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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]

Understanding the interfacial structure–property relationship of complex fluid–fluid interfaces is increasingly important for guiding the formulation of systems with targeted interfacial properties, such as those found in multiphase complex fluids, biological systems, biopharmaceuticals formulations, and many consumer products. Mixed interfacial flow fields, typical of classical Langmuir trough experiments, introduce a complex interfacial flow history that complicates the study of interfacial properties of complex fluid interfaces. In this article, we describe the design, implementation, and validation of a new instrument capable of independent application of controlled interfacial dilation and shear kinematics on fluid interfaces. Combining the Quadrotrough with both in situ Brewster angle microscopy and neutron reflectometry provides detailed structural measurements of the interface at the mesoscale and nanoscale in relationship to interfacial material properties under controlled interfacial deformation histories. [ABSTRACT FROM AUTHOR]

Light scattering measurements determined the ratio of the nonequilibrium to equilibrium static structure factor for suspensions of ‘‘hard spheres’’ undergoing shear flow as a function of volume fraction of the suspended particles, the shear rate, and a limited number of scattering angles. These data are compared with existing theoretical predictions for the static structure factor ratio in the limit of small shear flow rates. We conclude that the existing theories, some which are considered exact at low volume fractions, give increasingly poor estimates of the true structure factor at small scattering angles as the volume fraction increases. [ABSTRACT FROM AUTHOR]

The rheological characterization of a model, nearly ideal thixotropic carbon black suspension first proposed by Dullaert and Mewis (JNNFM 139: 21-30, 2006) is extended to include large amplitude oscillatory shear (LAOS) flow, shear flow reversal, and unidirectional LAOS flow (UD-LAOS). We show how this broader data set is useful for validating and improving constitutive models of thixotropy. We apply this new data to further test a recently developed structure-kinetics model, the Modified Delaware Thixotropy Model (Armstrong et al., J Rheol. 60: 433-450, 2016a), as well as to better understand the microstructural basis and validity of strain-rate superposition methods proposed for soft solids. This comparison identifies the limitations of models based on a scalar description of microstructure, which cannot fully capture the reversal of flow directionality inherent in LAOS flow. Further, we use the model to identify a possible microstructural justification for the hypothesized technique of strain-rate frequency superposition in soft solids. [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]

Shear banding under steady and dynamic deformation is examined using a combination of rheology and flow-small angle neutron scattering (SANS) in a model wormlike micelle solution comprised of a mixed cationic/anionic surfactant (cetyltrimethylammonium tosylate/sodium dodecyl benzene sulfonate) and sodium tosylate. Flow-vorticity plane rheo-SANS reveals long transients during shear band formation. Flowgradient plane spatially resolved flow-SANS measurements probe the microstructure during steady and dynamic shear banding. Under large amplitude oscillatory shear (LAOS) deformation, shear banding is dependent on the Deborah and Weissenberg numbers, validating recent theoretical predictions that include shear-induced breakage. Micelle segmental alignment in the flow-gradient plane during LAOS is a nonmonotonic function of cycle time, t/T, and radial position, r/H. The maximum segmental alignment under LAOS often exceeds that of the corresponding shear rate under steady shear, termed "over-orientation," which can help identify LAOS shear banding. The results of this study present new methods for identifying shear banding under steady and dynamic deformation, while providing an extensive data set for the development and further improvement of constitutive models. [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]

We study the medium amplitude oscillatory shear (MAOS) of a semi-dilute, hard-sphere colloidal dispersion. Through solution of the Smoluchowski equation governing the statistical distribution of suspended particles in the semi-dilute limit, we calculate the linear viscoelasticity and the normal stress differences that arise from oscillatory shear as an expansion in terms of small rates of deformation. The effects of hydrodynamic versus conservative interactions are studied via the excluded-annulus model for the hard-sphere interactions. This treats the hardsphere interactions as a steric barrier that resides beyond the hydrodynamic radius of the particles and allows for continuous variation of the strength of hydrodynamic interactions. We see remarkable distinctions between suspensions that are freely draining and those for which hydrodynamic lubrication among the particles dominate their motion. In oscillatory shear flow of a freely draining suspension at high-frequency, the normal stress differences are generated by hard-sphere stresses. In contrast, for a hydrodynamically interacting suspension at highfrequency, the first normal stress difference derives from Brownian stresses while the second normal stress difference is dominated by hydrodynamic stresses. When the first normal stress difference is plotted parametrically against the oscillatory rate of strain, the resulting Lissajous- Bowditch curves for freely draining suspensions and those with full hydrodynamic interactions take on distinct and disparate shapes. Additionally, we use an asymptotic analysis to predict that the third harmonic of the suspension stress for hard spheres is dominated by the hydrodynamic lubrication in the limit of high frequency oscillation. Our calculations demonstrate that under MAOS, hydrodynamic interactions play a central and qualitative role in the stress response of a semi-dilute colloidal dispersion. [ABSTRACT FROM AUTHOR]

