We review the diverse range of materials made up of rod-shaped colloids. A common feature of such suspensions is the strong and efficient contribution of rods to the material's solid-like rheological properties such as elastic modulus and yield stress. Colloidal rod suspensions span from biomaterials such as f-actin and fd virus to inorganic materials such as boehmite and hematite, and to commercial fibers such as cellulose. We argue that, depending on the strength of pair potential interactions, such rod suspensions form microstructures that vary between the two limits of heterogeneous fractal clusters and homogeneous fiber networks. The volume fraction range for transition between these two limiting cases is strongly aspect ratio dependent. The two limiting microstructures can be distinguished by differences in the scattering vector dependence of their structure factors, as long as the range of scattering vector probed is sufficient to span regimes both above and below qL ≈ 1. Here q is the scattering vector and L is the rod length. Theories of the Brownian dynamics of fractal clusters and fiber networks show that the two types of microstructure can lead to the arrested dynamics of gelation and the glass transition, respectively. The volume fraction and aspect ratio dependences of the dynamical slowing down of these two cases differ significantly; we suggest that probing these differences is a convenient way to distinguish between gels and glasses of colloidal rods. This distinction is important to clarify because these microstructures are determinants of rheological properties such as elasticity and yielding. By combining these structural and dynamical ideas about rods, we classify a set of literature measurements on more than fifteen different colloidal materials and thereby distinguish between regimes of gelation and vitrification. We conclude by suggesting directions for future research in the arrested dynamics, the non-linear rheology, and the absolute lower limit of gelation in colloidal rod suspensions.

Microstructural regimes of colloidal rod suspensions, gels, and glasses

A metal nanoparticle composition for the fabrication of conductive features. The metal nanoparticle composition advantageously has a low viscosity permitting deposition of the composition by direct-write tools. The metal nanoparticle composition advantageously also has a low conversion temperature, permitting its deposition and conversion to an electrical feature on polymeric substrates.

Metal nanoparticle compositions

Processes for the production of metal nanoparticles. In one aspect, the invention is to a process comprising the steps of mixing a heated first solution comprising a base and/or a reducing agent (e.g., a non-polyol reducing agent), a polyol, and a polymer of vinyl pyrrolidone with a second solution comprising a metal precursor that is capable of being reduced to a metal by the polyol. In another aspect, the invention is to a process that includes the steps of heating a powder of a polymer of vinyl pyrrolidone; forming a first solution comprising the powder and a polyol; and mixing the first solution with a second solution comprising a metal precursor capable of being reduced to a metal by the polyol.

Production of metal nanoparticles

Experimental observations of the effects of shear rates and particle concentration on the viscosity of Fe2O3–deionized water nanofluids

We report results on the rheology of a model polymer/clay system, laponite clay particles with added poly(ethylene oxide) (PEO), focusing on the kinetics of gel formation and on molecular weight effects. We examined solutions at both pH = 7, where interparticle attractions are present and a networked gel is formed; and at pH = 10, where repulsive forces dominate and laponite forms a colloidal glass. We found that PEO of low to moderate molecular weight significantly slows down gelation and decreases the complex viscosity and elastic modulus of the dispersion by several orders of magnitude for both pH = 7 and 10. In the former case, adsorbed PEO likely forms a steric barrier to the formation of an attractive gel. In the latter case, we propose that free PEO chains in solution induce a depletion attraction between particles, preventing or slowing the formation of a colloidal glass. At higher molecular weights, PEO chains are long enough to bridge between particles and form an associative network, enhancing the viscosity and elastic modulus for both pH = 7 and 10. Finally, we have shown the critical molecular weight for transitioning between these two types of behavior scales with the interparticle distance. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 233–240, 2005

T.M. Kwon

Papers

Viscosity of magnetic particle suspension