Abstract
We compare the morphological features of highly branched fractal viscoelastic fingering (FVF) patterns obtained by injecting air into aqueous colloidal suspensions in a Hele-Shaw cell with those obtained by injecting water. Apart from a larger average finger width in the immiscible (air) case, which can be accounted for by a lower cut-off introduced by surface tension, the general features of the growth process (fractal dimension; finger width vs. tip velocity; pattern width vs. tip velocity; displacement efficiency vs. tip velocity) are closely parallel in the immiscible and in the miscible case. This shows that a negligible surface tension is but a minor factor in the onset of FVF when the high-viscosity fluid has non-Newtonian, shear thinning properties. An effective control parameter, 
, is calculated by taking into account the non-Newtonian viscosity of the colloidal fluid. is always larger than 105 in our experiments, which is much larger than in Saffman-Taylor fingering.