Publications

The apparent wall slip phenomenon is inevitably encountered in the standard rheological measurement for concentrated suspensions. It may significantly underestimate the apparent viscosity in the experiments. Previously we have proposed a slotted rotor design to reduce such effects. The objective of this study is to validate this design by conducting 3-D computational fluid dynamics (CFD) simulation and analyzing the velocity and shear stress fields in the double concentric cylinder rheometer with and without slotted rotor and the vane rheometer. Both shear thinning and yield stress fluids, modeled by a continuous viscosity constitutive equation, are considered in the simulations. The wall slip effects are taken into account using the wall slip length method. The results indicate that the double concentric cylinder rheometer equipped with a slotted rotor can measure the fluid properties with enhanced accuracy and less sensitivity to the wall slip velocity than a rheometer with a non-slotted rotor. The wall slip effects can be further reduced by either increasing the slot ratio or adding more slots to the rotor. Although the vane rheometer can reduce the wall slip effects too, it is only suitable for high shear thinning or high yield stress fluids due to the large end effects. As a conclusion, the use of a slotted rotor in the double concentric cylinder rheometer may significantly reduce both wall slip and end effects, making this design an excellent choice for rheological measurements.

In Multiple-Particle-Tracking Microrheology (MPTM), rheological properties of fluids are determined from the Stokes-Einstein theory applied to Brownian motion of small suspended particles. As compared to conventional rheometers, this noncontact method does not have the problem of wall slip effects and requires a very small amount of a test fluid. MPTM measurements are typically performed in a quiescent fluid to ensure all particles are subject to random motion. Unfortunately, it is very difficult if possible to completely eliminate the deterministic motion of a test fluid during measurement because of thermal convection of the fluid, fluctuations and inclination of the experimental platform, and active transport of particles. In this work, we report our first results on development of MPTM that takes into account the deterministic velocity of a test fluid. In our approach, 0.1 or 0.9 um diameter Latex beads were suspended in a fluid located between a glass microscope slide and a glass coverslip. The movement of the particles was visualized through an inverted microscope with 40x and 60x objectives and recorded by a high-speed camera at 30 frames per second. The trajectories of particles were analyzed using a MATLAB code in which the viscosity of a test fluid was determined from the ensemble averaged MSD vs. lag time curves with the deterministic component of the fluid removed under the assumption that the ensemble-averaged velocity for randomly moving particles was equal to zero. We applied this approach to measure the viscosity of water and 5% dextran-water solution at different temperatures. With the elimination of the deterministic component, our results agree well with published viscosity data for these fluids.

Wall slip effects in a rheometer with double concentric cylinder geometry may lead to significant errors in measurement of the apparent viscosity. Previously, we proposed to use a slotted rotor design to reduce these effects. In this paper, we conduct two- as well as three-dimensional computational fluid dynamics (CFD) simulations to determine the differences in rheological measurements of yield stress fluids between the slotted and non-slotted rotor designs. The test fluid and the slip wall boundary of a rotor are characterized in our computational model by the constitutive equation of Zhu et al. (2005) and the wall-slip length method, respectively. The model has been validated against the existing rheological data measured using a vane rheometer. The results of this study indicate that the rheometer equipped with a slotted rotor can measure the fluid properties with enhanced accuracy and less sensitivity to the wall slip velocity than a rheometer with a non-slotted rotor. We also show that the wall slip effects can be further reduced by either increasing the slot ratio or adding more slots to the rotor. This work illustrates that CFD analysis can be a powerful tool in rheometer design.

Biological systems are characterized by a signicant level of heterogeneity and, on the macro-scale, behave as viscoelastic materials. To study the mechanical behavior of biological systems, we have developed a novel parallel algorithm for fully three-dimensional numerical simulation of multiphase viscoelastic ow. The algorithm consists of the second order Volume-of-Fluid method for tracking uid-uid interfaces, the projection method for solving the Navier-Stokes equations, and the semi-implicit factorized scheme for the constitutive equation for the stress tensor (Giesekus, Oldroyd-B, or Upper-Convected Maxwell uid). We will talk about the application of the algorithm to the problems in microvascular hemodynamics, such as leukocyte-endothelial cell adhesion and blood ow in channels with complex geometry. We will show that the code we developed can accurately predict leukocyte rolling on vascular endothelium and blood ow in sprouting vessels. Proposals for extending the algorithm to other biological problems will also be discussed.

Atherosclerosis is a progressive disorder of medium-to large-size arteries characterized by hardening and narrowing of the vessels due to formation and calcification of atheromatous plaques on the inside of the vessel walls. It is established that this disorder develops near vessel bifurcations and curvatures (where separation and reversal of blood flow occur) as a result of oxidative damage to vascular endothelium caused by oxidized low-density lipoproteins (oxLDL). Such endothelial dysfunction leads to increased adhesion of monocytes to endothelial cells and accumulation of monocytes/macrophages in the intimal layer of the arterial wall. In this talk, we present three-dimensional computational models of monocyte-endothelium interactions that take into account 1) monocyte viscoelasticity, 2) complex flow conditions existing at atherosclerosis-prone sites, and 3) chemokine-stimulated and multiple-receptor-mediated cell adhesion kinetics. We also discuss our in vitro experiments on oxLDL-induced adhesion of monocytic cell line THP-1 to HUVEC in a micro-fluidic flow chamber. Through comparison of in vitro and computational studies, we show that firm adhesion of monocytes to endothelial cells is very sensitive to monocyte rheological properties and flow conditions to which endothelial cells and monocytes are exposed.

Many biological fluids are viscoelastic and require a nonlinear constitutive equation to describe the evolution of the extra-stress tensor. We use an immersed boundary framework to model processes that involve the movement of immersed elastic boundaries interacting with a surrounding viscoelastic fluid. We present recent results on applications including dynamics of a closed membrane moving under surface tension, and phase-locking of swimming sheets.

White blood cells (WBCs), also known as leukocytes, migrate to sites of infection to destroy pathogenic microorganisms. The ability of WBCs to deform is essential for this function but it is also an important determinant of healthy vasculature. This chapter analyzes the effects of leukocyte deformability on leukocyte–endothelial interactions, presents the evidence for the critical role of the cytoskeleton in bulk mechanical properties of leukocytes, summarizes recent advances in rheological measurements of leukocytes, and discusses pathologies associated with leukocyte activation and reduced deformability of these cells.