Search

Determination of the rheological properties and the interfacial structure–property relationships for complex fluid–fluid interfaces can be of crucial importance for the understanding of physiological and biomedical systems, designing and engineering industrial processes, and developing environmental remediation technologies. For the measurement of interfacial shear rheological material functions, it has been determined that the control of the surface pressure during the application of deformation is essential for obtaining reproducible data especially when measuring complex interfaces, such as particle- and polymer-laden interfaces. Moreover, the study of complex fluid interfaces is complicated by kinematically mixed interfacial flow fields, which include both shear and dilatation (shape and area changes), leading to a possible complex flow history. To address this, specialized rheometers have been developed to provide clear kinematic conditions. For instance, a radial trough has been introduced to enhance the study of dilatational interfacial rheology, effectively solving the challenges posed by the mixed flow fields typical in standard rectangular Langmuir–Pockels (LP) troughs or pendant drops. In the present work, we utilize a new trough instrument, the Quadrotrough (QT), capable of performing on the same device (and sample) independent dilatational and shear deformations at the air/liquid interface under strain and surface pressure control. Brewster angle microscopy (BAM) imaging is carried out in situ simultaneously with rheological measurements. Thus, the QT embodies the combined advantages of the circular trough and the controlled surface pressure shear interfacial rheometer. The interfacial rheology of poly(tert-butyl methacrylate) at the air/water interface was measured for both pure dilatation and pure shear in steady and small amplitude oscillatory (SAO) dilatation (D) and shear (S) modes on the same interface. BAM images were obtained during shear and compression. The results obtained by the QT were highly reproducible and in good agreement with measurements performed previously using the LP trough-mounted double wall ring rheometer and the radial trough. The Hencky strain model was employed to derive steady shear and dilatational interfacial moduli. Very good agreement was observed between the steady and complex shear moduli. However, the dilatational moduli measured under steady compression were markedly smaller than those measured by small amplitude oscillatory dilatational at fixed molecular areas, further highlighting the complicating factor of deformation history on material properties for complex interfaces. In summary, the QT has been shown to be a valuable tool for exploring interfacial rheology and providing insights into complex interfacial systems. [ABSTRACT FROM AUTHOR]

Predicting the response of complex fluids to different flow conditions has been the focal point of rheology and is generally done via constitutive relations. There are, nonetheless, scenarios in which not much is known from the material mathematically, while data collection from samples is elusive, resource-intensive, or both. In such cases, meta-modeling of observables using a parametric surrogate model called multi-fidelity neural networks (MFNNs) may obviate the constitutive equation development step by leveraging only a handful of high-fidelity (Hi-Fi) data collected from experiments (or high-resolution simulations) and an abundance of low-fidelity (Lo-Fi) data generated synthetically to compensate for Hi-Fi data scarcity. To this end, MFNNs are employed to meta-model the material responses of a thermo-viscoelastic (TVE) fluid, consumer product Johnson's® Baby Shampoo, under four flow protocols: steady shear, step growth, oscillatory, and small/large amplitude oscillatory shear (S/LAOS). In addition, the time–temperature superposition (TTS) of the material response and MFNN predictions are explored. By applying simple linear regression (without induction of any constitutive equation) on log-spaced Hi-Fi data, a series of Lo-Fi data were generated and found sufficient to obtain accurate material response recovery in terms of either interpolation or extrapolation for all flow protocols except for S/LAOS. This insufficiency is resolved by informing the MFNN platform with a linear constitutive model (Maxwell viscoelastic) resulting in simultaneous interpolation and extrapolation capabilities in S/LAOS material response recovery. The roles of data volume, flow type, and deformation range are discussed in detail, providing a practical pathway to multifidelity meta-modeling of different complex fluids. [ABSTRACT FROM AUTHOR]

Electron transport in complex fluids, biology, and soft matter is a valuable characteristic in processes ranging from redox reactions to electrochemical energy storage. These processes often employ conductor-insulator composites in which electron transport properties are fundamentally linked to the microstructure and dynamics of the conductive phase. While microstructure and dynamics are well recognized as key determinants of the electrical properties, a unified description of their effect has yet to be determined, especially under flowing conditions. In this work, the conductivity and shear viscosity are measured for conductive colloidal suspensions to build a unified description by exploiting both recent quantification of the effect of flow-induced dynamics on electron transport and well-established relationships between electrical properties, microstructure, and flow. These model suspensions consist of conductive carbon black (CB) particles dispersed in fluids of varying viscosities and dielectric constants. In a stable, well-characterized shear rate regime where all suspensions undergo self-similar agglomerate breakup, competing relationships between conductivity and shear rate were observed. To account for the role of variable agglomerate size, equivalent microstructural states were identified using a dimensionless fluid Mason number, Mnf, which allowed for isolation of the role of dynamics on the flow-induced electron transport rate. At equivalent microstructural states, shear-enhanced particle-particle collisions are found to dominate the electron transport rate. This work rationalizes seemingly contradictory experimental observations in literature concerning the shear-dependent electrical properties of CB suspensions and can be extended to other flowing composite systems. [ABSTRACT FROM AUTHOR]

