The drag force experienced by bubbles, rising freely in an inelastic shear-
thinning fluid is calculated. In particular, the effect of contaminants is
considered when the continuous phase can be represented by the power-law or
the three parameter Carreau models. Two cases are examined in order to tak
e into account the effect of surfactants: (a) interfacial mass transfer is
taken into account via a perturbation around the thermodynamic equilibrium
values using Gibbs elasticity, (b) a continuous distribution of impurities
over the whole interface (uniform retardation) leading to Marangoni stresse
s. The relations obtained are based on perturbation methods around the Newt
onian solution. The drag force can be evaluated via the rheological model p
arameters, the physical properties of the system and the concentration of i
mpurities in the fluid. The presence of contamination was found to increase
the drag resulting in a decrease of the rise velocity. Inversely, the effe
ct of shear thinning is to decrease the drag, thus leading to a balance bet
ween both effects. (C) 1999 Elsevier Science B.V. All rights reserved.