Extensional rheology and elastic instabilities of a wormlike micellar solution in a microfluidic cross-slot device

Wormlike micellar surfactant solutions are encountered in a wide variety of important applications, including enhanced oil recovery and ink-jet printing, in which the fluids are subjected to high extensional strain rates. In this contribution we present an experimental investigation of the flow of a model wormlike micellar solution (cetyl pyridinium chloride and sodium salicylate in deionised water) in a well-defined stagnation point extensional flow field generated within a microfluidic cross-slot device. We use micro-particle image velocimetry (m-PIV) and full-field birefringence microscopy coupled with macroscopic measurements of the bulk pressure drop to make a quantitative characterization of the fluid’s rheological response over a wide range of deformation rates. The flow field in the micromachined cross-slot is first characterized for viscous flow of a Newtonian fluid, and m-PIV measurements show the flow field remains symmetric and stable up to moderately high Reynolds number, Re z 20, and nominal strain rate, _3nom z 635 s1. By contrast, in the viscoelastic micellar solution the flow field remains symmetric only for low values of the strain rate such that _3nom # lM1, where lM ¼ 2.5 s is the Maxwell relaxation time of the fluid. In this stable flow regime the fluid displays a localized and elongated birefringent strand extending along the outflow streamline from the stagnation point, and estimates of the apparent extensional viscosity can be obtained using the stressoptical rule and from the total pressure drop measured across the cross-slot channel. For moderate deformation rates (_3nom $ lM1) the flow remains steady, but becomes increasingly asymmetric with increasing flow rate, eventually achieving a steady state of complete anti-symmetry characterized by a dividing streamline and birefringent strand connecting diagonally opposite corners of the cross-slot. Eventually, as the nominal imposed deformation rate is increased further, the asymmetric divided flow becomes time dependent. These purely elastic instabilities are reminiscent of those observed in crossslot flows of polymer solutions, but seem to be strongly influenced by the effects of shear localization of the micellar fluid within the microchannels and around the re-entrant corners of the cross-slot

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

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 203-213).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.by Thomas J. Ober.S.M

Role of viscoelasticity and non-linear rheology in flows of complex fluids at high deformation rates

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2013.Some pages printed landscape orientation. Cataloged from PDF version of thesis.Includes bibliographical references (pages 365-399).We combine pressure, velocimetry and birefringence measurements to study three phenomena for which the fluid rheology plays a dominant role: 1) shear banding in micellar fluids, 2) extension-dominated flows in microfluidic devices, and 3) flow-induced particle migration in microchannels. Firstly, worm-like micellar solutions are model non-Newtonian fluids having a single relaxation time [beta]. At shear rates larger than ... however, these systems exhibit shear banding and non-linear rheological behavior, whose importance is characterized by the Weissenberg number ... We develop a stability criterion for the onset of a purely viscoelastic instability for shear-banding fluids, to establish the limitations of conventional rheometric techniques for studying these fluids. A second challenge for conventional rheometers is inertially-driven secondary flows. The onset of these flows is governed by the Reynolds number ... where U is the velocity, D is the flow geometry length and v is the fluid kinematic viscosity. We develop microfluidic devices to impose shear and extensional deformation rates up to ...at low Re. These experiments combine pressure measurements, micro-particle image velocimetry ([mu]-PIV) and birefringence measurements. We develop a microfluidic chip that enables applied rheologists to quantitatively differentiate between fluid formulations intended for applications at high deformation rates. Finally, we study the interplay between fluid inertia and elasticity on particle migration. The inertially-dominated case is governed by the channel Reynolds number Re, and particle Reynolds number ... where a is the particle diameter. In a microfluidic device, the particle and channel size are on the same order, and hence migration occurs at ... in the so-called 'inertial focusing' regime which may have applications in clinical medicine. However, most physiological fluids are viscoelastic and therefore particle migration in these fluids occurs at high Reynolds and Weissenberg numbers, which is a mostly unstudied regime. We combine pressure measurements, streak imaging, [my]-PIV and particle trajectory analysis (PTA) to study the migration of polystyrene beads. Inertia drives particles toward the channel walls, whereas elasticity drives particles toward the channel centerline even at Re, ~ 2000.by Thomas Joseph Ober.Ph. D

