Steady Planar Elongational Viscosity of CTAB/NaSal Aqueous Solutions Measured in a 4-Roll Mill Flow Cell.

Rheological properties in steady planar elongational flow of CTAB/NaSal aqueous solutions were investigated using a 4-roll mill flow cell. The planar elongational stress was measured by using a flow-birefringence technique, which is based on the stress-optic rule. On the other hand, the rate of planar elongation was accurately determined by the analysis of the tracer particles in the system. The planar elongational viscosity showed elongational thinning as well as shear thinning. The planar elongational viscosity measured in the 4-roll mill flow cell was compared with that measured in a squeeze flow cell, and both well agreed with each other. Trouton ratio was evaluated to be 4 in both the Newtonian and elongational thinning region. Flow instability is observed in the 4-roll mill flow cell at a planar elongation rate lower than the shear rate at which the shear flow instability takes place.