Local micelle concentration fluctuations in microfluidic flows and its relation to a flow-induced structured phase (FISP)

Abstract

Shear-induced structures (SIS) are known to form in flows of wormlike micellar solutions. In simple shear cases these structures (SIS) are temporary and disintegrate upon cessation of the flow; while in certain mixed-flow cases these flow-induced structured phases (FISPs) are stable and long-lived. Here, we compare the flow of a micellar solution in a microfluidic device containing an array of microposts, with that of Poiseuille flow in a microchannel. In the former case a stable permanent FISP can be produced, whereas in the latter no structured phase (SIS or FISP) occurs. Using the fluorescent dye Nile Red, we are able to observe the local micelle concentration of the solutions during flow. We find that in the microfluidic device containing microposts, we can generate local concentration variation on the order of up to 25% that strongly correlates with the FISP, while in the microchannel with Poiseuille flow, no measurable concentration differences are observed. The relevance of these concentration variations for the formation of the FISP are discussed. The use of Nile Red as a probe into the local micelle concentration may also serve as a simple and complementary optical tool (i.e. with birefrigence and light scattering) towards investigation of a variety of flow-induced structures, as opposed to more complicated and expensive scattering techniques that involve neutrons and X-rays.