An improved population balance‐based rheological constitutive framework for polydisperse aggregating suspensions is derived by incorporating detailed models for orthokinetic and perikinetic aggregation and shear breakage processes. The framework accounts for critical properties such as dynamic arrest, viscoelasticity, kinematic hardening, thixotropy, and yield stress to generate a full range of thixotropic elasto‐viscoplastic (TEVP) response. Additionally, the model is thermodynamically consistent because the dynamics and timescales are completely determined by internal structural and kinetic variables. The model connects the rheological response to the structural descriptors such as the size distribution of agglomerates, mean sizes, fractal dimension, and agglomerate volume fraction. Predictions are compared against a wide range of shear rheology measurements data for model thixotropic suspensions of fumed silica and carbon black, including large amplitude oscillatory shear (LAOS), as well as ultra‐small angle neutron scattering under steady shear (Rheo‐uSANS). [ABSTRACT FROM AUTHOR]

Hemorheology is the study of blood flow and the mechanical stresses and kinematics involved. The Casson constitutive equation is a popular and simple model used to describe the steady shear rheology of blood, with only two parameters that specify an infinite shear viscosity and a yield stress that depend on blood physiology. Previous literature has identified hematocrit and fibrinogen concentration as the two most important physiological factors that affect blood flow, but previous parameterizations of the Casson model may not be reliable due to the use of non-standardized data sets. This study uses machine learning and the largest standardized dataset to improve the parameterization of the Casson model with respect to hematocrit and fibrinogen concentration for healthy individuals. The study also employs machine learning to identify a potential additional factor, the mean corpuscular hemoglobin (MCH), that may affect blood rheology. The proposed approach demonstrates the potential for machine learning to improve the connection between physiology and blood rheology with possible implications in cardiovascular diagnostics. [ABSTRACT FROM AUTHOR]

Novel measurements of the spatiotemporal microstructure of a continuous shear thickening colloidal suspension under shear cessation and reversal using small angle neutron scattering in a 1–2 shear cell are presented for moderate to high Péclet numbers. In the shear cessation experiments from high Péclet shear flow, two-step relaxation mechanism is found. Particles first readily diffuse on a characteristic timescale of single particle Brownian motion, followed by slower relaxation dynamics on a timescale of short-time self-diffusion when caging effects start dominating. In the shear reversal experiment, the unchanged scattering intensity and the quick flip in anisotropic structure around the shear thickened state suggest the persistence of hydroclusters without transitioning through the equilibrium structure, at least in a time resolution of 0.01 s. Access to the time evolution of microstructure provides valuable insights into the spatiotemporal details of interparticle interactions governing colloidal suspension rheology, such as Brownian, hydrodynamic, and nanotribological forces. [ABSTRACT FROM AUTHOR]

In this work, we outline the development of a thermodynamically consistent microscopic model for a suspension of aggregating particles under arbitrary, inertia-less deformation. As a proof-of-concept, we show how the combination of a simplified population-balance-based description of the aggregating particle microstructure along with the use of the single-generator bracket description of nonequilibrium thermodynamics, which leads naturally to the formulation of the model equations. Notable elements of the model are a lognormal distribution for the aggregate size population, a population balance-based model of the aggregation and breakup processes and a conformation tensor-based viscoelastic description of the elastic network of the particle aggregates. The resulting example model is evaluated in steady and transient shear forces and elongational flows and shown to offer predictions that are consistent with observed rheological behavior of typical systems of aggregating particles. Additionally, an expression for the total entropy production is also provided that allows one to judge the thermodynamic consistency and to evaluate the importance of the various dissipative phenomena involved in given flow processes. [ABSTRACT FROM AUTHOR]

Two colloidal suspensions of paucidisperse, spherical silica particles with different surface chemistries leading to extreme limits of surface contact friction are studied to identify experimental differences in shear rheology and microstructure and quantitatively test theory and simulation models. The nonequilibrium microstructure in the plane of shear is measured by flow-small angle neutron scattering for steady shear states spanning the shear thinning and shear thickening regimes. The shear rheology and microstructure are compared against predictions from theory for Brownian hard sphere suspensions and state-of-the-art simulation methods that incorporate either contact friction or enhanced lubrication hydrodynamics. The first normal stress differences are confirmed to distinguish between these micromechanical mechanisms for stress enhancement in the shear thickened regime. The nonequilibrium microstructure in the plane of shear shows more anisotropy for the suspension with higher interparticle friction. A significant fourfold symmetry is confirmed and found to be amplified with increasing surface contact friction in the shear thickened state. The differences in shear-induced microstructures between suspensions with varying contact friction demonstrate that the nonequilibrium microstructure can distinguish between nanotribological interactions in the shear thickened state. Statistical comparison of experiments with simulations indicates that better resolution of microstructures in simulation models is required to be validated by the experimental data presented. Implications for the development of theories for colloidal suspension rheology are discussed. [ABSTRACT FROM AUTHOR]

