Migration and alignment of spherical particles in sheared viscoelastic suspensions. A quantitative determination of the flow-induced self-assembly kinetics.

Flow-Induced Self-Assembly (FISA) is the flow-driven formation of ordered structures in complex fluids. In this paper the effect of shear flow on the microstructure formation of dilute sphere suspensions in a viscoelastic fluid has been studied experimentally by optical microscopy techniques. The system is formed by Polymethylmethacrylate beads suspended in 20 wt.% aqueous solutions of Hydroxypropylcellulose at volume fractions ranging between 0.1% and 1.0%. Experiments show that, under the action of flow, beads migrate from the bulk to the shear walls, there forming strings aligned along the flow direction. Strings grow with time eventually reaching a steady-state final length. The alignment kinetics have been quantified by means of an alignment factor, which is a measure of the average length of the strings. The experimental results indicate that both shear rate and particle concentration are relevant factors in determining the alignment factor kinetics. In particular, it is shown that, upon increasing shear rate, strings grow both faster and longer. As a consequence, the characteristic time of the overall alignment process remains roughly constant. It is also shown that an increase in particle volume fraction determines effects similar to an increase of shear rate.