Spatially resolved measurements of kinematics and flow-induced birefringence in worm-like micellar solutions undergoing high rate deformations

Abstract

Worm-like micellar solutions are model non-Newtonian systems on account of their well understood linear viscoelastic behavior. Their high deformation rate, non-linear rheological response, however, remains inadequately characterized and poorly understood. In this study, two worm-like micellar systems composed of either cetylpyridinium chloride (CPyCl) with sodium salicylate (NaSal) or cetyltrimethylammonium bromide (CTAB) with NaSal have been characterized across several orders of magnitude of deformation rate (10-2 < i6 < 104 s- 1 ). This range enables us to span both the linear and non-linear regimes of rheological behavior for both systems. The low deformation rate rheology was characterized using conventional rheometer fixtures. The high deformation rate rheology was determined using microfluidic rheometric devices, which may be exploited to observe the response of a fluid undergoing very large deformation rates at moderate volumetric throughputs, on account of the small lengthscales associated with microfluidic devices. In these experiments, micro-particle image velocimetry (p-PIV) was used to measure the flow kinematics and a commercial birefringence microscopy instrument (ABRIOTM System, CRi., Inc.) was adapted for making full-field measurements of flow-induced birefringence (FIB) in order to obtain high-resolution measurements of the evolution of the average stress and molecular conformation in the fluids undergoing strong deformations. First, the shear banding response of the CPyCl:NaSal system and shear thinning response of the CTAB:NaSal system were observed in Poiseuille flow through a rectilinear microchannel. Then the corresponding behavior in an extension-dominated flow through a converging microchannel was characterized. Qualitative as well as quantitative features of the flow kinematics and conformation were assessed in order to understand how the linear rheological properties of these systems effect their respective constitutive responses in high rate extensional flows.