Many artificial and natural fluids contain macromolecules, particles or droplets that impart complex flow behaviour to the fluid. This complex behaviour results in a non-linear relationship between stress and deformation standing in between Newton's law of viscosity for an ideal viscous liquid and Hooke's law for an ideal elastic material. Such non-linear viscoelastic behaviour breaks down flow reversibility under creeping flow conditions, as encountered at the micro-scale, and can lead to flow instabilities. These instabilities offer an alternative to the development of systems requiring unstable flows under conditions where chaotic advection is unfeasible. Flows of viscoelastic fluids are characterized by the Weissenberg (W i) and Reynolds (Re) numbers, and at the micro-scale flow instabilities occur in regions in the W i-Re space typically unreachable at the macro-scale, namely high W i and low Re. In this paper, we review recent experimental work by the authors on the topic of elastic instabilities in flows having a strong extensional component, includ- * Corresponding author. Tel:+ 35 22 508 15 97 : flow through a hyperbolic contraction followed by a sudden expansion; flow in a microfluidic diode and in a flow focusing device; flow around a confined cylinder; flow through porous media and simplified porous media analogues. These flows exhibit different types of flow transitions depending on geometry, W i and Re, including: transition from a steady symmetric to a steady asymmetric flow, often followed by a second transition to unsteady flow at high W i; direct transition between steady symmetric and unsteady flows.

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