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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]

Colloidal dispersions with a short-range attraction and long-range repulsion can exhibit an intriguing intermediate range order, manifested in scattering experiments as a low-q peak in the structure factor. Monte Carlo simulations are performed on fluids that exhibit intermediate range order to explicitly determine its connection to a possible state of microphase separation, equilibrium clustering. This is accomplished by decomposing the structure factor into cluster-cluster, monomer-monomer, and cross-correlations that cannot be extracted from experimental scattering patterns. Our simulation results indicate that the intermediate range order arises from either monomeric or cluster species, depending on solution conditions, and reflects the presence of a preferred length scale that is not trivially related to the interparticle potential. Further, criteria are established to define monomer, cluster, and percolated states in these systems that facilitate further studies. Combining scattering techniques with simulations provides an effective method for identifying clustered states in complex fluids. [ABSTRACT FROM AUTHOR]

We derive a statistical mechanical model for colloidal interactions mediated by polymer-like micelles (PLMs) that adsorb at the colloid surface. The model considers the end-adsorption and reversible scission of ideal chains, and is based on experimentally measurable parameters relevant to PLMs. The model predicts interparticle attractions due to micellar bridging that are stronger and longer-range than those encountered in ordinary telechelic polymers. Mapping the analytical potential onto the more familiar Double Yukawa potential allows, for the first time, accurate, a priori prediction of suspension microstructure and phase behavior when compared to experimental data for model nanoparticles dispersed in wormlike micelles over a range of solution conditions. [ABSTRACT FROM AUTHOR]

Understanding the mechanisms of clustering in colloids, nanoparticles, and proteins is of significant interest in material science and both chemical and pharmaceutical industries. Recently, using an integral equation theory formalism, Bomont et al. [J. Chem. Phys. 132, 184508 (2010)] studied theoretically the temperature dependence, at a fixed density, of the cluster formation in systems where particles interact with a hard-core double Yukawa potential composed of a short-range attraction and a long-range repulsion. In this paper, we provide evidence that the low-q peak in the static structure factor, frequently associated with the formation of clusters, is a common behavior in systems with competing interactions. In particular, we demonstrate that, based on a thermodynamic self-consistency criterion, accurate structural functions are obtained for different choices of closure relations. Moreover, we explore the dependence of the low-q peak on the particle number density, temperature, and potential parameters. Our findings indicate that enforcing thermodynamic self-consistency is the key factor to calculate both thermodynamic properties and static structure factors, including the low-q behavior, for colloidal dispersions with both attractive and repulsive interactions. Additionally, a simple analysis of the mean number of neighboring particles provides a qualitative description of some of the cluster features. [ABSTRACT FROM AUTHOR]

The shear induced microstructure for electrostatic and Brownian suspensions are compared using in situ small angle neutron scattering (SANS). The dispersions consist of 75 nm Stöber silica coated with 3-(trimethoxysilyl) propyl methacrylate (TPM) and have a zeta potential of -42.6 ± 4.7 mV. Neutralizing the surface charge with 0.066 M— nitric acid yields stable hard-sphere dispersions. SANS is conducted over a range of shear rates on the charge-stabilized and Brownian suspensions to test the order-disorder transition and hydrocluster mechanisms for shear thickening, and demonstrate the influence of stabilizing forces on the shear induced microstructure evolution. Through treatment of the colloidal micromechanics, shear induced changes in the microstructure are correlated to the hydrodynamic component of the shear stress and the thermodynamic component of the normal stress, i.e., the method of "Rheo-SANS" is developed. The results demonstrate that hydrocluster formation accompanies the shear thickening transition. [ABSTRACT FROM AUTHOR]

Studies the effects of colloidal interactions on the kinetics and extent of adsorption using a dynamic simulation of colloidal adsorption. Diffusion by Brownian dynamics accounted for by the simulation; Characterization of particle-particle and particle-surface interactions; Classes of charged colloidal particles considered.

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 two-particle correlation function is related to the structure factor by an integral transform and serves as an important connection between suspension microstructure, scattering experiments, and macroscopic properties. The nonequilibrium structure factor for a shearing colloidal suspension of spherical particles is expanded in spherical harmonics. Through this expansion, we demonstrate formal relationships between the experimentally accessible static structure factor, bulk stresses, and optical dichroism and birefringence, demonstrating that a limited number of expansion terms are required to characterize the suspension rheology. Low shear limiting theories are expressed in terms of this expansion, demonstrating the utility of the expansion for comparison of experimental measurements of nonequilibrium structure factors to theoretical predictions. Although flow birefringence and the thermodynamic shear stress are calculated as weighted integrals over the nonequilibrium microstructure in the low shear limit, no simple stress-optical coefficient is found. The expansion is proposed as a consistent methodology for analysis of experimental data and presentation of theoretical predictions, providing a means of interpreting rheo-optical data. [ABSTRACT FROM AUTHOR]

Structure measurements of a suspension of charged polystyrene spheres with significant polydispersity (16%) are reported for a wide range of particle and added salt concentrations and scattering wave vector. Small angle neutron scattering yielded the structure factors of concentrated samples that have had extensive rheological characterization. A Schulz distribution was used as a model of the particle size distribution to reduce the intensity data and as a basis for the theoretical fits of the structure factors. Static light scattering measurements yielded the structure factors of diluted, deionized samples. Dynamic light scattering was also performed to study the relaxation of these structures. These measured structures were fit with a recent theory that accounts for both size and charge polydispersity in the suspension. These results and the previous extensive, rheological investigations and physical characterization yield a complete data set on a well-characterized, model system that can be used to test theories of the statics and dynamics of polydisperse colloidal suspensions. [ABSTRACT FROM AUTHOR]

Optical measurements were used to detect structural anisotropy in concentrated dispersions over a range of Peclet numbers. Silica spheres of 49 and 130 nm radii with grafted octadecyl chains were dispersed in cyclohexane at volume fractions from 0.1 to 0.4. The apparatus consisted of a Couette cell with the dispersion in the annulus probed by a HeNe laser beam parallel to the axis of rotation. The dichroism and birefringence of the transmitted beam varied linearly with shear rate at low Peclet numbers with an orientation coincident with the principle direction of shear. Increasing the Peclet number, by increasing the particle size, produced a nonlinear response with the orientation tending to align in the direction of flow. A theory coupling the nonequilibrium microstructure under shear to the optical properties of the suspension enables direct interpretation of the dichroism. Comparison of sample dichroism calculations for two different forms of the theory demonstrates that the optical technique can be used to discriminate between theories which predict the microstructure. [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]