In press at Rheol. Acta

The Normal Stress Behaviour of Suspensions with Viscoelastic Matrix Fluids

April 25, 2001

Susanne E. Mall-Gleižle*(1), Wolfgang Gleižle(2), Gareth H. McKinley(3), Hans Buggisch(2)

(1) Universitaet Karlsruhe (TH), Institut fuer Technische Thermodynamik und Kaeltetechnik, Engler-Bunte-Ring 21, D-76131 Karlsruhe, DE
(2) Universitaet Karlsruhe (TH), Institut fuer Mechanische Verfahrenstechnik und Mechanik, D-76128 Karlsruhe, DE
(3) Massachusetts Institute of Technology (MIT), Department of Mechanical Engineering, 77 Massachusetts Avenue, Cambridge, MA 02139-4307, USA

* Corresponding author; e-mail: susanne.mall-gleissle@ciw.uni-karlsruhe.de

Abstract

We investigate the variations in the shear stress and the first & second normal stress differences of suspensions formulated with viscoelastic fluids as the suspending medium. The test materials comprise two different silicone oils for the matrix fluids and glass spheres of two different mean diameters spanning a range of volume fractions between 5 and 25%. In agreement with previous investigations, the shear-stress đ shear-rate functions of the viscoelastic suspensions were found to be of the same form as the viscometric functions of their matrix fluids, but progressively shifted along the shear rate axis to lower shear rates with increasing solid fraction. The normal stress differences in all of the suspensions examined can be conveniently represented as functions of the shear stress in the fluid. When plotted in this form, the first normal stress difference, as measured with a cone and plate rheometer, is positive in magnitude but strongly decreases with increasing solid fraction. The contributions of the first and the second normal stress differences are separated by using normal force measurements with parallel plate fixtures in conjunction with the cone-and-plate observations. In this way it is possible for the first time to successfully quantify the variations in the second normal stress difference of viscoelastic suspensions for solid fractions of up to 25 volume percent. In contrast to measurements of the first normal stress difference, the second normal stress difference is negative with a magnitude that increases with increasing solid content. The changes in the first and second normal stress differences are also strongly correlated to each other: The relative increase in the second normal stress difference is equal to the relative decrease of the first normal stress difference at the same solid fraction. The variations of the first as well as of the second normal stress difference are represented by power law functions of the shear stress with an unique power law exponent that is independent of the solid fraction. The well known edge effects that arise in cone-and-plate as well as parallel plate rheometry and limit the accessible measuring range in highly viscoelastic materials to low shear rates could be partially suppressed by utilizing a custom-designed guard-ring arrangement.  A procedure to correct the guard-ring influence on torque and normal force measurements is also presented.