Model aluminosilicate (N‐A‐S‐H) gel systems give insight to the complex reaction occurring in alkali‐activated binders, allowing for more precise control over chemical composition and exhibiting simpler kinetics. Small angle neutron scattering, oscillatory rheology, and solid‐state NMR were used to characterize N‐A‐S‐H gel samples and relate structure, chemical composition, and rheology. Aluminum concentration was varied in the gel samples at fixed silica and sodium concentration. Past the percolation point, the gel develops in two regimes distinct in structure and rheological behavior. The first gel regime is characterized by increase in modulus and initial formation of fractal structures following a diffusion‐limited monomer‐cluster aggregation mechanism. The second gel regime at higher aluminum concentrations is characterized by a modulus plateau and structure development via reaction‐limited aggregation of existing clusters. NMR data indicate a saturation of aluminum in the network at the transition between gel regimes, which may be pH driven. Understanding the structure‐property relationships critical to formation of the N‐A‐S‐H gel can give valuable insight to the reaction mechanism in alkali‐activated binders and the influence of chemical composition on resultant structure and mechanical properties. [ABSTRACT FROM AUTHOR]

Rheological experiments on model and industrial colloidal dispersions that exhibit reversible shear thickening are compared to a recent model that correlates simulation results for the shear rheology of bidisperse suspensions of particles with varying levels of contact friction. Comparisons are presented with rheological results for a variety of colloidal dispersions varying from model systems to industrially relevant, commercial dispersions where both shear and first normal stress differences are available. Particle-scale measurements of particle friction coefficients are provided through a survey of the literature. The comparisons show under which conditions the model can quantitatively describe the measured rheology and how the model can be used to correlate data for systems with more complex interactions when allowances are made for effective volume fractions and adjustments in friction coefficients. Discrepancies evident in comparisons for the first normal stress differences highlight the importance of enhanced hydrodynamic friction in many systems, especially for continuous shear thickening, in qualitative agreement with recent simulations that incorporate enhanced lubrication hydrodynamics due to elastohydrodynamics or particle roughness. Finally, a literature survey of measurements of jamming volume fractions and particle friction coefficients indicates that the model overestimates the experimentally observed jamming volume fraction. The comparisons presented in this work provide quantitative information valuable for those modeling suspension processing as well as suggestions for further model improvement. [ABSTRACT FROM AUTHOR]

Stabilized-emulsions often undergo Ostwald ripening owing to the higher solubility of the smaller droplets. Their rheological properties are a direct result of the interplay between their microstructure and flow, which in turn affect their processing as well as their applications. In a recent study [Mwasame et al., "On the macroscopic modelling of dilute emulsions under flow," J. Fluid Mech. 831, 433–473 (2017)], the use of a unit determinant conformation tensor to represent morphological changes has been advanced within the context of a thermodynamically consistent theory for a dilute monodisperse emulsion. The resulting model was validated against previously developed asymptotic theory that was also used to obtain all model parameters. In the present work, the first key innovation is to show how this theory can be extended to allow for multiple conformation tensors, subject to a single joint constraint originating from the mass conservation of the dispersed phase, in order to allow for the representation of dilute polydisperse emulsions undergoing Ostwald ripening. The second enabling innovation of this work is to show how to introduce correct dissipative terms into the dissipation bracket to account for mass transfer processes. The approach is illustrated for the particular case of a bidisperse emulsion. The model illustrates how multiple transport phenomena and thermodynamic effects can be seamlessly combined within a thermodynamically consistent approach. This work allows for not only the coexistence of a bimodal population of droplets but also the evaluation of their deformability and effective rheology in the presence of Ostwald ripening as the largest size droplets increase in mass at the expense of the smaller ones. [ABSTRACT FROM AUTHOR]

