Rheological behavior of a bentonite mud

Predicting drilling fluids rheology is crucial to control/optimize the drilling process and the gas extraction from drilling fluids in logging systems. A Couette viscometer measured the apparent viscosity of a bentonite mud at various shear rates and temperatures. The bentonite mud behaved as a yield-pseudoplastic fluid, and a modified Herschel-Bulkley model predicted the shear rate and temperature effects upon the shear stress. A pipe viscometer was built to seek a correlation between the mud flow rate and the pressure drop and thereby determine refined Herschel-Bulkley parameters. Coupling a rheological model to a pipe viscometer enables the continuous acquisition of apparent viscosities of Newtonian or non-Newtonian fluids at a rig-site surface.

E-Selectin Expression on Endothelial Cells in the Presence of Platelets and Cigarette Smoke Extract

Cigarette smoking is a risk factor for development of cardiovascular (CV) disease [1], with increased platelet activation due to cigarette smoke involving a major part of this risk.[2] We have shown that this smoke-induced platelet activation is largely due to the non-nicotine smoke components and their effects can be effectively modulated in the presence of nicotine.[3] However, the effects of nicotine and non-nicotine cigarette smoke components need to be confirmed more physiologically in the presence of endothelial cells (ECs). Prior in-vitro studies have shown that high concentrations of cigarette smoke extract (CSE) increase adhesion molecule expression on ECs.[4] These studies however preclude the involvement of physiological shear stresses and are performed on ECs alone. To overcome these limitations and investigate ECs-platelets together in one system under shear stress, we use a hemodynamic shear device (HSD) that combines features of the cone and plate, and the annular Couette viscometer (to facilitate platelet sampling). We test the following hypotheses — (1) smoke-activated platelets and the nicotine-free extract would confer a synergistic E-selectin expression on ECs, and (2) in contrast to conventional cigarette extracts, nicotine-free smoke extracts would increase platelet activation more significantly, and that this effect may be independent of the presence of ECs.

Complex Rheology in Particle-Laden Composite Materials

The rheology of curing composite materials is important to many manufacturing processes. At Sandia, we work with particle-filled epoxy systems for a variety of applications. One particular system, designated as “459,” exhibits complex, counter-intuitive rheological dependence on temperature, flow history and particle-concentration. Despite the particles being relatively large (10 μm), 459 exhibits shear-thinning hysteretic behavior reminiscent of colloidal systems. It is hypothesized that the thixotropy arises from reaction of the 459 curative with the surface of the particulates. Under certain conditions, the addition of filler offsets the effect of epoxy polymerization on the viscosity, resulting in a viscosity that is constant with time. For this reason, we have developed an experimental model system that can be used to separate the effects of particle aggregation from the effects of cure on the viscosity. We have done experiments with this model suspension to determine its time-dependent response in step shear. In addition, we are working on a computational model that can be used to predict the behavior of the aggregating suspension. Preliminary modeling efforts focus on using a generalized Newtonian constitutive equation that relates the viscosity to the local aggregate concentration, defined by a dimensionless structure factor, to capture the time-dependence. Results from this constitutive equation are presented and compared to the experimental data from step-shear experiments in a Couette viscometer. Modeling results are also presented for a transient fiber spinning problem where the free surface evolves as the viscosity develops over time.

Ferrohydrodynamics in Time-Varying Magnetic Fields

Measurements of magnetic-field-induced ferrofluid flow and torque in uniform rotating magnetic fields are presented and compared to theoretical analyses in order to understand observed paradoxical behavior. The viscous torque from this fluid flow is measured using a cylindrical Couette viscometer, as a function of magnetic field amplitude, frequency, and direction of rotation. The first set of experiments measures the torque on the outer wall of a polycarbonate spindle that is attached to a viscometer, which functions as a torque meter. The spindle is immersed in the ferrofluid, which is centered in the gap of a three-phase AC 2-pole motor stator winding. Anomalous behavior, such as negative effective viscosity, is demonstrated and discussed. The second set of experiments measures the viscous torque on the inner wall of a hollow spindle attached to the torque meter and filled completely with ferrofluid so that there is no free surface. These measurements show that magnetic fluid effects arise even in the absence of free surfaces. These observations are then shown to agree with a recently derived analysis of spin-up flow in ferrofluids.