Rheological Behavior of Concentrated Suspensions after Rupture

Abstract

Rheological behavior after yielding to steady flow of concentrated suspensions, Bentonite, Kaolin (pH=3.9, rapidly flocculated and pH=5.0, weakly flocculated), and Kanto-Loam, was studied using a cone-and-plate rheometer. The plate was rotated at a constant acceleration velocity, θ_p, from rest up to a certain steady-state velocity, θ_p. In addition to stress measurements, deformation profiles on the side surface of the suspension were visually observed.
For all the samples except for Kaolin of pH=5.0, the average shear rate in actually flowing zone, γ, which was estimated from the visualization, was almost equal to the applied shear rate, γ_ap, until a certain critical rotation angle, N_c, was reached. During this period, however, the shear stress, τ, increased drastically with shear rate. Thus the thixotropic nature of the suspensions was apparent.
At rotation angles larger than N_c, γ was always larger than γ_ap and attained the steady value depending on θ_p. This behavior is due to the existence of non-flowing layers and their growth with time. This also gives a time dependence of τ different from that in the period of smaller rotation angles, and the apparent viscosity evaluated by using showed an abrupt decrease with time.
The Kaolin suspension of pH=5.0 did not show such behavior; γ was always larger than γ_ap, and the τ-γ relation in the accelaration period can be well estimated from the flow properties in the steady state.