Nonequilibrium Brownian dynamics simulations are used to study the effect of polydispersity on the thermodynamics, rheology, microstructure, and shear-induced disorder–order transition in suspensions of charged colloids. Approximately 43 000 particles with 2000 different components of a discretized Schulz distribution at polydispersities from 0% to 30% are simulated on a massively parallel computer. Recent advances in the integral equation theory for polydisperse suspensions are tested and verified with respect to both structure and equilibrium mechanical properties. The low shear rate rheology for both monodisperse and polydisperse suspensions is found to be well represented by the Ree–Eyring model. At higher shear rates an ordered ‘‘string’’ phase is shear induced for low polydispersities (<10%). Increasing the polydispersity further (>=20%) inhibits the ordering, suggesting the existence of a critical polydispersity beyond which a colloidal suspension cannot be induced into an ordered state by shearing. The validity of a generalized, nonequilibrium Stokes–Einstein relationship for polydisperse colloids is also investigated. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The simple shearing of a suspension of charge-stabilized, colloidal particles close to the melting line is investigated by massively parallel, nonequilibrium Brownian dynamics (NEBD) simulation. The suspension undergoes a discontinuous transition from a distorted fluid structure to an ordered ‘‘string’’ phase. Comparisons between simulations of 43 000, 4725 particles, and previous NEBD work on ≤500 particles proves that shear-induced ordering is not an artifact of the small system sizes. We also show that the shear-rate dependence of the rheological properties obtained from NEBD is different than those obtained from nonequilibrium molecular dynamics (NEMD), a consequence of the solvent damping not being present in NEMD. The validity of the Ree–Eyring model for viscosity and the stress-optic law for colloids are tested. Further, a type of generalized Stokes–Einstein relationship is discovered for systems under shear. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The linear viscoelasticity and steady shear properties of bitumen are established; however, bitumen used in pavement also supports time-varying loading in the nonlinear regime. Consequently, recent research focuses on the nonlinear rheological behavior of bitumen under time-varying flows that is essential for establishing the constitutive model. Here, we propose a protocol to obtain the nonlinear rheological behavior of bitumen under a large amplitude oscillatory shear (LAOS) stress. The LAOS stress rheological response of both a neat bitumen and a modified bitumen is analyzed by Fourier-transform rheology and strain decomposition to an orthogonal set of Chebyshev polynomials. We find that the relative nonlinearity of bitumen increases with increasing stress and decreasing frequency; the relationship between I3/I1 and stress amplitude can obey the sigmoidal function; the intrinsic nonlinearity Q0 decreases with increasing frequency and decreasing temperature. Both the FT analysis results and decomposition results show that the nonlinearity of modified bitumen B is much more significant than neat bitumen A. Bitumen A exhibits stress softening and stress thinning under all the studied test conditions, but bitumen B exhibits stress stiffening under some conditions. [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]

The flow behavior of mixtures of micron-sized cubic particles suspended in a concentrated colloidal dispersion is investigated across a broad range of cubic particle concentrations. In the semi-dilute regime, the qualitative shape of the dynamic moduli and flow curves reflect those of the underlying colloidal dispersion medium. These curves are superimposed with the underlying colloidal dispersion using shift factors that are found to be larger than those obtained in a recent study of suspensions of non-colloidal spherical particles in the same colloidal dispersion medium. At higher concentrations of cubic particles, deviations from this shifting procedure are apparent. Scaling calculations suggest depletion interactions are responsible for the increase in the low shear viscosity and confinement of the underlying colloidal dispersion can be expected to enhance the shear thickening behavior at high shear stresses. The results of this study provide guidance for formulating suspensions through control of particle shape and mixture concentration. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1091-1101, 2017 [ABSTRACT FROM AUTHOR]

A population balance model appropriate for the shear flow of thixotropic colloidal suspensions with yield stress is derived and tested against experimental data on a model system available in the literature. Modifications are made to account for dynamic arrest at the onset of the yield stress, in addition to enforcing a minimum particle size below which breakage is not feasible. The resulting constitutive model also incorporates a structural based relaxation time, unlike existing phenomenological models that use the inverse of the material shear rate as the relaxation time. The model provides a reasonable representation of experimental data for a model thixotropic suspension in the literature, capturing the important thixotropic timescales. When compared to prevalent structure kinetics models, the coarse-grained population balance equation is shown to be distinct, emphasizing the novelty of utilizing population balances as a basis for thixotropic suspension modeling. © 2016 American Institute of Chemical Engineers AIChE J, 63: 517-531, 2017 [ABSTRACT FROM AUTHOR]

The structural anisotropy and shear rheology of colloidal gels under startup of shear flow are calculated by Brownian dynamics simulations modified to include particle surface-mediated attractions, which is a model for thermoreversible colloidal gels. Shear-induced structural changes are analyzed in both real and reciprocal space through computation of the pair distribution function and structure factor. A distinct structural anisotropy is evident as alignment along the compressional and vorticity axes. A spherical harmonics expansion of pair distribution function is calculated to analyze structural anisotropy. In reciprocal space, structural anisotropy is quantified through an alignment factor, which shows overshoot behavior similar to the stresses. Based on the microstructure analysis, the evolution of the structural anisotropy is explained by the anisotropic rupture of the colloidal gel microstructure. This result provides evidence for how shearing creates structural anisotropy. The structural anisotropy during flow startup is quantitatively related to rheological behavior via a stress-small angle neutron scattering (SANS) rule that is modified to consider the influence of the rupture of the colloidal gel structure. A simple structure kinetic equation accounting for bond creation and rupture provides a unifying view of the results. This new, modified stress-SANS rule quantitatively links the shear-induced structural anisotropy to the nonlinear rheology. [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]

