A revolution in novel nanoparticles and colloidal building blocks has been enabled by recent breakthroughs in particle synthesis. These new particles are poised to become the 'atoms' and 'molecules' of tomorrow's materials if they can be successfully assembled into useful structures. Here, we discuss the recent progress made in the synthesis of nanocrystals and colloidal particles and draw analogies between these new particulate building blocks and better-studied molecules and supramolecular objects. We argue for a conceptual framework for these new building blocks based on anisotropy attributes and discuss the prognosis for future progress in exploiting anisotropy for materials design and assembly.

The melt-state linear and nonlinear shear rheological properties of hybrid materials of polypropylene and amine-exchanged montmorillonite were studied. The materials were prepared by melt mixing with maleic anhydride functionalized polypropylene as the compatibilizer. The clay interlayer spacing (as determined by wide-angle X-ray scattering) increased upon melt mixing; however, the short-range ordering of the clay layers was preserved. Above inorganic loadings of 2.0 wt % the hybrid materials exhibited apparent low-frequency plateaus in the linear viscoelastic moduli. The hybrid storage modulus was sensitive to the chemistry of the amine exchanged into the clay. The amount of stress overshoot observed in flow reversal experiments was found to be a function of the rest time allowed between the reversal. The transient stress in start-up of steady shear scaled with the applied strain. These observations allow features of the polypropylene/montmorillonite hybrid structure to be deduced. The transient nonlinear rheology is consistent with an anisometric, non-Brownian structure. These anisometric particulate domains are mesoscopic, and internally, they contain multiple, ordered platelets. This mesoscopic structure is itself thermodynamically unstable, because the rheology indicates that quiescent structural evolution whose origin is not Brownian relaxation is observed. The demonstration of the sensitivity of melt-state rheological measurements to interparticle structure and chemistry of the hybrid materials indicates the potential usefulness of such studies for the development of new nanocomposite materials.

We review the sequences of structural states that can be induced in colloidal suspensions by the application of flow. Structure formation during flow is strongly affected by the delicate balance among interparticle forces, Brownian motion and hydrodynamic interactions. The resulting nonequilibrium microstructure is in turn a principal determinant of the suspension rheology. Colloidal suspensions with near hard-sphere interactions develop an anisotropic, amorphous structure at low dimensionless shear rates. At high rates, clustering due to strong hydrodynamic forces leads to shear thickening rheology. Application of steady-shear flow to suspensions with repulsive interactions induces a rich sequence of transitions to one-, two-and threedimensional order. Oscillatory-shear flow generates metastable ordering in suspensions with equilibrium liquid structure. On the other hand, shortrange attractive interactions can lead to a fluid-to-gel transition under quiescent suspensions. Application of flow leads to orientation, breakup, densification and spatial reorganization of aggregates. Using a non-Newtonian suspending medium leads to additional possibilities for organization. We examine the extent to which theory and simulation have yielded mechanistic understanding of the microstructural transitions that have been observed.

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