Prior work has demonstrated that colloidal suspensions can be ordered with large amplitude oscillatory shear (LAOS), showing that the final type and degree of ordering depends on the frequency and amplitude of the applied oscillation. Here, we present LAOS results for the kinetics of ordering for colloidal particles (d=1 μm) at [lowercase_phi_synonym]=0.54 from a shear disordered state over a range of frequencies and shear amplitudes. A custom rheolight scattering (rheo-LS) device is used to quantify the type and degree of crystalline order observed either as a six spot Bragg pattern or a three streak twinning pattern. The dynamic shear stress and the alignment factor derived from the rheo-LS show the same time-dependent behavior indicative of a primary relaxation that is exponential in time. The rate of ordering to the r-hcp state is found to scale with the amplitude of oscillation and inversely with the frequency over strains from 100% to 300% and frequencies from 0.5 to 5 Hz. These results show that the rate of LAOS ordering does not simply reflect the rate of deformation, but rather a more complex mechanism pointing toward competition between shear ordering and shear-induced defect generation. [ABSTRACT FROM AUTHOR]

The device described measures a two-dimensional projection of the light scattered from a sheared, optically clear colloidal suspension and decomposes the pattern to identify quantitatively the effects of shear on the structure of the dispersion. The evolution of the nonequilibrium structure for a hard-sphere suspension is traced from low to high shear rates at moderate to dense concentrations and is related qualitatively to rheological properties. At high shear rates shear induces order in dense suspensions. Comparison of the measured nonequilibrium structure factor at low shear rates to theoretical predictions based on pairwise additive interactions confirms the importance of many-body hydrodynamics, as suggested by similar discrepancies between measurements of the steady shear viscosity and predictions from the same theory. Scalings that correlate the shear rate dependence of the viscosity appear to work for the microstructure as well. Measurements of a similar suspension with weak long-range repulsions demonstrates the sensitivity of the nonequilibrium microstructure to the form of the interparticle potential. [ABSTRACT FROM AUTHOR]

• Application of a multiscale, population balance‐based thixotropy model to blood. • More physical description of the coin‐stack like rouleaux aggregate structures. • Meaningful microscopic structural information obtained by fitting bulk rheology. Modeling blood rheology remains challenging in part because of its multiphase, aggregating colloidal nature that gives rise to complex viscoplastic and time-dependent (thixotropic) behavior. Here, we demonstrate that a multiscale approach incorporating a direct coupling of coarse-graining particle-level modeling to the macroscopic phenomenological modeling can provide new insights and a promising methodology. Specifically, a general population balance-based, multiscale, thixotropic modeling approach, first proposed by Mwasame et al., AIChE J. 63 (2017) 517–531, is applied to account for the rouleaux-induced thixotropy in human blood in shear flow. Population balances offer a compelling alternative to previously proposed structure-based heuristic kinetics models of aggregating colloidal suspensions as they use a statistical approach to describe the aggregate size distribution with well-defined processes for either shear-induced or Brownian aggregation and breakup under shear flow. When applied to human blood, the population balance approach offers a first attempt to model the size evolution of predominantly coin-stack like rouleaux structures of the red blood cells that are the primary source behind the observed yield stress and thixotropy of blood at low shear rates. This microscopic information, suitably coarse-grained, is then introduced into a semi-phenomenological macroscopic model that expresses the total stress in terms of an elastic and viscous contribution. Shear thinning introduced due to the red blood cell deformation at high shear rates is accounted for by following Horner et al., J. Rheol. 62 (2018) 577–591. An advantage of this modeling approach is that the parameters have specific physical meaning that allows for independent estimates and/or evaluations through appropriately designed independent experiments. Conversely, parameters with specific microscopic interpretations, such as the fractal dimension of the aggregates, d f , are obtained fits of macroscopic shear experiments. Fitting and predictions use steady shear, and unidirectional large amplitude oscillatory shear (UD-LAOS) experiments on whole blood samples of two healthy donors, as reported in Horner et al. We obtain values for d f in the range of 1.5 ± 0.2 which is consistent with the rod-like shape of rouleaux structures reported in the literature. Furthermore, the shear predictions compare favorably against the experiments. While this approach is not as accurate as the fits of prior structure kinetics modeling of Horner et al., these promising results provide a pathway for model improvement by including independently verified physical properties of blood. This work demonstrates a new particle-level approach for describing and predicting the non-Newtonian, thixotropic rheology of human blood. [ABSTRACT FROM AUTHOR]

