Particle chaining and chain dynamics in viscoelastic liquids

Viscoelastic fluids can dramatically change the stress field around rigid spherical particles in shear flows, and these changes can cause microstructure and particle chain formation as a result of hydrodynamic interaction between particles. This paper introduces a new microscopic based approach for quantifying the kinetics of particle chaining. By monitoring the number of particles in the form of singlets, doublets, triplets, and multiplets, an absolute measure of string formation is presented. This approach enables finding the shear induced microstructural length scales in the medium. The concept of a String Factor (SF) is defined as a direct and reliable measure of chaining strength. The SF, which can be plotted over time, considers and accordingly weights the long range orientational correlations within each cluster. Real-time monitoring of particles is carried out by confocal microscopy imaging in a cone and plate setup that ensures a constant shear rate all over the field. Monodisperse polystyrene spheres with 5 mu m diameter in a viscoelastic solution of polyethylene oxide in DI water (0.8% PEO, 1.7% PEO) are used as test suspensions to demonstrate quantifiable differences in chain dynamics over a small range of Weissenberg number. (C) 2012 Elsevier B.V. All rights reserved.