Stability of a viscous pinching thread Phys. Fluids 24, 072103 (2012) Libration driven elliptical instability Phys. Fluids 24, 061703 (2012) Dynamic mode decomposition analysis of detonation waves Phys. Fluids 24, 066101 (2012) On the necessary conditions for bursts of convection within the rapidly rotating cylindrical annulus Phys. Fluids 24, 066604 (2012) Additional information on Phys. FluidsWe study theoretically the Saffman-Taylor instability of an air bubble expanding into a non-Newtonian fluid in a Hele-Shaw cell, with the motivation of understanding suppression of tip-splitting and the formation of dendritic structures observed in the flow of complex fluids, such as polymeric liquids or liquid crystals. A standard visco-elastic flow model is simplified in the case of flow in a thin gap, and it is found that there is a distinguished limit where shear thinning and normal stress differences are apparent, but elastic response is negligible. This observation allows formulation of a generalized Darcy's law, where the pressure satisfies a nonlinear elliptic boundary value problem. Numerical simulation shows that shear-thinning alone modifies considerably the pattern formation and can produce fingers whose tip-splitting is suppressed, in agreement with experimental results. These fingers grow in an oscillating fashion, shedding ''side-branches'' from their tips, closely resembling solidification patterns. A careful analysis of the parametric dependencies of the system provides an understanding of the conditions required to suppress tip-splitting, and an interpretation of experimental observations, such as emerging length-scales.

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