Effect of the viscoelasticity of the suspending fluid on structure formation in suspensions

Particles suspended in viscoelastic media are known to develop string-like structures when being sheared at sufficiently high shear rates. The occurrence of these strings has been attributed to normal stress effects, and it has been suggested that a critical Weissenberg number of 10 governs their appearance. This assumption is tested here, using suspensions of monodisperse polystyrene spheres. The level of viscosity and elasticity, as well as the degree of shear thinning of the suspending fluid, have been varied over a wide range of values. The critical Weissenberg number for the onset of string formation, as detected by microscopy, was not constant but ranged from 0.5 to 16.5 in contrast with literature results. Suspensions in highly elastic Boger fluids did not display any alignment at all, even at Weissenberg numbers as high as 260. Small angle light scattering (SALS) was used to calculate an alignment factor. It provides an objective and quantitative measure for the degree of alignment of the particles. It also makes it possible, in some cases, to determine precisely the interparticle spacing in the strings. The kinetics of string formation can be studied as well, they seem to be strain-controlled. SALS experiments with different gap spacings have been used to demonstrate that the string formation occurs in the bulk of the liquid rather than at the walls of the cell. (C) 2004 Elsevier B.V. All rights reserved.