Andrea W. Chow

Shear‐induced particle migration in Couette and parallel‐plate viscometers: NMR imaging and stress measurements

Couette and parallel plate viscometers are two commonly used flow geometries to characterize shear viscosity of concentrated suspensions. In Couette flow, it is well documented that prolonged shearing causes a decrease in the apparent viscosity of concentrated suspensions due to shear‐induced particle migration from the annulus region to the stagnant region under the bob. In this study, the technique of nuclear magnetic resonance imaging (NMRI) was used to measure the evolution of suspension concentration profiles in Couette and parallel‐plate flow devices upon shearing. Neutrally buoyant suspensions of nearly monodisperse, non‐Brownian spherical particles at a volume fraction of 0.5 in a Newtonian fluid were used. The same flow cells and suspensions were also used in a rheometer to measure the changes in shear stress under identical experimental conditions such that a direct comparison can be made between the stress and concentration data. For Couette flow, the NMRI data correlated very well with the str...

NMR flow imaging of fluids and solid suspensions in Poiseuille flow

The application of nuclear magnetic resonance (NMR) flow imaging to the study of Poiseuille flows of single‐phase fluids and solid/liquid suspensions is demonstrated and investigated. Two fluids were studied: a mixture of a low‐molecular‐weight polyether oil and water which is Newtonian and an aqueous solution of a high‐molecular‐weight polymer which is non‐Newtonian. The velocity profiles of these fluids in Poiseuille flow measured by NMR are in excellent agreement with those predicted from viscometric data using a power law model. The suspensions investigated consisted of density‐matched compositions of solid poly(methyl methacrylate) beads of various concentrations up to 52% in a Newtonian, polyether‐based liquid. At particle loadings above 40% these suspensions exhibited rheological behavior akin to what has been reportedly observed for some solid‐rocket motor propellant compositions. NMR‐derived flow velocities of these suspensions at high particle loadings show clear deviations from the Newtonian parabolic behavior and some unusual intensity features which are not yet fully understood. The capabilities and limitations of NMR flow imaging as well as considerations for data interpretation in the special case of highly filled suspensions is discussed.The application of nuclear magnetic resonance (NMR) flow imaging to the study of Poiseuille flows of single‐phase fluids and solid/liquid suspensions is demonstrated and investigated. Two fluids were studied: a mixture of a low‐molecular‐weight polyether oil and water which is Newtonian and an aqueous solution of a high‐molecular‐weight polymer which is non‐Newtonian. The velocity profiles of these fluids in Poiseuille flow measured by NMR are in excellent agreement with those predicted from viscometric data using a power law model. The suspensions investigated consisted of density‐matched compositions of solid poly(methyl methacrylate) beads of various concentrations up to 52% in a Newtonian, polyether‐based liquid. At particle loadings above 40% these suspensions exhibited rheological behavior akin to what has been reportedly observed for some solid‐rocket motor propellant compositions. NMR‐derived flow velocities of these suspensions at high particle loadings show clear deviations from the Newtonian pa...

Response of Moderately Concentrated Xanthan Gum Solutions to Time‐Dependent Flows Using Two‐Color Flow Birefringence

A new optical arrangement for the flow birefringence experiment, based on a two‐color laser, is described. Using this system, flow birefringence measurements can be performed in transient flow conditions with the birefringence Δn and orientation angle χ of the sample being obtained simultaneously in time. Unlike previous designs which have been devised to accomplish this capability, only a single experimental run is necessary. Measurements using a quarter‐wave retarder indicate that the experimental accuracy is 2.2% for the birefringence and 0.15 ° for the orientation angle. The minimum birefringence detectable by this system is estimated to be 2×10−7. Using this technique. Δn and χ of moderately concentrated xanthan solutions were measured following the inception and cessation of simple shear. The effects of increased ionic strength by adding NaCl to the xanthan solution were also examined. A qualitative comparison of measurements between the no salt and 0.1M NaCl solutions indicates that the macromolecu...

Direct observation of particle microstructure in concentrated suspensions during the falling‐ball experiment

It is demonstrated in this study that suspension microstructure and mean fall velocity can be obtained simultaneously in real time during the falling‐ball experiment using nuclear magnetic resonance imaging (NMRI). When the size of the suspending particles is significantly different from that of the falling ball (at least by a factor of 2), the motions of both the falling ball and the surrounding neutrally buoyant particles can be tracked by monitoring the proton NMR signals from the suspending liquid. Using a 50 vol % suspension of approximately 700 μm diameter spheres and a 9 mm diameter cylinder, the suspension microstructure around a 4.76 mm (3/16 in.) diameter falling ball was found to be significantly perturbed by the motions of the ball, whereas the fall velocity decreases with time. The suspension concentration increases at the leading edge and decreases behind the falling ball. These effects diminish as the diameter of the falling ball decreases to 1.59 mm (1/16 in.). Possible mechanisms to accou...

Some experimental results on the development of Couette flow for non-Newtonian fluids

Abstract This paper reports some experimental results on the time development of a Couette flow following the start-up of shear flow using the technique of two-color flow birefringence. Measurements obtained on collagen solutions are consistent with two theoretical studies which predict that for some viscoelastic liquids, momentum is transferred from the moving Couette cell boundary to the interior of the fluid through a velocity wave propagating and reflecting between the cell boundaries. This non-Newtonian phenomenon, exhibited as an oscillatory response in the measured birefringence and orientation angle, is observed at shear rates above a critical value when the response time of the polymer solution approaches the flow development time in the Couette flow cell.

Structure‐property relations in processing high‐performance polyisoimide–imide resins

The processing of high‐performance polyimides via their processible isoimide modifications is of great interest to aeronautical and aerospace applications. Rheological characterization during the isoimide‐to‐imide conversion, essential for identifying the processing conditions of such reactive polymers and their polymer blends, is reported. Using dynamic shear measurements, we obtained rheological information for a series of polyisoimide–imide resins as a function of temperature and cure history. Both homopolymers and copolymers have been investigated. We found that the processibility (e.g., melt viscosity of isoimide) and ultimate product properties (e.g., Tg of polyimide) can be systematically varied by changing the chemical constituents and rigidity of the polymer backbone. Such correlations of chemical structures, rheological behavior, and processing characteristics provide the necessary database for tailor making new polymer systems with desirable processibility and mechanical properties. Practical c...