The low-earth orbit environment contains small micrometeoroid and orbital debris (MMOD) particles traveling at characteristic velocities of several kilometers per second. In addition to being a direct threat to astronauts and spacecraft, upon impact with the exterior surface of a space vehicle, these highly energetic MMOD particles can create cut and puncture hazards for astronauts performing extra-vehicular activities (EVA). In this work, we demonstrate that replacing the standard neoprene-coated nylon absorber layers with woven aramid textiles intercalated with colloidal shear thickening fluids, i.e., STF-Armor™, can provide a meaningful enhancement to the cut and puncture resistance of the thermal micrometeoroid garment (TMG). Quasi-static puncture testing is performed using hypodermic needles of varying gauge to simulate the cutting and puncture hazards at deformation rates characteristic of human motion. At equal areal densities, we find that a TMG lay-up containing STF-Armor™ greatly improves puncture protection with a reduction in weight and comparable flexibility. [ABSTRACT FROM AUTHOR]

The formation of multilamellar vesicles(MLVs) from the lamellar phase of nonionic surfactant system C12E5/D2O under shear flow is studiedby time-resolved small angle neutron and light scattering during shearflow. A novel small angle neutron scattering sample environment enablesthe tracking of the lamellae alignment in the velocity–velocitygradient (1–2) plane during MLV formation, which was trackedindependently using flow small angle light scattering commensuratewith rheology. During the lamellar-to-multilamellar vesicle transition,the primary Bragg peak from the lamellar ordering was observed totilt, and this gradually increased with time, leading to an anisotropicpattern with a primary axis oriented at ∼25° relativeto the flow direction. This distorted pattern persists under flowafter MLV formation. A critical strain and critical capillary numberbased on the MLV viscosity are demonstrated for MLV formation, whichis shown to be robust for other systems as well. These novel measurementsprovide fundamentally new information about the flow orientation oflamellae in the plane of flow that cannot be anticipated from thelarge body of previous literature showing nearly isotropic orientationin the 2,3 and 1,3 planes of flow. These observations are consistentwith models for buckling-induced MLV formation but suggest that theinstability is three-dimensional, thereby identifying the mechanismof MLV formation in simple shear flow. [ABSTRACT FROM AUTHOR]

We present experiments and simulations that show a fundamental scaling for both the rheology and microstructure of flowing gels. Unique flow-SANS measurements demonstrate that the structure orients along both the neutral and compression axis. We quantify the anisotropy using a single parameter, α, that scales by a dimensionless number, M', that arises from a force balance on a particle. Simulations support the scalings and confirm the results are independent of the shape and range of the potential suggesting a universal for colloidal gels with short-ranged attractions. [ABSTRACT FROM AUTHOR]

Shear-induced structural transitions of a micellar cubic phase during large amplitude oscillatory shear flow is studied with time-resolved oscillatory rheological small angle neutron scattering. This technique allows us to resolve the structural changes within a cycle of oscillation. By applying a strain rate near the critical melting shear rate, melting and recrystallization occurs in a cyclic mode. The maximum degree of order is observed when the shear stress reaches a plateau value during the large amplitude oscillatory shear cycle, whereas melting is maximized at the strain rate wave peaks. This structural evolution confirms the cyclic mechanism of sticking and sliding of 2D hexagonal close-packed layers [I. W. Hamley et al, Phys. Rev. E 58, 7620 (1998)]. [ABSTRACT FROM AUTHOR]