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To estimate yield stress and other rheological properties relevant to modeling of wax deposition in pipelines where flow is continuous, but stress varies across the pipe radius, model wax-oil mixtures are cooled at multiple cooling rates under constant shear stress until a temperature is reached, at which flow is arrested by gelation due to wax crystal formation. From these data, combined with a measurement of temperature-dependence of precipitated wax concentration by differential scanning calorimetry, an apparent yield stress σ y , below which flow is arrested at each temperature, is related to the concentration of precipitated wax C p and the cooling rate. Results are reported for multiple concentrations in oil of two independent wax mixtures: a many-component commercial wax mimicking the composition of field oil, and a simpler six-alkane mixture. These transient rheological data are fit to a pseudo "Herschel–Bulkley" constitutive equation from which it is found that the yield stresses obtained during flow under cooling are generally an order of magnitude, or more, lower than the yield stresses obtained in the previous work in flow at a comparable constant temperature after cooling in the absence of flow. We also find a strong decrease in the arrest temperature with a decreasing cooling rate, with no convergence even at the lowest cooling rate of 0.0625 °C min−1, indicating that under slower cooling, flow continues even under low stresses. The cooling-rate-dependent yield stress obtained in our study under constant stress provides a challenge to the recent models of gelation under flow stress and is of relevance to wax deposition in pipelines. [ABSTRACT FROM AUTHOR]

The mechanism producing enhanced thermal conductivities of nanofluids has been the subject of much debate. The formation of aggregates allowing for percolation paths within the fluid has shown the most promise. This work studies the aggregate formation of a nanofluid and compares the results to earlier thermal conductivity measurements and Monte Carlo simulation results. Static light scattering is employed to measure the fractal dimension of aggregates formed in the nanofluid over time at various temperatures and concentrations. As expected, aggregates form more quickly at higher concentrations and temperatures, which explains the increased enhancement with temperature reported by other research groups. The permanent aggregates in the nanofluid are found to have a fractal dimension of 2.4 and the aggregate formations that grow over time are found to have a fractal dimension of 1.8, which is consistent with diffusion limited aggregation. Predictions indicate that as aggregates grow the viscosity increases at a faster rate than thermal conductivity making the highly aggregated nanofluids unfavorable, especially at the low fractal dimension of 1.8. [ABSTRACT FROM AUTHOR]

Investigates the distribution of stresses in the shear-thinning regime using simulations of the flow of concentrated aggregated colloidal systems at a particulate level. Distribution of shear stress; Manifestation of the microstructural mechanisms associated with large stresses; Highest forces occurring towards the end of the shear thinning.

The viscosity of colloidal dispersions is a highly sensitive function of the microstructure of the dispersion and the nature of the interactions among the particles. While the viscosity of solutions of colloidal hard spheres is well understood, the viscosity in a broader array of colloidal solutions remains poorly studied. Here, we use immersed boundary simulations to investigate the viscosity of a prototypical colloidal solution, adhesive hard spheres, which under the right conditions can percolate to form sample spanning networks. We examine how rigidity of the particle–particle contacts in these dispersions can enhance the dispersion viscosity. By rigidly constraining a fraction of the adhered particles, we establish a reaction coordinate, the extent of rigidity, along which we can track the development of a gelled state characterized by a diverging viscosity. The fraction of rigid bonds is analogous to the extent of reaction for cross-linking in chemical gels, which suggests a close connection between chemical gelation with polymers and physical gelation with adhesive hard spheres. A critical gel point is identified when particles connected by rigid bonds begin to percolate the sample. In the vicinity of critical gel point, we observe a diverging viscosity and critical scaling behavior in agreement with previous studies on adhesive hard spheres. We use these results to formulate empirical equations for the viscosity across a broad range of volume fractions as a function of the extent of rigidity. Most importantly, through these simulations, we show that rigid constraints among the bonded particles are essential to producing the large viscosities measured experimentally in dispersions of adhesive hard spheres. Hydrodynamic interactions alone are insufficient for reproducing experimental observations. [ABSTRACT FROM AUTHOR]