Rheological measurements on a model thixotropic suspension by Dullaert and Mewis [J. Non-Newtonian Fluid Mech. 139(1-2), 21-30 (2006); Rheol. Acta 45, 23-32 (2005)] are extended to include large amplitude oscillatory shear (LAOS) flow, shear flow reversal, and a novel unidirectional LAOS flow to provide an extended rheological data set for testing constitutive models. We use this extended data set to test a new structure-based model developed by improving the Delaware thixotropic model [A. Mujumdar et al., J. Non-Newtonian Fluid Mech. 102, 157-178 (2002); A. J. Apostolidis et al., J. Rheol. 59, 275-298 (2015)]. Model parameters are determined from steady, small amplitude oscillatory, and step shear rate tests. Holding those parameters fixed, model predictions are compared to LAOS experiments. Similar comparisons are made for three contemporary models from the literature. Two of these models use a scalar internal structural parameter and include the modified Jeffreys model proposed by de Souza Mendes and Thompson [Rheol. Acta 52, 673-694 (2013)]. The third model is based on fluidity additivity [F. Bautista et al., J. Non-Newtonian Fluid Mech. 80, 93-113 (1999)]. A common weakness in all models is shown to be the use of scalar order parameters that cannot account for the reversal of flow directionality inherent in LAOS flow. This is further illustrated by comparison with flow reversal and unidirectional LAOS experiments. [ABSTRACT FROM AUTHOR]

The shear flow of non-Brownian glass spheres suspended in a concentrated colloidal dispersion that exhibits non-Newtonian rheology is investigated. At low volume fractions, the addition of non-Brownian spherical particles to the colloidal dispersion leads to an increase in the steady shear viscosity as well as the dynamic moduli. The flow curves of these suspensions are qualitatively similar to the suspending colloidal dispersion medium, and as such, in this semidilute regime, the suspension data can be shifted on to that of the colloidal dispersion medium at constant shear stress with shift factors comparable to those predicted for spherical particles in a Newtonian fluid. At higher volume fractions of non-Brownian spheres, the shear thickening power law exponent increases with the addition of non-Brownian particles. This increase in the shear thickening power law exponent is shown to be consistent with the effects of confinement on the shear thickening colloidal dispersion by the larger non-Brownian particles. [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 rheology and three-dimensional microstructure of a concentrated viscoelastic solution of the triblock copolymer poly(ethylene oxide)106-poly(propylene oxide)6 8-poly(ethylene oxide)106 (Pluronic F127) in the protic ionic liquid ethylammonium nitrate are measured by small angle neutron scattering (SANS) under flow in three orthogonal planes. This solution's shear-thinning viscosity is due to the formation of two-dimensional hexagonal close-packed (HCP) sliding layer structure. Shear-melting of the crystalline structure is observed without disruption of the selfassembled micelles, resulting in a change in flow properties. Spatially resolved measurements in the 1-2 plane reveal that both shear-melting and sliding are not uniform across the Couette gap. Melting and recrystallization of the HCP layers occur cyclically during a single large amplitude oscillatory shear (LAOS) cycle, in agreement with the "stick-slip" flow mechanism proposed by Hamley et al. [Phys. Rev. E 58, 7620-7628 (1998)]. Analysis of 3D "structural" Lissajous curves show that the cyclic melting and sliding are direct functions of the strain rate amplitude and show perfect correlation with the cyclic stress response during LAOS. Both viscosity and structural order obey the Delaware-Rutgers rule. Combining rheology with in situ spatiotemporally resolved SANS is demonstrated to elucidate the structural origins of the nonlinear rheology of complex fluids. [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]

Micellar solutions of hexadecyltrimethylammonium bromide(CTAB)in a protic ionic liquid, ethylammonium nitrate (EAN), are studiedby shear rheology, polarizing optical microscopy (POM), conductivitymeasurements, and small angle neutron scattering (SANS). Three concentrationregimes are examined: A dilute regime (with concentrations [CTAB]< 5 wt %) consisting of noninteracting spherical micelles, a semidiluteregime (5 wt % ≤ [CTAB] ≤ 45 wt %) where micelles interactvia electrostatic repulsions, and a concentrated regime (45 wt % <[CTAB] ≤ 62 wt %) where a reversible, temperature-dependentisotropic (L1) to hexatic (Hex) phase transition is observed.The L1–Hex transition, which has been predictedbut not previously observed, is characterized by (1) a sharp increasein the shear viscosity, (2) the formation of focal conical birefringencetextures (observed by POM), and (3) enhancement of the crystallineorder, evidenced by the appearance of Bragg reflections in the SANSprofiles. Ionic conductivity is not sensitive to the L1–Hex transition, which corroborates the absence of topologicaltransitions. [ABSTRACT FROM AUTHOR]

