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

TL;DR

This study investigates the unique buckling, folding, and reverse swell behaviors of viscoelastic wormlike-micelles jets, revealing novel flow phenomena and proposing physical mechanisms supported by experimental and theoretical analysis.

Contribution

It introduces the first detailed analysis of folding and reverse swell in non-Newtonian micellar jets, contrasting their behavior with Newtonian fluids and proposing mechanisms for these phenomena.

Findings

Wormlike-micelles jets exhibit folding instead of coiling.

Identification of reverse swell phenomenon in micellar jets.

Comparison of flow regimes between micellar solution and silicone oil.

Abstract

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.