Transient normal stress response in a concentrated suspension of spherical particles

Abstract

The transient normal force response in a concentrated suspension of spherical particles upon startup of shear, following a period of rest, was found to depend on the direction in which shear was restarted. When shear was restarted in the same direction, the normal force signal rapidly grew to its positive steady-state value. However, when shear was restarted in the opposite direction, the normal force signal was initially negative and decreased, within the response time of the instrument, to a negative minimum, from which it gradually increased, passing through zero, to its positive steady-state value. This is believed to be the first experimental confirmation of this phenomenon, which had been suggested by the numerical simulations of previous investigators.

Deterministic solution of the kinetic theory model of colloidal suspensions of structureless particles

“…8 Evolution of g along a trajectory for FE calculation Ohl and Gleissle 1993;So et al 2001), as well as the negative value of the second stress coefficient. However, for large Pe, thickening behavior which has been observed experimentally (Kolli et al 2002;Ohl and Gleissle 1993;So et al 2001) and numerically (Brady 2001;Foss and Brady 2000) in concentrated suspensions cannot be reproduced by the model here considered.…”

Microstructure and velocity fluctuations in sheared suspensions

“…Their oscillatory experiments showed similar results, in that the measured dynamic viscosity µ ′ , although independent of the frequency of oscillation at low frequency, was consistently smaller that the shear viscosity µ of the suspension, stressing again the presence of a microscopic structure induced by the shear. In recent experiments, Kolli et al (2002) used a parallel ring geometry that allowed them to measure the normal stress response to shear reversal in concentrated suspensions, in addition to measuring the shear stress behavior, and found a transient response in both the normal and the shear stresses when the shear was restarted in the opposite direction. Moreover, the absolute value of both the normal and the shear stresses changed at the very instant of flow reversal, which means that the fore-aft asymmetry in the microstructure alone is not enough to explain the observed response in the stress upon shear reversal, but that non-hydrodynamic forces must also have been acting on the system, either in the form of repulsion forces or of rough contacts between spheres.…”

A new rate-independent tensorial model for suspensions of noncolloidal rigid particles in Newtonian fluids

“…In 2006, Goddard [28] revisited this approach, and proposed a model involving twelve material parameters and two tensors for describing the anisotropy. By a systematic fitting procedure of the parameters, he obtained numerical results in quantitative agreement with shear reversal experiments [7,31]. Also in 2006, Stickel et al [34] (see also [35,36]) defined the conformation tensor on the base of particle mean free path, and simplified the expression of the stress to be linear in the deformation rate and the conformation tensor.…”

“…[29]). For concentrated suspensions of spherical particles, Phan-Thien [30] proposed a differential constitutive equation for the conformation tensor, that led to prediction qualitatively in agreement with time-dependent experimental observations in shear reversal [6,7,31]. The structural unit used to define the conformation tensor was taken as the unit vector n joining two neighboring particles, thereby encoding a direct connection with the pair distribution function g(x).…”

“…However, a wealth of experimental evidence also showed the existence of non-Newtonian rheological effects as soon as φ exceeds 0.2, typically. One of the most prominent examples is the the existence of transient viscosity drops upon reversal of the shearing direction [6][7][8]. There is nowadays a general agreement to relate these non-Newtonian characteristics to flow-induced changes in the microstructure of the suspension [3,9,10].…”