A simple and versatile method for making chemically patterned anisotropic colloidal particles is proposed and demonstrated for two different types of patterning. Using a combination of thermo/mechanical stretching followed by a wet chemical treatment of a sterically stabilized latex, both patchy ellipsoidal particles with sticky interactions near the tips as well as particles with tunable fluorescent patterns could be easily produced. The potential of such model colloidal particles is demonstrated, specifically for the case of directed self-assembly.

Current theoretical attempts to understand the reversible formation of stable microtubules and virus shells are generally based on shape-specific building blocks or monomers, where the local curvature of the resulting structure is explicitly built-in via the monomer geometry. Here we demonstrate that even simple ellipsoidal colloids can reversibly self-assemble into regular tubular structures when subjected to an alternating electric field. Supported by model calculations, we discuss the combined effects of anisotropic shape and field-induced dipolar interactions on the reversible formation of self-assembled structures. Our observations show that the formation of tubular structures through self-assembly requires much less geometrical and interaction specificity than previously thought, and advance our current understanding of the minimal requirements for self-assembly into regular virus-like structures.

We present a computer simulation study on the crystalline phases of hard ellipsoids of revolution. For aspect ratios ജ3 the previously suggested stretched-fcc phase ͓Frenkel and Mulder, Mol. Phys. 55, 1171 ͑1985͔͒ is replaced by a different crystalline phase. Its unit cell contains two ellipsoids with unequal orientations. The lattice is simple monoclinic. The angle of inclination of the lattice, ␤, is a very soft degree of freedom, while the two right angles are stiff. For one particular value of ␤, the close-packed version of this crystal is a specimen of the family of superdense packings recently reported ͓Donev et al., Phys. Rev. Lett. 92, 255506 ͑2004͔͒. These results are relevant for studies of nucleation and glassy dynamics of colloidal suspensions of ellipsoids. DOI: 10.1103/PhysRevE.75.020402 PACS number͑s͒: 82.70.Dd, 64.60.Cn, 61.50.Ah, 82.20.Wt Classical, hard particles such as nonoverlapping spheres, rods, or ellipsoids are widely used as models for granular matter, colloidal and molecular fluids, crystals, and glasses. Their success-and their appeal-lies in the fact that the problem of evaluating a many-body partition function is reduced to a slightly simpler, geometrical problem, namely, the evaluation of entropic contributions only. This is an advantage, in particular, for computer simulations. Hence one of the first applications of computer simulations was a study of the liquid-solid phase transition in hard spheres ͓1͔.In this Rapid Communication, we reexamine the highdensity phase behavior of hard ellipsoids of revolution with short aspect ratios. This system has been studied in Monte Carlo simulations by Frenkel and Mulder in 1985 ͓2͔. Since then, the focus of attention has been on the nematic phase and the isotropic-nematic transition ͓3-5͔. Biaxial hard ellipsoids have also been studied ͓6,7͔. But, to our knowledge, the high-density phases have not been investigated further. Knowledge of these phases is relevant for studies of elongated colloids in general, and it is crucial for the study of nucleation ͓8͔ and glassy dynamics ͓9͔ in hard ellipsoids.At high densities, Frenkel and Mulder assumed that the most stable phase was an orientationally ordered solid which can be constructed in the following way. A face-centered cubic ͑fcc͒ system of spheres is stretched by a factor x in an arbitrary direction n. This transformation results in a crystal structure of ellipsoids of aspect ratio x, which are oriented along n. As the transformation is linear, the density of closest packing is the same as for the closest packing of spheres = / ͱ 18Ϸ 0.7405. Recently, Donev and co-workers showed that ellipsoids can be packed more efficiently if non-latticeperiodic packings ͑i.e. packings in which a unit cell contains several ellipsoids at different orientations͒ are taken into account ͓10͔. For unit cells containing two particles, they constructed a family of packings which reach a density of = 0.770 732 for aspect ratios larger than ͱ 3.We have performed Monte Carlo simulations of hard ellipsoids of rev...