UNEXPECTED PHENOMENA OBSERVED IN PARTICLE SETTLING IN NON-NEWTONIAN MEDIA

Particle settling experiments are seemingly simple but a number of sur prising phenomena have been observed in the past. A review of observat ions on exceedingly long (when compared to the relaxation time of the fluid) velocity transients and time effects is presented. This is comp lemented with new experimental results on time effects in shear-thinni ng fluids which are very surprising indeed for Carbopol which on a tim e scale of hours shows no relaxation of the wake structure at all. In fact the opposite occurs: the time effects seems to become stronger. A lso the effect of shear flow on single particle settling is considered . It is shown that for shear-thinning gel-like fluids it is possible t o calculate the settling velocity if the viscosity at the prevailing s hear rate is entered into Stokes' Law. For shear-thinning fluids which display a low shear rate viscosity plateau this appeared to be possib le only in the Newtonian range of shear rates whereas at higher rates, in the shear-thinning regime, the particles settle more slowly than c alculated on the basis of the viscosity alone. Experiments with two, o r more, settling particles indicate that the interaction between parti cles is complicated by the time effects in the wake of the leading par ticle. Also elastic effects are shown to be important and in our exper iments seem to cause particle dispersion by themselves.