simon haward

Wormlike micelles are elongated semi-flexible aggregates resulting from the self-assembly of amphiphilic molecules in solution. In the semi-dilute and concentrated regimes, wormlike micellar solutions are entangled and exhibit viscoelasticity in a way similar to polymer solutions, with a typical relaxation time (λ) on the order of milliseconds to seconds. However, a significant difference between wormlike micelles and conventional polymers with covalently bonded back-bone chains is their ability to break and reform dynamically, resulting in unusual stress relaxation behaviour and allowing the rheological properties of the solutions to recover subsequent to degradation in strong flows. In addition the physical properties of the micelles (e.g. contour length, persistence length and relaxation time) are extremely sensitive to factors including surfactant concentration, ionic strength and temperature, which allows an exquisite control over the bulk fluid rheological properties. The resulting unique properties of these fluids have led to their use in a wide range of applications in including jetting and spraying, turbulent drag reduction, enhanced oil recovery. Such processes involve complex flow fields with strongly extensional components, which can cause large and rapid deformation of the fluid microstructure. The resulting stretching and alignment of micelles leads to a range of poorly understood phenomena including flow-induced chain scission and structure formation, and elastic instabilities. We have been using our microfluidic cross-slot device to study the nature of purely elastic flow instabilities in wormlike micellar solutions in this well-defined extensional flow with a view understanding and optimizing industrial uses of these fascinating fluids.