Oscillatory settling in wormlike-micelle solutions: bursts and a long time scale

TL;DR

This paper investigates the complex oscillatory settling behavior of a sphere in wormlike-micelle solutions, revealing long time scales and bursts linked to viscoelastic stress and flow interactions, supported by a new model and experimental evidence.

Contribution

It introduces a novel model incorporating a slow structural variable to explain oscillatory settling in wormlike-micelle solutions, supported by experimental validation.

Findings

Oscillatory bursts in settling velocity for large spheres

Identification of a long time scale in the dynamics

Experimental evidence supporting the structural variable model

Abstract

We study the dynamics of a spherical steel ball falling freely through a solution of entangled wormlike-micelles. If the sphere diameter is larger than a threshold value, the settling velocity shows repeated short oscillatory bursts separated by long periods of relative quiescence. We propose a model incorporating the interplay of settling-induced flow, viscoelastic stress and, as in M. E. Cates, D. A. Head and A. Ajdari, Phys. Rev. E, 2002, 66, 025202(R) and A. Aradian and M. E. Cates, Phys. Rev. E, 2006, 73, 041508, a slow structural variable for which our experiments offer independent evidence.