Large amplitude oscillatory shear (LAOS) is an increasingly popular nonlinear rheological test method, and the interpretation of the measurements is still an active area of research. Here, we demonstrate a new method whereby the nonlinear parameters of a popular constitutive equation used to model polymeric and other viscoelastic systems, the Giesekus model, may be determined directly from LAOS measurements without complicated, nonlinear model fitting. We define the stress response of the sample as an expansion in deformation strain and oscillation frequency. To leading order in strain, we derive the explicit analytic expressions for the first three harmonics of the stress response during LAOS for the Giesekus constitutive model. This allows for rapid determination of the Giesekus model parameters, including the nonlinear coupling parameter, directly from plots of the response obtained from modern rheological instruments. We demonstrate the validity, utility, and limits of this new method on a well-studied wormlike micelle (WLM) surfactant solution, cetyltrimethylammonium bromide (CTAB), and water. This robust and simple approach is especially advantageous for systems where steady shearing in the nonlinear regime is problematic and can provide rapid determination of parameters in rheological constitutive equations. [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 split Hopkinson pressure bar (SHPB) technique is implemented to evaluate the transient response of a colloidal suspension exhibiting shear thickening at strain rates and timescales never before explored in a laboratory instrument. These suspensions are shown to exhibit a discontinuous transition from fluid-like (shear thinning) to solid-like (shear thickening) behavior when evaluated using rotational rheometry. The effect of loading rate on this transition time is studied for a particle volume fraction of 0.54 using the SHPB technique. It is shown that the time required for transition to occur decreases logarithmically with loading rate. From these results, we conclude that transition is not triggered by a characteristic shear rate, but rather a critical shear strain is required. Results from SHPB experiments performed up to Peclet numbers of order 107 are presented and discussed for 0.50, 0.52, and 0.54 particle volume fraction suspensions. [ABSTRACT FROM AUTHOR]

We present the first combined study of spatially resolved structure and shear rheology for a model shear banding fluid comprised of cetyltrimethylammonium bromide wormlike micelles. Combining conventional rheometry, velocimetry, flow birefringence, and flow-small angle neutron scattering (flow-SANS) in the 1–2 (flow-gradient) plane of shear completely characterizes shear banding in the system and enables comparison of local flow kinematics to local segmental orientation and alignment within the bands. The Giesekus constitutive equation with stress diffusion is shown to successfully model the viscoelasticity, steady shear viscosity, and shear banding kinematics. Flow-SANS measurements in the 1–2 plane exhibit a critical alignment and orientation required for shear banding, followed by a first order orientational transition to a paranematic state in the high-shear band. Master curves of the segmental orientation and alignment are constructed by comparing the local structural features to the locally observed shear rate. The Giesekus-diffusion model successfully predicts the measured segmental orientation and alignment, connecting the microstructure to the macroscopic rheology and shear banding kinematics. In doing so, a stress-SANS rule is developed, analogous to the stress-optic rule, that relates micellar flow alignment to the shear and normal stresses. The results confirm that shear banding is driven by a nonequilibrium shear-induced isotropic-nematic transition and suggest that the underlying phase behavior of the material is important in determining fluid microstructure and rheology during banding. [ABSTRACT FROM AUTHOR]

Abstract: The complex rheological behaviour of suspensions is very strongly affected by the details of the microstructure and the interparticle forces by which it is controlled. The difficulties involved in characterizing the microstructure, in particular during flow, caused the development of suspension rheology to lag behind that of polymer rheology. Progress in theory, experiments and simulation is rapidly changing the picture, providing opportunities for further significant developments. Possible evolutions in six areas of suspension rheology are discussed here. [Copyright &y& Elsevier]

Abstract: Flow can induce reversible and irreversible structural changes in dispersions. The analysis of flow effects on microstructure and rheology remains one of the challenging problems in colloid science. The rheological manifestation of flow-induced structural changes is a variable viscosity. If the changes are reversible and time dependent, the effect is called thixotropy. The basic elements of this concept are reviewed here, including its definition and the relation with nonlinear viscoelasticity. The omnipresence of thixotropy is illustrated with a wide range of examples from natural and manmade colloidal systems. Its various rheological manifestations are reviewed as well as possible measurement procedures. The microstructural changes due to flow are quite complex and not fully understood. Existing models for thixotropic suspension rheology are categorized and evaluated. [Copyright &y& Elsevier]

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]

