We study the random packing of non-spherical particles by computer simulation to investigate the effect of particle shape and aspect ratio on packing density and microstructure. Packings of cut spheres (a spherical segment which is symmetric about the centre of the sphere) are simulated to assess the influence of a planar face on packing properties. It turns out that cut spheres, in common with spherocylinders and spheroids, pack more efficiently as the particle's aspect ratio is perturbed slightly from unity (the aspect ratio of a sphere) to reach a maximum density at an aspect ratio of approximately 1.25. Upon increasing the aspect ratio further the cut spheres pack less efficiently, until approximately an aspect ratio of 2, where the particles are found to form a columnar phase. The amount of ordering is sensitive to simulation parameters and for very thin disks the formation of long columns becomes frustrated, resulting in a nematic phase, in marked contrast to the behavior of long thin rods which always randomly pack into entangled isotropic networks. With respect to coordination numbers it appears that cut spheres always pack with significantly fewer contacts than required for isostatic packing.

Random packings of non-spherical granular particles are simulated by combining mechanical contraction and molecular dynamics, to determine contact numbers as a function of density. Particle shapes are varied from spheres to thin rods. The observed contact numbers (and packing densities) agree well with experiments on granular packings. Contact numbers are also compared to caging numbers calculated for sphero-cylinders with arbitrary aspect-ratio. The caging number for rods arrested by uncorrelated point contacts asymptotes towards γ = 9 at high aspect ratio, strikingly close to the experimental contact number C ≈ 9.8 for thin rods. These and other findings confirm that thin-rod packings are dominated by local arrest in the form of truly random neighbor cages. The ideal packing law derived for random rod-rod contacts, supplemented with a calculation for the average contact number, explains both absolute value and aspect-ratio dependence of the packing density of randomly oriented thin rods.

We have measured the random packing density of monodisperse colloidal silica ellipsoids with a well-defined shape, gradually deviating from a sphere shape up to prolates with aspect ratios of about 5, to find for a colloidal system the first experimental observation for the density maximum (at an aspect ratio near 1.6) previously found only in computer simulations of granular packings. Confocal microscopy of ellipsoid packings, prepared by rapidly quenching ellipsoid fluids via ultra-centrifugation, demonstrates the absence of orientational order and yields pair correlation functions very much like those for random sphere packings. The density maximum, about 12% above the Bernal random sphere packing density, also manifests itself as a maximum in the hydrodynamic friction that resists the swelling osmotic pressure of the ellipsoid packings. The existence of the density maximum is also predicted to strongly effect the dynamics of colloidal non-sphere glasses: slightly perturbing the sphere shape in a sphere glass will cause it to melt.

focal point of a germanium lens which directed thermal energy from a heat source 30 cm away onto the circuit board while also filtering out extraneous visible light. A multimeter (Keithley, model 2700) was used to record the changes in resistance of the films.Film percentages by weight: PVA (60.0 %), PEG (24.3 %), XC72 (10.0 %), TlpA8 (4.3 %), and X-100 (1.4 %). Materials with a periodic modulation of refractive index on the (sub)-micrometer scale interact strongly with light and can exhibit a photonic bandgap, the optical analogue of the electronic bandgap in semiconductors.[1] Colloidal suspensions of monodisperse micro-spheres that self-organize, analogously to atomic crystals, into periodic structures with the lowest free energy, are promising as three-dimensional photonic materials.[2±6] However, growing large single-domain colloidal crystals without an overlying fluid layer is difficult. We present a technique to grow millimeter-scale (3 mm 0.5 mm) electricfield-induced colloidal single crystals, and a polymerization process that immobilizes them, allowing drying and reversal of the refractive-index contrast. A 70 V mm ±1 (rms) electric field switches the crystal structure from close-packed to bodycentered tetragonal (bct). Lower values increase the area of single-domain close-packed crystals and the preference for face-centered cubic (fcc) packing over hexagonal closepacked (hcp). Intermediate fields produced mixed crystals with a lower fcc layer and a connected upper bct layer. Most photonic applications require periodic structures with a low filling fraction of the high dielectric-constant component. This can be achieved by preparing wet colloidal crystals with a high particle volume fraction and ªinverting the contrastº by drying the crystal, re-infiltrating it with a high-index material, and ultimately removing the solid spheres by etching or burning.[2] High-volume fraction colloidal crystals can be made by allowing colloids in suspension to sediment in gravity and densify. When colloids interact with each other as hard spheres or as slightly charged spheres (with a hard core plus a repulsive inter-particle interaction) the resulting equilibrium structure is an fcc crystal. However, for hard spheres, the COMMUNICATIONS 596