David C. Venerus

A benchmark study on the thermal conductivity of nanofluids

This article reports on the International Nanofluid Property Benchmark Exercise, or INPBE, in which the thermal conductivity of identical samples of colloidally stable dispersions of nanoparticles or “nanofluids,” was measured by over 30 organizations worldwide, using a variety of experimental approaches, including the transient hot wire method, steady-state methods, and optical methods. The nanofluids tested in the exercise were comprised of aqueous and nonaqueous basefluids, metal and metal oxide particles, near-spherical and elongated particles, at low and high particle concentrations. The data analysis reveals that the data from most organizations lie within a relatively narrow band (±10% or less) about the sample average with only few outliers. The thermal conductivity of the nanofluids was found to increase with particle concentration and aspect ratio, as expected from classical theory. There are (small) systematic differences in the absolute values of the nanofluid thermal conductivity among the various experimental approaches; however, such differences tend to disappear when the data are normalized to the measured thermal conductivity of the basefluid. The effective medium theory developed for dispersed particles by Maxwell in 1881 and recently generalized by Nan et al. [J. Appl. Phys. 81, 6692 (1997)], was found to be in good agreement with the experimental data, suggesting that no anomalous enhancement of thermal conductivity was achieved in the nanofluids tested in this exercise.

SEGMENT CONNECTIVITY, CHAIN-LENGTH BREATHING, SEGMENTAL STRETCH, AND CONSTRAINT RELEASE IN REPTATION MODELS. II. DOUBLE-STEP STRAIN PREDICTIONS

Predictions for double-step strain flows are presented using a newly proposed reptation theory that accounts for segment connectivity, chain-length breathing, segmental stretch and constraint release in a self-consistent, full-chain theory. In this part of the work emphasis is on double-step shear strains where the second step is reversed and the imposition time of the second strain is earlier than the estimated retraction time, for which the Doi–Edwards model and single-integral models have been found to be incapable of describing experimental trends. Transient stress relaxation properties of two types of reversing flows, types B and C, have been examined and compared to the predictions obtained from the Doi–Edwards model and a single-integral model. The simulations show excellent agreement with the experimental trends based on recent mechanical and optical measurements.

Study of thermal transport in nanoparticle suspensions using forced Rayleigh scattering

Thermal diffusivity measurements on two nanofluids and their base fluids were made using an optical technique called forced Rayleigh scattering. The nanofluids studied were a citrate-stabilized Au nanoparticle suspension in water and an Al2O3 nanoparticle suspension in a petroleum oil. Thermal diffusivity measurements on the nanofluids and base fluids were made at temperatures in the range of 25–75°C. From these data, it was possible to estimate the thermal conductivity enhancement in the nanofluids as a function of temperature. In contrast to previous reports on similar systems, our experiments are consistent with thermal conductivity enhancement predictions from effective medium theory. In particular, we find that the level of thermal conductivity enhancement is independent of temperature.

Analysis of diffusion-induced bubble growth in viscoelastic liquids

Abstract Transport models of diffusion-induced bubble growth in viscoelastic liquids are developed and evaluated. A rigorous model is formulated that can be used to describe bubble growth or collapse in a non-linear viscoelastic fluid, and takes into account convective and diffusive mass transport as well as surface tension and inertial effects. Predictions for bubble growth dynamics demonstrating the importance of fluid elasticity are presented. These predictions indicate that for diffusion-induced bubble growth in viscoelastic liquids, the lower bound for growth rate is given by growth in a Newtonian fluid and the upper bound by diffusion-controlled growth. The influence of non-linear fluid rheology on bubble growth dynamics is examined and found to be relatively minor in comparison to fluid elasticity. It is shown how previously published models employing various approximations can be derived from the rigorous model. Comparisons of predicted bubble growth dynamics from the rigorous and approximate models are used to establish the ranges of applicability for two commonly-used approximations. These comparisons indicate that models using a thin boundary layer approximation have a rather limited range of applicability. An analysis of published experimental bubble growth data is also carried out using appropriate transport models.

Characterization of type I collagen gels modified by glycation

Chronic exposure to reducing sugars due to diabetes, aging, and diet can permanently modify extracellular matrix (ECM) proteins. This non-enzymatic glycosylation, or glycation, can lead to the formation of advanced glycation end products (AGE) and crosslinking of the ECM. This study investigates the effects of glycation on the properties of type I collagen gels. Incubation with glucose-6-phopshate (G6P), a reducing sugar that exhibits similar but more rapid glycation than glucose, modified the biological and mechanical properties of collagen gels. Measures of AGE formation that correlate with increased complications in people with diabetes, including collagen autofluorescence, crosslinking, and resistance to proteolytic degradation, increased with G6P concentration. Rheology studies showed that AGE crosslinking increased the shear storage and loss moduli of type I collagen gels. Fibroblasts cultured on glycated collagen gels proliferated more rapidly than on unmodified gels, but glycated collagen decreased fibroblast invasion. These results show that incubation of type I collagen gels with G6P increases clinically relevant measures of AGE formation and that these changes altered cellular interactions. These gels could be used as in vitro models to study ECM changes that occur in diabetes and aging.

