Stability Of Concentrated Suspensions Research Articles

Stability of concentrated suspensions under Couette and Poiseuille flow

The stability of two-dimensional Poiseuille flow and plane Couette flow for concentrated suspensions is investigated. A linear stability analysis of the two-phase flow model for both flow geometries shows the existence of a convectively driven instability with increasing growth rates of the unstable modes as the particle volume fraction of the suspension increases. In addition it is shown that there exists a bound for the particle phase viscosity below which the two-phase flow model may become ill-posed as the particle phase approaches its maximum packing fraction. The case of two-dimensional Poiseuille flow gives rise to base state solutions that exhibit a jammed and unyielded region, due to shear-induced migration, as the maximum packing fraction is approached. The stability characteristics of the resulting Bingham-type flow is investigated, and the connections to the stability problem for the related classical Bingham flow problem are discussed.

Fracture Behaviour of Pressureless Sintered Nickel-Reinforced Alumina Composites

Nickel-reinforced alumina composites have been manufactured by aqueous slip casting and pressureless sintered under flowing atmosphere of argon with 0,36 and 1% of oxygen in order to force interfacial reactions leading to the formation of a nickel-aluminum spinel. Colloidal stability of concentrated suspensions of alumina with 5, 10 and 15 vol% of nickel has been studied in terms of zeta potential, rheometry and packing density. The processed composites show a high dispersion of the nickel into the alumina matrix and green densities of 60-70 %th. The effect of sintering temperature and atmosphere on the mechanical behaviour of the composites has been investigated through Vickers indentation and fractographic SEM observations.

Preventing ageing on Al2O3 casting slips dispersed with polyelectrolytes

Slip casting process needs a highly stabilized and well-characterized suspension, which involves not only the traditional rheological measurements, but an estimation of the colloidal stability in terms of interaction potentials between particles. For electrosterically stabilized ceramic suspensions the calculation of the steric contribution is not well described in the available theories because they are developed for ideal conditions where many variables affecting the stability of concentrated suspensions are neglected. In this work, alumina suspensions were prepared at 75 wt% solids and characterized considering the potential energy curves and the rheological behaviour. The main objective is to select the proper dispersing conditions in order to prevent ageing of the slips, which is an important problem in any manufacturing cycle. For the polyacrylate dispersant used it was shown that a concentration of 0.8 wt% provided the lowest viscosity, allowed ageing prevention and resulted in higher green and sintered densities.