Complex flows of complex fluids

After a pedagogical introduction to the rheology (deformation and flow properties) of complex fluids and soft materials, we discuss recent research concerning viscoelastic instabilities in these materials, where an initially homogeneous flow field gives way to a spatially more complicated one. We focus particularly on extensional flow protocols and present new results for an age-dependent switching between two distinct modes of failure in disordered soft "glassy" materials subject to extensional deformations. Our calculations predict a smooth, liquid-like failure mode in young samples, and a more sudden, solid-like failure mode in old samples. This is the first instance, of which we are aware, in which the physics of a material's mode of failure is predicted to be governed by a switch between two distinct modes of instability, set simply by the age of the sample.