We have examined both the structure and surface chemistry of gold clusters formed by the kinetic aggregation of colloidal gold particles. The highly disordered, ramified aggregates can be very accurately described as self-similar or fractal objects with a fractal dimension equal to 1.75. Spectroscopic studies performed with surface-enhanced Raman scattering (SERS), clearly indicate that colloidal gold surfaces are highly heterogeneous, consisting both of donor and acceptor sites which can be identified as Au(0) and Au(I). respectively. Aggregation occurs when negatively charged species are displaced from the gold surface by more strongly bound molecular adsorhates, with the rate determined by the nature and concentration of the displacing species. The new insights afforded by the fractal description of the structure of the aggregates and the SERS probe of the chemical nature of the colloid surface should lead to a more complete understanding of the basic mechanisms of colloid aggregation. This potential is illustrated with a quantitative description of the dynamics of aggregate growth measured by dynamic light scattering.​​​​​​

We show that there are two regimes of irreversible, kinetic aggregation of aqueous colloids, determined by the short-range interparticle potential, through its control of the sticking probability upon approach of two particles. Each regime has different rate-limiting physics, aggregation dynamics, and cluster-mass distributions, and results in clusters with different fractal dimensions. These results set limits on the fractal dimension, df, for gold aggregates of 1.75<~df<~2.05(±0.05).

The large lattice spacings in colloidal crystals produce elastic constants ~ 10¹⁰ less than conventional solids. It is therefore easy to study flow properties at stress/elastic constant ratios higher than previously available. The highly nonlinear plastic flow regime studied in oscillating and steady state flow yields periodic patterns. These patterns correspond to alternating regions of ordered crystallites which are mirror image structures. The pattern observed in oscillatory flow in a tube is also unusual in that the core is liquid while at larger radius one finds a solid on the tube wall. This traditionally unstable configuration may be the result of an anomalous stress-rate relation at the fluid-solid boundary. Experiments in couette geometry produce vertical stripes which correspond to coherent motion of dislocations at the boundaries separating two mirror image structures. The stripes move in the direction opposite from the rotation of the inner cylinder, at velocities close to the transverse sound velocity.

Four-probe resistivity measurements on organic conductors have been extended to 6 GPa. The organic metal HMTSF-TCNO appears to undergo a phase transition to a three-dimen-sionaliy ordered conducting state near 4 GPa. X-ray and Raman scattering confirm the transition. Unexpectedly, the degree of charge transfer in HMTSF-TCNO is relatively insensitive to pressure.