A microrheological model of aggregating dispersions is proposed in which the shear stress is estimated as the sum of hydrodynamic and structural parts. The former is attributed to the hydrodynamic cores of fractal aggregates, which behave as a suspension of impermeable spheres. The latter accounts for the forces transmitted by chains of particles linking neighboring aggregates into a transient network. To calculate the structural part the concept of fractal aggregation is incorporated into a transient network theory, to account for the creation and breakup of chains of colloidal particles connecting the aggregates. Rigid and soft chains are distinguished. The former have multiply connected backbones which deform as contorted elastic rods, while the latter have at least one soft junction and deform without elastic resistance until fully loaded. The contribution of the soft chains to the stress tensor is neglected. The calculations treat two different mechanisms for the evolution of rigid chains: a purely mechanical one, which corresponds to a shear-controlled structure built up in flow, and a thermal mechanism, which pertains to a quasiequilibrium structure undisturbed by shear. We calculate steady-shear viscosities in the former case and viscoelastic functions in the latter. The model can be fitted satisfactorily to the experimental results for a well-characterized polystyrene latex dispersion with physically acceptable parameters. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We experimentally study the behavior of suspensions of silica particles in aqueous solution. Despite many studies on these materials, the dependence of the overall rheological properties of the suspension on particle size, solid volume fraction, ionic strength, and strain history remains debated. In this paper, we manage to manufacture materials and develop procedures that allow us (i) to approach this problem in the best possible way and (ii) to check that the results obtained with well controlled systems (monodisperse silica spherical particles) also apply to less controlled suspensions (crushed silica particles). We find that the elastic modulus-particle size and yield stress-particle size relationships follow power laws that disagree with classical models from the literature. We also show that elastic modulus versus yields stress data fall on a single master curve when rescaled by particle size, whatever are solid volume fraction, resting time, and ionic strength. This suggests that the rescaled elastic modulus can play the role of a parameter in a structural kinetics model of the behavior of thixotropic suspensions. Furthermore confocal observations of the system provided evidence that the evolution of the overall properties of the material with resting time cannot be ascribed to changes in the particle network. [ABSTRACT FROM AUTHOR]

Gelling colloidal suspensions represent an important class of soft materials. Their mechanical response is characterized by a solid-to-liquid transition at a given shear stress level. Moreover, they often exhibit a complex time-dependent rheological behavior known as thixotropy. The viscosity changes find their origin in the microstructure, which depends on flow history. Yet, the structural response of colloidal gels to flow differs fundamentally from most complex fluids, where flow induces orientation. Upon yielding, low to intermediate volume fraction gels break down in a spatially anisotropic way. Bonds in the velocity-velocity gradient plane are broken, whereas microstructural features in other planes are less affected. The subsequent flow-induced microstructural anisotropy is characterized by typical butterfly scattering patterns. However, as yet there was no evidence for the pertinence of this anisotropy for the rheological properties of these systems. In the present work, orthogonal superposition rheometry was first used to evaluate how the flow-induced microstructure affects the viscoelastic properties. It was shown to retain significant elasticity in the velocity-vorticity plane, even when the structure liquefied. Further, the shearinduced mechanical anisotropy was measured using two-dimensional small amplitude oscillatory shear, exploiting the fact that for suitable thixotropic samples the recovery after arresting the flow is relatively slow. It was hence possible to measure the anisotropy of the moduli upon cessation of flow. The mechanical anisotropy was shown to be spectacular, with the storage moduli in perpendicular directions differing by as much as 2 orders of magnitude. [ABSTRACT FROM AUTHOR]