The rheology and particle alignment of suspensions of anisotropic-precipitated calcium carbonate particles of various aspect ratios is investigated using small-angle neutron scattering simultaneous with rheological measurement (Rheo-SANS). Rheo-SANS experiments were performed at concentrations from dilute to those exhibiting continuous and discontinuous shear thickening behavior. Long axis flow alignment is evident in all systems over the range of shear rates investigated. The flow alignment is shown to increase with particle aspect ratio, the applied shear stress (up to the point of shear thickening), and particle loading. For samples exhibiting shear thickening behavior, the highest degree of flow alignment occurs at the critical stress associated with the onset of shear thickening. Irrespective of particle aspect ratio, a gradual reduction in flow alignment with increasing applied stress is observed beyond the critical stress in continuously shear thickening samples, whereas discontinuously shear thickening samples maintain nearly the same degree of flow alignment within the shear thickening regime. The results are shown to be consistent with hydrocluster formation as the mechanism of shear thickening. The critical volume fraction for discontinuous shear thickening is shown to coincide with the equilibrium isotropic-nematic phase transition at higher aspect ratios. [ABSTRACT FROM AUTHOR]

A micromechanical model for the onset of shear thickening in polymer stabilized colloidal dispersions is derived. The model incorporates both the stabilizing forces of the polymer brush or “coat” and the associated modification of the hydrodynamic interactions. Comparison is made to simulations and experiments on well-characterized dispersions. The model is shown to provide a quantitative prediction for the onset of shear thickening in polymer coated colloidal dispersions as compared to simulations. For comparison with experiments, where independent characterization of some parameters is not available, the onset of shear thickening can be predicted with realistic assumptions for the model parameters. The model quantifies the sensitivity of the shear thickening transition to both the interaction potential and the hydrodynamic permeability of the polymer coat. [ABSTRACT FROM AUTHOR]

[Display omitted] Colloidal suspensions of anisotropic particles are ubiquitous in particle-based industries. Consequently, there is a need to quantify the effects of particle shape on equilibrium phases and kinetic state transitions, particularly at lower aspect ratios (L / D ≈ 1–10). We present a new, colloidal system comprised of hollow, octadecyl-coated silica rods with 40 nm diameter with controlled aspect ratio and thermoreversible short-range attractions. Rheology and dynamic light scattering measurements on suspensions of these hollow adhesive hard rods with nominal aspect ratio ≈3 suspended in tetradecane exhibit thermoreversible gelation without complicating effects of gravitational settling. Small angle neutron scattering measurements of the microstructure are analyzed to determine the effective strength of attraction in the form of Baxter sticky parameter. Quantitative agreement is found with simulation predictions of the thermoreversible gel transition as a function of volume fraction, further validating a universal state diagram and providing guidance for the effects of aspect ratio on gelation. [ABSTRACT FROM AUTHOR]

The microstructure and rheological properties of colloidal suspensions depend on particle size and shape. This work aims to further control the size, shape, and polydispersity of anisotropic silica colloids, to reduce particle size, and to provide additional mechanistic insights on a prevalent, water-in-oil emulsion synthesis method. Key findings show that the dimensions of anisotropic silica particles can be systematically varied by approximately fivefold, with a limiting minimum particle size (D ≈ 60 nm, L ≈ 300 nm) obtained from emulsions with excess polyvinylpyrrolidone (PVP) and sodium citrate. The synthesis conditions are identified and discussed for which the emulsion composition, temperature, sonication, polymer entanglements, mixing, and other perturbations may induce or mitigate emulsion instabilities, citrate precipitation, a competing mechanism of templated growth, termination of anisotropic growth, irregular silica structures, and fiber formation. An improved mechanistic understanding will expand the roadmap for rational design and synthetic control of anisotropic colloids using sol-gel silica chemistry confined within water-in-oil emulsions. [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]

The self-assembly of an end-functionalized PEO106-PPO70-PEO106 triblock copolymer (BCP) with acrylic end groups (Pluronic F127 diacrylate or FDA) in a protic ionic liquid (deuterated ethylammonium nitrate, dEAN) is studied using small-angle neutron scattering, shear rheology, and dielectric spectroscopy. This functionalization constitutes less than 1 wt % of the copolymer and has no effect on micelle formation in dEAN. The sol-gel transition and the supramolecular crystal structure of the concentrated FDA/dEAN solutions are also found to be unaffected by the acrylation of the end groups. Photo-cross-linking results in viscoelasticity enhancement of the FDA/dEAN solutions with FDA contents below the gel point. Cross-linking gels produces soft elastomers with exceptional elasticity and conductivity comparable to the un-cross-linked solutions. Tuning BCP microstructure via self-assembly in ionic liquids followed by chemical cross-linking is shown to be a promising route for the design of materials with specific mechanical properties and ionic conductivities. [ABSTRACT FROM AUTHOR]

