found to fail through a dramatic rupture near the axial midplane at a constant stress independent f imposed extension rate. The result is an extensional viscosity that decays linearly with ncreasing extension rate. This filament failure likely stems from the local scission of individual ormlike micelle chains. For the more concentrated solutions, as the imposed extension rate is educed, a critical extension rate is found below which the filament does not rupture, but instead lastocapillary pinch off is recovered and the elastic tensile stresses achieved in the fluid filament row far beyond the value observed at rupture. This dramatic upturn in the elastic tensile stress and he extensional viscosity at low extension rates is not intuitively expected and is most likely a esult of structural changes to the entangled wormlike micelle solution. Strain hardening is also bserved in capillary breakup rheometry experiments, however, when the results of filament tretching and capillary breakup rheometry measurements at nominally the same extension rate are uperimposed, the results do not agree; the extensional viscosity measurements from filament tretching are in some instances more than an order of magnitude larger. This result calls into uestion the use of capillary breakup rheometry for quantitatively measuring the extensional iscosity of wormlike micelle solutions. © 2007 The Society of Rheology. DOI: 10.1122/1.2718974