Within the field of nanotechnology, nanoparticles are one of the most prominent and promising candidates for technological applications. Self-assembly of nanoparticles has been identified as an important process where the building blocks spontaneously organize into ordered structures by thermodynamic and other constraints. However, in order to successfully exploit nanoparticle self-assembly in technological applications and to ensure efficient scale-up, a high level of direction and control is required. The present review critically investigates to what extent self-assembly can be directed, enhanced, or controlled by either changing the energy or entropy landscapes, using templates or applying external fields.

Directed Self-Assembly of Nanoparticles

The ability to assemble nanoscopic components into larger structures and materials depends crucially on the ability to understand in quantitative detail and subsequently "engineer" the interparticle interactions. This Review provides a critical examination of the various interparticle forces (van der Waals, electrostatic, magnetic, molecular, and entropic) that can be used in nanoscale self-assembly. For each type of interaction, the magnitude and the length scale are discussed, as well as the scaling with particle size and interparticle distance. In all cases, the discussion emphasizes characteristics unique to the nanoscale. These theoretical considerations are accompanied by examples of recent experimental systems, in which specific interaction types were used to drive nanoscopic self-assembly. Overall, this Review aims to provide a comprehensive yet easily accessible resource of nanoscale-specific interparticle forces that can be implemented in models or simulations of self-assembly processes at this scale.

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Nanoscale Forces and Their Uses in Self‐Assembly

Colloidal processing of ceramics is reviewed with an emphasis on interparticle forces, suspension rheology, consolidation techniques, and drying behavior. Particular attention is given to the scientific concepts that underpin the fabrication of particulate-derived ceramic components. The complex interplay between suspension stability and its structural evolution during colloidal processing is highlighted.

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Colloidal Processing of Ceramics

http://static.sif.it/SIF/resources/public/files/va2012/likos_0710.pdf

Effective interactions in soft condensed matter physics

Nanoemulsions are kinetically stable liquid-in-liquid dispersions with droplet sizes on the order of 100 nm. Their small size leads to useful properties such as high surface area per unit volume, robust stability, optically transparent appearance, and tunable rheology. Nanoemulsions are finding application in diverse areas such as drug delivery, food, cosmetics, pharmaceuticals, and material synthesis. Additionally, they serve as model systems to understand nanoscale colloidal dispersions. High and low energy methods are used to prepare nanoemulsions, including high pressure homogenization, ultrasonication, phase inversion temperature and emulsion inversion point, as well as recently developed approaches such as bubble bursting method. In this review article, we summarize the major methods to prepare nanoemulsions, theories to predict droplet size, physical conditions and chemical additives which affect droplet stability, and recent applications.

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Nanoemulsions: formation, properties and applications

1. Introduction to colloid science and rheology 2. Hydrodynamic effects 3. Brownian hard spheres 4. Stable colloidal suspensions 5. Non-spherical particles 6. Weakly flocculated suspensions 7. Thixotropy 8. Shear thickening 9. Rheometry of suspensions 10. Suspensions in viscoelastic media 11. Advanced topics.

Colloidal Suspension Rheology

The yield stress of many yield stress fluids has turned out to be difficult to determine experimentally. This has led to various discussions in the literature about those experimental difficulties, and the usefulness and pertinence of the concept of yield stress fluids. We argue here that most of the difficulties disappear when taking the thixotropy of yield stress fluids into account, and will demonstrate an experimental protocol that allows reproducible data to be obtained for the critical stress necessary for flow of these fluids. As a bonus, we will show that the interplay of yield stress and thixotropy allows one to account for the ubiquitous shear localization observed in these materials. However, due to the thixotropy the yield stress is no longer a material property, since it depends on the (shear) history of the sample.

Yield stress and thixotropy: on the difficulty of measuring yield stresses in practice

Yield stress and thixotropy : on the difficulty of measuring yield stresses in practice

The non-Newtonian viscosities of polymerically stabilized colloidal suspensions are usually predicted and correlated on the basis of data and scaling principles for Brownian hard spheres. Here, the specific effect of the stabilizer layer is investigated using suspensions of monodisperse PMMA particles with a chemically attached stabilizer layer. The ratio between particle radius and stabilizer layer thickness is changed between 5 and 61. At high values of this ratio the data show Brownian hard sphere behavior. At lower values deviations appear. As a first approximation, the “softness” of the particles can be characterized through the concentration at maximum packing. A more detailed comparison with hard sphere data provides a measure for softness that changes with concentration and shear rate. A theoretical estimate of the concentration effect is in line with the experiments. The critical shear stress (or Peclet number) is not a constant but goes through a maximum when the concentration is increased.

The rheology of suspensions containing polymerically stabilized particles

Thixotropic systems are usually characterized by measuring the stress transients resulting from sudden changes in shear rate. By using a procedure that takes into account the transfer function of the transducer on a strain-controlled rheometer, the initial part of the transients could be accessed. In this manner complete transient stress curves and accurate constant-structure curves could be recorded on a model system consisting of fumed silica particles dispersed in a Newtonian medium. In agreement with previously obtained stress jump data [Dullaert and Mewis (2005b)], an initial viscoelastic response is detected that precedes the structural time effects in either build-up or breakdown experiments. The effect of flow history on the characteristic times and on the shape of the transient curves is investigated. It is shown that common assumptions in thixotropic models, such as a structure that can be characterized by a single parameter or the existence of exponential stress transients, do not generally hold. Although the material response is quite complex, the time constants of the stress transients obey a simple scaling law. The present data set is very suitable for model evaluation. Here, two representative models, those of Houska and of Coussot, have been tested. The initial parts of the curves are not well described. The time constants for the thixotropic structure build-up are reasonably well predicted, although shear history effects are not properly modeled. For the breakdown curves larger errors are recorded.

Jan Mewis

Papers

Colloidal Suspension Rheology

Yield stress and thixotropy: on the difficulty of measuring yield stresses in practice

Yield stress and thixotropy : on the difficulty of measuring yield stresses in practice

The rheology of suspensions containing polymerically stabilized particles

Thixotropy : Build-up and breakdown curves during flow