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

We characterize the transient response of semi-dilute wormlike micellar solutions under an imposed steady shear flow in a cone-plate geometry. By combining conventional rheometry with 2-D Particle Image Velocimetry (PIV), we can simultaneously correlate the temporal stress response with time-resolved velocimetric measurements. By imposing a well defined shear history protocol, consisting of a stepped shear flow sweep, we explore both the linear and nonlinear responses of two surfactant solutions: cetylpiridinium chloride (CPyCl) and sodium salicylate (NaSal) mixtures at concentrations of [66:40] mM and [100:60] mM, respectively. The transient stress signal of the more dilute solution relaxes to its equilibrium value very fast and the corresponding velocity profiles remain linear, even in the strongly shear-thinning regime. The more concentrated solution also exhibits linear velocity profiles at small shear rates. At large enough shear rates, typically larger than the inverse of the relaxation time of the fluid, the flow field reorganizes giving rise to strongly shear-banded velocity profiles. These are composed of an odd number of shear bands with low-shear-rate bands adjacent to both gap boundaries. In the non-linear regime long transients (much longer than the relaxation time of the fluid) are observed in the transient stress response before the fluid reaches a final, fully-developed state. The temporal evolution in the shear stress can be correlated with the spatiotemporal dynamics of the multiple shear-banded structure measured using RheoPIV. In particular our experiments show the onset of elastic instabilities in the flow which are characterized by the presence of multiple shear bands that evolve and rearrange in time resulting in a slow increase in the average torque acting on the rotating fixture

Microfluidic extensional rheometry using a hyperbolic contraction geometry

Microfluidic devices are ideally suited for the study of complex fluids undergoing large deformation rates in the absence of inertial complications. In particular, a microfluidic contraction geometry can be utilized to characterize the material response of complex fluids in an extensionally-dominated flow, but the mixed nature of the flow kinematics makes quantitative measurements of material functions such as the true extensional viscosity challenging. In this paper, we introduce the ‘extensional viscometer-rheometer-on-a-chip’ (EVROC), which is a hyperbolically-shaped contraction-expansion geometry fabricated using microfluidic technology for characterizing the importance of viscoelastic effects in an extensionally-dominated flow at large extension rates (λ[. over ε][subscript a] ≫ 1, where λ is the characteristic relaxation time, or for many industrial processes . over ε][subscript a] ≫ 1 s[superscript −1]). We combine measurements of the flow kinematics, the mechanical pressure drop across the contraction and spatially-resolved flow-induced birefringence to study a number of model rheological fluids, as well as several representative liquid consumer products, in order to assess the utility of EVROC as an extensional viscosity indexer.National Science Foundation (U.S.). Graduate Research FellowshipUnited States. National Aeronautics and Space Administration (Microgravity Fluid Sciences Grant NNX09AV99G)European Commission. Marie Curie Actions (FP7-PEOPLE-2011-IIF Grant 298220

Visualization of microscale particle focusing in diluted and whole blood using particle trajectory analysis

Inertial microfluidics has demonstrated the potential to provide a rich range of capabilities to manipulate biological fluids and particles to address various challenges in biomedical science and clinical medicine. Various microchannel geometries have been used to study the inertial focusing behavior of particles suspended in simple buffer solutions or in highly diluted blood. One aspect of inertial focusing that has not been studied is how particles suspended in whole or minimally diluted blood respond to inertial forces in microchannels. The utility of imaging techniques (i.e., high-speed bright-field imaging and long exposure fluorescence (streak) imaging) primarily used to observe particle focusing in microchannels is limited in complex fluids such as whole blood due to interference from the large numbers of red blood cells (RBCs). In this study, we used particle trajectory analysis (PTA) to observe the inertial focusing behavior of polystyrene beads, white blood cells, and PC-3 prostate cancer cells in physiological saline and blood. Identification of in-focus (fluorescently labeled) particles was achieved at mean particle velocities of up to 1.85 m s[superscript −1]. Quantitative measurements of in-focus particles were used to construct intensity maps of particle frequency in the channel cross-section and scatter plots of particle centroid coordinates vs. particle diameter. PC-3 cells spiked into whole blood (HCT = 45%) demonstrated a novel focusing mode not observed in physiological saline or diluted blood. PTA can be used as an experimental frame of reference for understanding the physical basis of inertial lift forces in whole blood and discover inertial focusing modes that can be used to enable particle separation in whole blood

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

In this study, we use microparticle 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, optical rheometry of two wormlike 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 ≤ math ≤ 104s-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 wormlike 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 breakdown in the stress-optical rule as the characteristic rate of deformation is increased.National Science Foundation (U.S.) (Graduate Research Fellowship

Rotational 3D printing of damage-tolerant composites with programmable mechanics

Natural composites exhibit exceptional mechanical performance that often arises from complex fiber arrangements within continuous matrices. Inspired by these natural systems, we developed a rotational 3D printing method that enables spatially controlled orientation of short fibers in polymer matrices solely by varying the nozzle rotation speed relative to the printing speed. Using this method, we fabricated carbon fiber–epoxy composites composed of volume elements (voxels) with programmably defined fiber arrangements, including adjacent regions with orthogonally and helically oriented fibers that lead to nonuniform strain and failure as well as those with purely helical fiber orientations akin to natural composites that exhibit enhanced damage tolerance. Our approach broadens the design, microstructural complexity, and performance space for fiber-reinforced composites through site-specific optimization of their fiber orientation, strain, failure, and damage tolerance

Centerscope

Centerscope, formerly Scope, was published by the Boston University Medical Center "to communicate the concern of the Medical Center for the development and maintenance of improved health care in contemporary society.