Spatially resolved quantitative rheo-optics of complex fluids in a microfluidic device

Abstract

In this study, we use micro-particle image velocimetry (µ-PIV) and adapt a commercial birefringence microscopy system for making full-field, quantitative measurements of flow-induced birefringence (FIB) for the purpose of microfluidic, opticalrheometry of two worm-like micellar solutions. In combination with conventional rheometric techniques, we use our microfluidic rheometer to study the properties of a shear-banding solution of cetylpyridinium chloride (CPyCl) with sodium salicylate (NaSal) and a nominally shear thinning system of cetyltrimethylammonium bromide (CTAB) with NaSal across many orders of magnitude of deformation rates (10 −2 ≤γ ≤ 10 4 s −1 ). We use µ-PIV to quantify the local kinematics and use the birefringence microscopy system in order to obtain high-resolution measurements of the changes in molecular orientation in the worm-like fluids under strong deformations in a microchannel. The FIB measurements reveal that the CPyCl system exhibits regions of localized, high optical anisotropy indicative of shear bands near the channel walls, whereas the birefringence in the shear-thinning CTAB system varies more smoothly across the width of the channel as the volumetric flow rate is increased. We compare the experimental results to the predictions of a simple constitutive model, and we document the break-down in the stress optical rule as the characteristic rate of deformation is increased.

Spatiotemporal dynamics of multiple shear-banding events for viscoelastic micellar fluids in cone-plate shearing flows

“…Above this critical shear rate, viscometric flow becomes unstable against the formation of different shear bands [20,22,1,31]. These results are consistent with previous studies that used identical or comparable wormlike micellar solutions [2,49,23,7,36,24].…”

“…Different bands have different microstructural organization, corresponding to different degrees of alignment of the wormlike micelles under shear. These microstructural variations can be detected using birefringence measurements [23,24].…”

Rheo-PIV of a shear-banding wormlike micellar solution under large amplitude oscillatory shear

“…Additional bulk rheological measurements of an identical test fluid (eg. measurements of the normal stress data N 1 ) are provided in a recent study by Ober et al (2011). Fig.…”

“…However, an identical fluid composition (i.e. one without salt) has been used in a wide range of previous studies that have covered topics from bulk rheology (Rehage and Hoffmann, 1991), to microfluidic rheometry and steady channel flow (Ober et al, 2011) to shear banding in cone-plate geometries (Britton and Callaghan, 1997). In figure 1 we summarize the shear rheology of the fluid (measured at 25 • C) and show that it is well described by a single mode Maxwell model for low frequencies ω ≤ 1/λ, for which the viscoelastic moduli G and G are given by the following expressions:…”

“…In addition to studies of banding in wall-bounded Couette cells, there have also been velocimetric studies of the onset of shear banding in these fluids within pressure driven microchannel flows (Mendez-Sanchez et al, 2003;Marín-Santibáñez et al, 2006;Nghe et al, 2008;Ober et al, 2011). Although the kinematics of steady complex fluid flow in a microchannel are spatially inhomogeneous, these experiments have been useful because they can be used to probe the nonlinear rheology of the fluids at much higher shear rates than is usually possible and can probe the onset of "non-local" effects when the characteristic length scale of the geometry and of the shear band become comparable (Masselon et al, 2008(Masselon et al, , 2010.…”