The folding motion of an axisymmetric jet of wormlike-micelles solution

The problem of buckling and coiling of jets of viscous, Newtonian liquids has received a substantial level of attention over the past two decades, both from experimental and theoretical points of view. Nevertheless, many industrial fluids and consumer products are non-Newtonian, and their rheological properties affect their flow behavior. The present work aims at studying the dynamics of cylindrical jets of a viscoelastic, shear-thinning solution of cetylpyridinium salt (CPyCl). In concentrated solutions, CPyCl surfactant molecules have been shown to assemble in long wormlike micellar structures, which gives the fluid its non-Newtonian properties. Jets of this fluid show novel features compared to their Newtonian counterparts, including a type of motion, in which the jet folds back and forth on itself in a fashion similar to sheets of viscous fluids, instead of coiling around the vertical axis as cylindrical Newtonian jets do. Another novel feature of CPyCl micellar fluid jets is a widening of the jet above the plate reminiscent of the die-swell phenomenon that we call \emph{reverse swell}. We propose physical mechanisms for both folding and reverse swell, and compare theoretical predictions to experimental measurements. In addition, we systematically explore different flow regimes in the parameter space of the height of fall and flow rate and compare regime maps of a CPyCl micellar solution and a Newtonian silicone oil.