WHITE-METZNER MODELS FOR ROD CLIMBING IN A1

Measurements of rod climbing in Al give rise to an apparent linear rel ation between the height rise h and the angular velocity OMEGA of the rod. We use a White-Metzner model to fit the data and we find that the height rise on the rod deviates from the quadratic dependence on the angular velocity when the viscosity or relaxation time vary with shear rate. When both the relaxation time and the viscosity change simultan eously with the shear rate, the climbing profile on the rod deviates m ore from the standard theory for rod climbing. A simplified argument b ased on the upper-convected Maxwell model and using power laws for the viscosity and the relaxation time gives rise to h(OMEGA) infinity OME GA(n+m), where n and m are power-law indices which can be chosen to fi t the data. The height rise data for Al, which is linear rather than q uadratic in the shear rate at low shears, can be fitted to a White-Met zner model using measured values for the viscosity function and a long time of relaxation which decreases strongly with the rate of shear. T his result suggests that a shear-decreasing relaxation time function m ay be useful for describing the rheology of fluids with small quadrati c range.