The topology of self-assembled surfactant solutions includes varying degrees of micellar branching, ranging from linear wormlike micelles to a micellar network. Micellar branching acts as an effective attraction between micelles such that network condensation can lead to phase separation. Unlike chemical branching in polymers, micellar branches are labile. Movement of branches along a micelle contour has therefore been proposed as a mechanism of stress relaxation that leads to a reduction in the structural relaxation time and thus, the zero-shear viscosity. Branching is also thought to suppress flow alignment, and for lower levels of branching, may also suppress instabilities such as shear banding. The suppression of shear banding can lead to a lesser degree of shear-thinning in the apparent viscosity at higher shear rates, as well as a reduction in extensional thickening. However, for higher levels of branching, shear can induce branching for samples in proximity to such a phase transition, which can result in shear banding due to shear-induced phase separation. Recent modeling and simulations of the energetics of branching, as well as experiments on model systems, show that the reduction in zero-shear viscosity is due to micelle branching. Current research includes efforts to develop a more mechanistic, quantitative understanding of micellar branching and more generally, its effects on micellar solution rheology. [ABSTRACT FROM AUTHOR]

The microstructure and rheologicalproperties of micellar solutionsof Pluronic block copolymers (PEOx–PPOy–PEOx)in deuterated ethylammonium nitrate (dEAN, a protic ionic liquid)are studied by small- and ultrasmall-angle neutron scattering (SANSand USANS), fluorescence microscopy, and shear rheology. A homologousseries of three Pluronics with similar PPO block length (F127 (PEO106–PPO70–PEO106), P123(PEO20–PPO70–PEO20),and L121 (PEO5–PPO70–PEO5)) are examined to explore the effect of the relative balance ofsolvophilic to solvophobic block size on the self-assembled microstructure.Both F127 and P123 form spherical micelles in dEAN that assemble intoface-centered and body-centered cubic supramolecular crystals, respectively,upon increasing concentration or temperature. The transition fromdisordered to crystalline (ordered) structures manifests as a sol–gelrheological transition. A richer phase diagram is observed in theL121/dEAN system: due to its low solvophilicity in dEAN, L121 self-assemblesinto structures with low curvatures, namely, vesicles in the diluteregime, elongated (wormlike) micelles at intermediate concentrationsand low temperatures, a nematic phase in concentrated solutions, andmultilamellar phases at high temperatures. Highly viscoelastic solutionsare created by the self-assembly of entangled L121 wormlike micelles.Structural length scales of the three Pluronic/dEAN systems in thevarious mesophases are determined from SANS and USANS. [ABSTRACT FROM AUTHOR]

Abstract: Complex fluids with high concentrations of crystallizing fatty acids are important for consumer care products. The key features of these materials are their ability to support their weight under gravity and to yield or flow beyond a critical applied strain. To better understand the post-processing behavior of these systems, we have utilized a combination of Rheology, Differential Scanning Calorimetry, X-ray scattering, and Polarized Light Microscopy to study a model formulation consisting of two synthetic surfactants and a fatty acid (n-hexadecanoic acid) in water. Homogeneous samples with high storage modulus (G′) are realized due to the formation of a space filling solid fatty acid crystal network. A minimum of 5–7wt% of fatty acid is required to achieve the formation of a percolating crystal network. The storage modulus, yield stress and strain of these formulations show a non-monotonic increase with fatty acid content. This is attributed to the change in the fatty acid crystal habit from large to small aspect ratio plates. We hypothesize that the fatty acid crystal network consists of crystal aggregates linked by surfactant-fatty acid gels, rendering the desired paste-like characteristics to the formulation. The combination of methods utilized in this research provides a framework for understanding crystallizing systems. [Copyright &y& Elsevier]

Structure-property relationships of alkali-activated aluminosilicate gels are studied using rheology, neutron scattering, solid-state NMR, and TEM to develop a pseudo-ternary state diagram. Empirical gel states are classified by the rheological behavior (sol-gel transition and scaling of storage modulus with aluminum concentration). The state boundaries are compared to alkali-activated materials from the literature including denser binders. This work focuses on two compositions of dilute gels, formulated to specifically quantify the effects of pH and aluminum concentration on gel microstructure and rheology. Gels formulated at constant pH of 12.5 show a change in morphology and silicon coordination, but consistent mass fractal dimension as aluminum concentration is increased. This microstructural behavior contrasts the characterization of gels previously formulated with a variable pH (range 3–11) which is dependent on aluminum concentration. These results inform the complicated interactions in the polycondensation reaction of alkali-activated binders and lead to a state diagram which unifies many structure-property observations reported for these materials. Structure-property relationships for two formulations of N-A-S-H gels are compared and plotted in a pseudo-ternary state diagram which includes other compositions of N-A-S-H gels and alkali-activated binders from the literature, shaded according to the scaling of the storage modulus where applicable. [Display omitted] [ABSTRACT FROM AUTHOR]