Time dependent viscosity of concentrated alumina suspensions

Viscometric investigations of concentrated aqueous alumina suspensions with particles smaller than 5 mu m have been performed. Experimental flow curve s indicate thixotropy in the shear rate interval between (gamma) over dot = 20 and 640 s(-1). In the range smaller than (gamma) over dot = 200 s(-1) w e found pseudoplastic flow behavior, in the higher range the material shows dilatancy. The non-Newtonian behavior results from a small content of sodi um aluminum oxide in the alumina suspension. This gives rise to interpartic le forces that can drive the suspension into a gel-like state. The time sca le of this process is some days. On the short-time scale of some hours the material ages slowly increasing moderately the apparent viscosity. Studying the relaxation process after a shear rate jump, the shear stress time depe ndency at constant shear rate follows an exponential law. There is a single particular relaxation time for each shear rate. The relaxation towards a s teady state occurs asymptotically over some 10(3) s. Flow curves calculated from steady state data after relaxation processes are below the experiment al flow curves which were measured during some 100 s. The flow curves follo w the Herschel-Bulkley formula. The shape of the viscosity curves indicates changes of suspension structure at ca, (gamma) over dot = 200 and 400 s(-1 ). At constant shear rates in the interval between (gamma) over dot = 400 a nd 450 s(-1) the apparent viscosity of the alumina suspension fluctuates pe riodically in time in the same manner found for other suspensions. This eff ect is interpreted as periodic organization of agglomeration and deglomerat ion processes. Supposing, that the stabilisation energy of agglomerates is of the order of the energy introduced by the mechanical shear field, the ob servation of oscillations at (gamma) over dot = 400 s(-1) is in agreement w ith the drastic slope change in the viscosity curves. (C) 1999 Elsevier Sci ence B.V. All rights reserved.