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Scientific questions surrounding the shear-dependent microstructure of carbon black suspensions are motivated by a desire to predict and control complex rheological and electrical properties encountered under shear. In this work, direct structural measurements over a hierarchy of length scales spanning from nanometers to tens of micrometers are used to determine the microstructural origin of the suspension viscosity measured at high shear rates. These experiments were performed on a series of dense suspensions consisting of high-structured carbon blacks from two commercial sources suspended in two Newtonian fluids, propylene carbonate and light mineral oil. The shear-induced microstructure was measured at a range of applied shear rates using Rheo-VSANS (very small angle neutron scattering) and Rheo-USANS (ultra-small angle neutron scattering) techniques. A shear-thinning viscosity is found to arise due to the self-similar break up of micrometer-sized agglomerates with increasing shear intensity. This self-similarity yields a master curve for the shear-dependent agglomerate size when plotted against the Mason number, which compares the shear force acting to break particle-particle bonds to the cohesive force holding bonds together. It is found that the agglomerate size scales as R g , agg ∼ M n − 1 . Inclusion of the particle stress contribution extends the relevance of the Mason number to concentrated suspensions such as those relevant to the processing of carbon black suspensions for various applications. [ABSTRACT FROM AUTHOR]

We present new dynamic rheological tests of human blood that motivate an improved model for the thixotropic shear rheology of human blood. Unidirectional large amplitude oscillatory shear (UD-LAOS) is proposed as a dynamic test relevant for understanding in vivo arterial pulsatile blood flow, and data are presented for two healthy donors studied previously using more traditional steady and step-rate experiments. The model analyzes the overall extra stress of the system in two components. A thixotropic component is represented by a structure-kinetics model coupled with a stress expression containing both viscoelastic and thixotropic elements to account for the presence of linear aggregates of red blood cells, known as rouleaux, which form at low shear rates. Another component is viscoelastic, described through an extend White–Metzner approach. It corresponds to the stresses that arise due to the isolated red blood cells, which deform at high shear rates. The combined model is fit to experimental steady shear and UD-LAOS data for two healthy donors and is subsequently used to predict the stress response of the blood sample to UD-LAOS experiments over a broad range of strain amplitudes and frequencies. Comparison to the previous models for transient blood rheology shows a significantly improved fit over a range of physiologically relevant flow conditions. In addition, this analysis demonstrates the usefulness of UD-LAOS tests with regards to understanding the microstructural evolution in blood under transient flow. [ABSTRACT FROM AUTHOR]

A stable dispersion of silica nanoparticles in a common ionic liquid [C4mim][BF4] that exhibits significant shear thickening is formulated by controlling the strength of hydrogen bonding between the nanoparticle surface and the anion of [C4mim][BF4]. Colloidal stability is demonstrated to be due to the formation of solvation layers, the properties of which are determined by scattering measurements. Timetemperature superposition measurements and the onset of yielding behavior in steady shear indicate the increase of particle-particle attraction and loss of stability of colloidal dispersions above 30 °C. Small angle neutron scattering and dynamic light scattering measurements confirm the reduction in solvation layering at elevated temperatures, leading to a transition from stable dispersion to unstable, attractive gel with increasing temperature (i.e., an inverse melting transition). It is demonstrated how controlling specific surface-anion interactions through chemical functionality can be used to formulate nanoparticle dispersions in ionic liquids with specific rheological properties. [ABSTRACT FROM AUTHOR]