We propose a rheological model for ideal thixotropy, defined as a reversible time-dependent shear-thinning viscous response, in both shearrate- controlled (RC Model) and stress-controlled (SC Model) forms. The model introduces a spectrum of structure parameters that collectively relaxes as a stretched exponential. It retains time-invariance symmetry with only a stretching exponent β as an additional model parameter relative to conventional single-structure-parameter models. The kinetic equations are nonlinear in both the structure parameters and the flow parameters of strain rate or stress. We demonstrate that introducing multiple structure parameters can successfully capture the stretched-exponential and nonmonotonic evolution of stress or shear rate in general step tests; and using nonlinear kinetic equations can explain the relaxation time's dependence on both the initial and final values of shear rate or stress in step tests. We present rheological data for a fumed silica dispersion in a number of shear histories including steady state, step shear rate, step stress, shear-rate ramp, and stress ramp. A systematic way to parameterize the models is provided. Both models fit experimental data well although the SC Model provides better agreement with the measurements. The ideal thixotropic models can be combined with existing methods that incorporate viscoelasticity so as to extend their validity into the region of low shear rates. [ABSTRACT FROM AUTHOR]

This paper is concerned with the rheology of algae suspensions relevant to algae biofuel processing for a range of concentrations up to 15 vol. % using mostly a piezoaxial vibrator (PAV) rheometer as a method of measuring rheological properties. Linear viscoelastic (LVE) measurements of a Scenedesmus obliquus [culture collection of algae and protozoa (CCAP) 276/7] living algae strain were obtained and a curve for complex viscosity (η*) as a function of concentration/volume fraction derived. The PAV complex viscosity data increased exponentially with cell concentration and elasticity (G′) developed in a similar way with increasing concentration. The results indicated the presence of interaction between algae cells at all measured concentrations. For concentrations above ∼5 vol. %, steady shear data obtained using a Couette geometry showed non-Newtonian 'shear-thinning' behavior and at higher concentrations there was a divergence from the Cox-Merz rule. A difference in the LVE rheological measurements was found for cells that were either alive or dead indicating that cell motility and significant interparticle contact and interactions influenced levels of viscoelasticity. The results are of potential scientific relevance and also useful in relation to the design of algae bioprocessing for the production of biofuels. [ABSTRACT FROM AUTHOR]

This paper is concerned with the experimental ability to measure viscoelasticity of low viscosity ink jet fluids and demonstrates the capability of both a piezo axial vibrator and torsion resonator rheometer to capture high frequency rheological data for both model and commercial ink jet fluids. Results are presented for polymer and particle laden suspensions together with a commercial ink. The data demonstrate that high frequency linear viscoelastic rheology can be captured using both rheometers and that both the presence of polymer and particles can induce viscoelasticity within the fluid. It is believed that the physical origin of viscoelastic effects produced by the presence of polymer or particles is different, and this results in a different high frequency limiting slope for the G′ data. [ABSTRACT FROM AUTHOR]

The effect of fractal dimensions and Feret diameter of aggregated nanoparticle on predicting the thermophysical properties of nanofluids is demonstrated. The fractal dimensions and Feret diameter distributions of particle agglomerates are quantified from scanning electron and probe microscope imaging of yttria nanofluids. The results are compared with the fractal dimensions calculated by fitting the rheological properties of yttria nanofluids against the modified Krieger-Dougherty model. Nanofluids of less than 1 vol. % particle loading are found to have fractal dimensions of below 1.8, which is typical for diffusion controlled cluster formation. By contrast, an increase in the particle loading increases the fractal dimension to 2.0-2.2. The fractal dimensions obtained from both methods are employed to predict the thermal conductivity of the nanofluids using the modified Maxwell-Garnet (M-G) model. The prediction from rheology is found inadequate and might lead up to 8% error in thermal conductivity for an improper choice of aspect ratio. Nevertheless, the prediction of the modified M-G model from the imaging is found to agree well with the experimentally observed effective thermal conductivity of the nanofluids. In addition, this study opens a new window on the study of aggregate kinetics, which is critical in tuning the properties of multiphase systems. [ABSTRACT FROM AUTHOR]