Instabilities in Complex Fluid Flows: understanding, exploitation and cure {ICOMPFF}

Complex fluid flows are ubiquitous in both the natural and man-made worlds. From the pulsatile flow of blood through our bodies, to the pumping of personal products such as shampoos or conditioners through complex piping networks as they are processed. For such complex fluids the underlying microstructure can give rise to flow instabilities which are often totally absent in "simple" Newtonian fluids such as water or air. For example, many wormlike micellar surfactant ("soap/detergent") systems are known to exhibit shear-banding where the homogenous solution splits into two (or more) bands of fluid: such flows are often unstable to even infinitesimally small perturbations. At higher pump speeds the flows can develop chaotic motion caused by the elastic normal-stresses developed in flow. Such "elastic turbulence" can also develop for other flowing complex fluids, such as polymer solutions and melts, and give rise to new phenomena. Often such instabilities are unwelcome, for example in rheometric devices when the aim is to measure material properties or in simple pumping operations when they can give rise to unacceptably large pressure drops and prevent pumping. In other cases they can give rise to enhanced mixing of heat and mass which would otherwise be difficult to achieve (e.g. microfluidics applications).