Colloidal gels exhibit time-dependent bulk properties. However, the processes and mechanisms by which aging occurs are poorly understood, which complicates the prediction of macroscopic behavior in these systems. Using a model, thermoreversible, adhesive hard sphere system consisting of octadecyl-coated silica nanoparticles dispersed in tetradecane, rheological aging is quantitatively related to structural aging. By simultaneously measuring the bulk properties and gel microstructure using rheometry and small angle neutron scattering, respectively, we show a one-to-one correspondence between the time-dependent storage modulus and the microstructure, and further, that this correspondence is independent of the gel's thermal and shear history. At the working volume fraction, the gel is homogeneous and, unlike phase-separated gels, aging behavior is not due to heterogeneous coarsening. Instead, the results presented here are consistent with homogeneous, local particle rearrangements as the mechanism of rheological aging. By establishing a quantitative and predictive relationship between the underlying microstructure and bulk mechanical properties, the results of this study may be (1) industrially relevant to products that age on commercially relevant timescales, (2) applicable to other dynamically arrested systems, such as metallic glasses, and (3) valuable in the design of new materials. [ABSTRACT FROM AUTHOR]

In this work, we assess the capability of two thermodynamically consistent, microstructure-based models to predict the rheology and microstructure of a model wormlike micellar surfactant solution known to exhibit shear banding undergoing transient simple shear and large amplitude oscillatory shear deformations in a cylindrical Couette rheometer. The microstructure of the surfactant solution was simultaneously measured during the rheometric tests using small angle neutron scattering and this data is used along with the bulk rheometry to critically evaluate the models. The first model is a new, two-species model specifically developed for wormlike micelles, whereas the second model is the well-known Giesekus model with solvent and diffusion. The new model accounts for the microstructural dynamics including flow-induced micellar breakage and recombination, which is shown to be necessary to predict some of the measured rheology and microstructure. More species should be included to improve the quantitative comparison with experiments, as well as nonlinear relaxation, as judged by the success of the Giesekus model to represent certain features of the time-dependent data. More experimental research is needed to validate the role of flow-induced breakage in the nonlinear rheology of wormlike micellar surfactant solutions. [ABSTRACT FROM AUTHOR]

The shear rate-dependent rheological properties of soft to rigid colloidal suspensions are studied using computational models. We show that a contact force defined based on an elastohydrodynamic deformation theory captures an important rheological behavior of colloidal suspensions: While near hard-sphere particles exhibit a strong and continuous shear thickening the evolves to a constant viscosity state, soft suspensions undergo a second shear-thinning regime at high Peclet numbers when the hydrodynamic stresses become larger than the modulus of the colloidal particles. We measure N1 and N2 to be large and negative in the shear-thickening regime; however, for soft spheres at the onset of second shear-thinning N2 reduces in magnitude and eventually becomes positive. We show that for near hard-sphere suspensions, colloidal pressure, shear stress, and normal stress difference coefficients tend to diverge near the maximum packing fraction while P > σ > N1 > N2. [ABSTRACT FROM AUTHOR]

The rheology and shear-induced structures of a series of self-assembled surfactant wormlike micelles (WLMs) with varying levels of branching are measured using rheo- and flow-small angle neutron scattering (SANS). The degree of branching in the mixed cationic/anionic surfactant (cetyltrimethylammonium tosylate/sodium dodecyl benzene sulfonate) WLMs is controlled via the addition of the hydrotropic salt sodium tosylate and verified by cryo-transmission electron microscopy. The linear viscoelasticity of the low salt (linear) micellar solutions is well described as an extended Maxwell (Oldroyd-B) fluid, and samples exhibit shear banding under steady-shear flow. The linear viscoelasticity of more highly branched solutions deviates from Maxwellian behavior, where the plateau in G' gradually increases in slope with increasing salt content. The higher salt solutions exhibit a shear thinning regime, followed by a shear thickening regime at high shear rates. Micelle segmental alignment in the flow-gradient plane is a nonmonotonic function of salt level and radial position. Spatially resolved measurements of the segmental alignment corroborate shear banding in the linear WLMs, and the absence of shear banding with branching. Rheo-SANS measurements show that the onset of shear thickening at high rates corresponds to a structural transition. The results of this study link micellar microstructure and topology to the measured shear rheology of WLM solutions. [ABSTRACT FROM AUTHOR]