Stress and birefringence measurements during the uniaxial elongation of polystyrene melts

A rheometer for generating uniaxial elongations in molten polymers has been modified to allow for the simultaneous measurement of stress and flow-induced birefringence. Tensile stress σ and birefringence Δn′ data in flows at constant strain rates up to 1 s−1 were collected on a polydisperse polystyrene melt at temperatures of 160 and 170 °C. From these data, the stress-optic rule was followed for stresses below roughly 1 MPa. For stresses less than 1 MPa, the stress-optic coefficient |C|=|Δn′|/σ was found to have a value of 4.8×10−9 Pa−1, which was independent of strain, strain rate, and temperature. At stress levels higher than 1 MPa, |C| decreased indicating a failure of the stress-optic rule. A criteria for failure of the stress-optic rule was formulated using simple arguments from network models and characteristic times suggested by the tube model. This criteria, which is based on the hypothesis that failure of the stress-optic rule is the result of significant chain stretching, was found to be consis...

Segment connectivity, chain-length breathing, segmental stretch, and constraint release in reptation models. III. Shear flows

A previously proposed self-consistent reptation model that includes chain stretching, chain-length fluctuations, segment connectivity, and constraint release is used to predict transient and steady shearing flows. Quantitative comparisons are made with the concentrated solution data considered in the previous papers of the series. The model is able to capture quantitatively all features of experimental data considered, including overshoot in both shear and first normal stresses, the strain-rate dependence of the strain magnitude at maximum stress, the steady-state viscosity and first-normal-stress coefficient as functions of shear rate, the viscosity curves for different molecular weight, the transient and steady-state behavior of the extinction angle, and the stress relaxation in cessation of steady shear flow. The model can describe all aspects of the data very well except the magnitude of the overshoot in stress at high shear rates, where the model is somewhat over-predictive. A new method of analysis for shear stress decay following cessation of steady shear is proposed, based on the physics of the model.

A critical evaluation of step strain flows of entangled linear polymer liquids

A critical evaluation of step strain flow experiments on entangled, linear polymer liquids is performed. Roughly one-half of the published shear stress relaxation modulus data for these systems are consistent with the predictions of the well-known tube model. A model of step strain flow experiments is developed to determine whether the remaining published data, which are qualitatively different from tube model predictions, are simply artifacts caused by slip, an imperfect strain step strain history, or transducer compliance. Modeling results suggest that these factors are capable of producing the types of behavior observed in experiments deemed anomalous. New criteria based on the retraction time τR, or longest Rouse relaxation time, for avoiding anomalies caused by imperfect strain step strain history and transducer compliance are proposed. These simple criteria are, in a majority of cases, found to be capable of predicting the type of observed stress relaxation behavior for 60 published step strain expe...

Flow field visualization of entangled polybutadiene solutions under nonlinear viscoelastic flow conditions

Using self-designed particle visualization instrumentation, startup shear and step-strain tests were conducted under a series of systematically varied rheological and geometrical conditions, and the velocity profiles in three different well-entangled polybutadiene/oligomer solutions were obtained. For startup shear tests, in the regime of entanglement densities up to 89 and nominal reptation Weissenberg numbers up to 18.6, we generally observe either wall slip and a linear velocity/strain profile or simply the linear profile with no wall slip unless a massive edge fracture or instability has occurred in the sample. Meanwhile, step-strain tests conducted at similar and higher step Weissenberg numbers revealed little particle motion upon cessation. These results lead us to a conclusion that there is no compelling evidence of shear banding or nonquiescent relaxation in the range of entanglement density and Wi investigated; we interpret the results to imply that any observed banding probably correlates with edge effects.

Comparison of optical and mechanical measurements of second normal stress difference relaxation following step strain

Relaxation of the second normal stress difference (N2) following step strain of a concentrated monodisperse polystyrene solution has been studied using mechanical and optical rheometry. Measurements of normal thrust in a parallel plate geometry are corrected for strain inhomogeneity and combined with independent measurements of the first normal stress difference (N1) to determine N2. Optical experiments were performed using a novel configuration where flow birefringence data collected using multiple light paths within the shear plane are combined with the stress-optical law to determine all three independent stress components for shearing deformations. This technique eliminates end effects, and provides an opportunity to oversample the stress tensor and develop consistency checks of experimental data. N2 is found to be nonzero at all accessible times, and relaxes in roughly constant proportion to N1. This reflects nonaffine distribution of chain segments, even well within the regime of chain retraction at short times. Data collected with the two techniques are reasonably consistent with each other, and with results of previous studies, generally lying between the predictions of the Doi-Edwards model with and without the independent alignment approximation. The normal stress ratio −N2/N1 shows pronounced strain thinning in the nonlinear regime.