Flow and stability of wormlike micellar and polymeric solutions in converging and T-Shaped microchannels

The flow and stability of wormlike micellar and polymeric solutions is investigated in two prototypical converging and elongational geometries; (i) microfabricated hyperbolic contractions and (ii) converging flow in T-shaped microchannels. Understanding the flow behavior of such fluids at the microscale is important to the design and optimization of microfluidic devices for lab-on-a-chip processes and fluidic computing applications as well as to industrial applications such as extensional flow through porous media. The controlled flow rates and very well-defined geometries achievable with microfluidic fabrication technologies enable us to gain insight into the extensional rheology of complex fluids at high extension rates and to investigate the onset of elastically-driven flow asymmetries. In the present study, celyltrimethyl ammonium bromide (CTAB) wormlike micelles in aqueous solutions of sodium salicylate as well as dilute polyethylene oxide (PEO) solutions are selected as test fluids. Using the micellar fluids, it is possible to quantify the two-dimensional distribution of both the velocity and stress fields in hyperbolic-shaped micro-contractions using a new micro scope-based flow-induced birefringence technique in conjunction with microparticle imaging velocimetry (mu PIV). The knowledge of both the deformation and velocity data allows us to better understand the behavior of shear-banding fluids in inhomogeneous extensional flows. In the case of dilute PEO solutions flowing through perfectly symmetric T-shaped microchannels, a local extensional flow develops where the two streams meet. The resulting birefringent strand of highly-oriented material can lead to symmetry-breaking bifurcations in the flow at high Weissenberg number which can be quantified using microparticle imaging velocimetry. The spatio-temporal characteristics of these purely elastic flow asymmetries can also be compared to predictions of numerical simulation.