The microstructure-rheology relationship for a model, thermoreversible nanoparticle gel is investigated using a new technique of time-resolved neutron scattering under steady and timeresolved large amplitude oscillatory shear (LAOS) flows. A 21 vol. % gel is tested with varying strength of interparticle attraction. Shear-induced structural anisotropy is observed as butterfly scattering patterns and quantified through an alignment factor. Measurements in the plane of flow show significant, local anisotropy develops with alignment along the compressional axis of flow, providing new insights into how gels flow. The microstructure-rheology relationship is analyzed through a new type of structure-Lissajous plot that shows how the anisotropic microstructure is responsible for the observed LAOS response, which is beyond a response expected for a purely viscous gel with constant structure. The LAOS shear viscosities are observed to follow the "Delaware-Rutgers" rule. Rheological and microstructural data are successfully compared across a broad range of conditions by scaling the shear rate by the strength of attraction, providing a method to compare behavior between steady shear and LAOS experiments. However, important differences remain between the microstructures measured at comparatively high frequency in LAOS experiments and comparable steady shear experiments that illustrate the importance of measuring the microstructure to properly interpret the nonlinear, dynamic rheological response. [ABSTRACT FROM AUTHOR]

The rheological behavior and microstructure of branched, cationic wormlike micellar (WLM) solutions of 40 mM erucyl bis(hydroxyethyl)methylammonium chloride (EHAC) are studied as a function of added salt (sodium salicylate) concentration, temperature, and shear rate via Rheosmall-angle light scattering (Rheo-SALS). These WLM solutions exhibit shear-enhanced concentration fluctuations leading to shear-induced phase separation (SIPS), manifested as visual turbidity under shear and the appearance of a characteristic 'butterfly' scattering pattern in Rheo-SALS experiments. Flow kinematics measurements in a Couette geometry are used to determine the relationship between SIPS and shear banding, i.e., the splitting of the flow into shear bands with different local shear rates. Modeling using the Giesekus constitutive equation aids in discrimination between banding and nonbanding solutions. The combination of Rheo-SALS, dynamic rheology, velocimetry, and constitutive equation modeling allows detailed exploration of the relationship between SIPS, shear banding, fluid microstructure, and the equilibrium phase behavior. [ABSTRACT FROM AUTHOR]

The shear rheology of a model wormlike micellar solution exhibits moderate shear thinning and curved flow velocity profiles without discontinuity (nonbanding case). The shear rheology and the flow kinematics are analyzed within the framework of the Giesekus constitutive equation. Macroscopically, the steady state flow curve of the solution exhibits shear thinning with a shear exponent <1 without hysteresis, indicative of a sample that does not shear band. The microstructure of the micellar network is probed by the combination of dynamic rheology, rheo-optics, and SANS. Flow kinematics in a Couette geometry are measured by particle tracking velocimetry and found to be consistent with predictions of the Giesekus constitutive equation fit to the bulk shear rheology. 1-2 plane SANS measurements of the segmental alignment under shear are also found to be in agreement with predictions of the constitutive equation, providing a coherent picture of the mechanisms by which wormlike micelles flow and shear thin. The degree of segmental alignment is found to lag behind predictions of the model, which is postulated to be a consequence of the long, branched topology of the wormlike micelles. [ABSTRACT FROM AUTHOR]

A stable dispersion of silica nanoparticles in a common ionic liquid [C4mim][BF4] that exhibits significant shear thickening is formulated by controlling the strength of hydrogen bonding between the nanoparticle surface and the anion of [C4mim][BF4]. Colloidal stability is demonstrated to be due to the formation of solvation layers, the properties of which are determined by scattering measurements. Timetemperature superposition measurements and the onset of yielding behavior in steady shear indicate the increase of particle-particle attraction and loss of stability of colloidal dispersions above 30 °C. Small angle neutron scattering and dynamic light scattering measurements confirm the reduction in solvation layering at elevated temperatures, leading to a transition from stable dispersion to unstable, attractive gel with increasing temperature (i.e., an inverse melting transition). It is demonstrated how controlling specific surface-anion interactions through chemical functionality can be used to formulate nanoparticle dispersions in ionic liquids with specific rheological properties. [ABSTRACT FROM AUTHOR]