We present an approach to fabricate solid capsules with precise control of size, permeability, mechanical strength, and compatibility. The capsules are fabricated by the self-assembly of colloidal particles onto the interface of emulsion droplets. After the particles are locked together to form elastic shells, the emulsion droplets are transferred to a fresh continuous-phase fluid that is the same as that inside the droplets. The resultant structures, which we call "colloidosomes," are hollow, elastic shells whose permeability and elasticity can be precisely controlled. The generality and robustness of these structures and their potential for cellular immunoisolation are demonstrated by the use of a variety of solvents, particles, and contents.Efficient encapsulation of active ingredients such as drugs, proteins, vitamins, flavors, gas bubbles, or even living cells is becoming increasingly important for a wide variety of applications and technologies, ranging from functional foods to drug delivery to biomedical applications (1-8). Increasingly sophisticated techniques are being developed to create physical structures that can meet the demanding requirements of these applications. A versatile technique should provide efficient encapsulation in structures whose size, permeability, mechanical strength, and compatibility can be easily controlled. Control of the size allows flexibility in applications and choice of encapsulated materials; control of the permeability allows selective and timed release; control of the mechanical strength allows the yield stress to be adjusted to withstand varying of mechanical loads and to enable release by defined shear rates; and control of compatibility allows encapsulation of fragile and sensitive ingredients, such as biomolecules and cells. Precise control of all these features would allow the strategic design of possible release mechanisms. Ideally, it should be feasible to construct these capsules from a wide variety of inorganic, organic, or polymeric materials to provide flexibility in their uses.A variety of techniques has been developed to address specific encapsulation requirements: Coacervation, or controlled gelation, of polymers at the surface of water drops can be used to fabricate nano-or microporous capsules (1-5, 9); other fluid extrusion methods can also be used to create the polymer coating (6, 7). Coating immiscible templates by electrostatic deposition of alternating layers of charged polymers or particles can be used to fabricate nanoporous capsules (10-18). Microfabrication technology can be used to create submillimeter-sized silicon capsules with exquisitely precise nanometer-scale holes for selective permeability and slow release (19). However, despite the enormous progress in encapsulation technologies, these methods can be limited in their applicability, in the range of materials that can be used, in the uniformity of pore sizes, in the accessible permeabilities and elasticities, or in the ease of synthesis, filling efficiency, and yield. We present a flexi...

The self-assembly of particles at fluid interfaces, driven by the reduction in interfacial energy, is well established. However, for nanoscopic particles, thermal fluctuations compete with interfacial energy and give rise to a particle-size-dependent self-assembly. Ligand-stabilized nanoparticles assembled into three-dimensional constructs at fluid-fluid interfaces, where the properties unique to the nanoparticles were preserved. The small size of the nanoparticles led to a weak confinement of the nanoparticles at the fluid interface that opens avenues to size-selective particle assembly, two-dimensional phase behavior, and functionalization. Fluid interfaces afford a rapid approach to equilibrium and easy access to nanoparticles for subsequent modification. A photoinduced transformation is described in which nanoparticles, initially soluble only in toluene, were transported across an interface into water and were dispersed in the water phase. The characteristic fluorescence emission of the nanoparticles provided a direct probe of their spatial distribution.

We describe experimental investigations of the structure of two-dimensional spherical crystals. The crystals, formed by beads self-assembled on water droplets in oil, serve as model systems for exploring very general theories about the minimum energy configurations of particles with arbitrary repulsive interactions on curved surfaces. Above a critical system size we find that crystals develop distinctive high-angle grain boundaries, or scars, not found in planar crystals. The number of excess defects in a scar is shown to grow linearly with the dimensionless system size. The observed slope is expected to be universal, independent of the microscopic potential.Spherical particles on a flat surface pack most efficiently in a simple lattice of triangles, similar to billiard balls at the start of a game. Such six-fold coordinated triangular lattices [1] cannot, however, be wrapped on the curved surface of a sphere; instead, there must be extra defects in coordination number. Soccer balls and C 60 fullerenes [2,3] provide familiar realizations of this fact -they have 12 pentagonal panels and 20 hexagonal panels. The necessary packing defects can be characterized by their topological or disclination charge, q, which is the departure of their coordination number c from the preferred flat space value of 6 (q = 6 − c); a classic theorem of Euler [4,5] shows that the total disclination charge of any triangulation of the sphere must be 12 [6]. A total disclination charge of 12 can be achieved in many ways, however, which makes the determination of the minimum energy configuration of repulsive particles, essential for crystallography on a sphere, an extremely difficult problem. This was recognized nearly 100 years ago by J.J. Thomson [7], who attempted, unsuccessfully, to explain the periodic table in terms of rigid electron shells. Similar problems recur in fields as diverse as multi-electron bubbles in superfluid helium [8], virus morphology [9, 10, 11], protein s-layers [12,13] and coding theory [14,15]. Indeed, both the classic Thomson problem, which deals with particles interacting through the Coulomb potential, and its generalization to other interaction potentials remain largely unsolved after almost 100 years [16,17,18].The spatial curvature encountered in curved geometries adds a fundamentally new ingredient to crystallography, not found in the study of order in spatially flat systems. To date, however, studies of the Thomson and related problems have been limited to theory and computer simulation. As the number of particles on the sphere grows, isolated charge 1 defects are predicted to induce too much strain; this can be relieved by introducing additional dislocations, consisting of pairs of tightly bound 5-7 defects [19] which still satisfy Euler's theorem since their net disclination charge is zero. Dislocations, which are point-like topological defects in two dimensions, disrupt the translational order of the crystalline phase but are less disruptive of orientational order [19]. While they play an essential rol...

Drying aqueous suspensions of monodisperse silica nanoparticles can fracture in remarkable patterns. As the material solidifies, evenly spaced cracks invade from the drying surface, with individual cracks undergoing intermittent motion. We show that the growth of cracks is limited by the advancement of the compaction front, which is governed by a balance of evaporation and flow of fluid at the drying surface. Surprisingly, the macroscopic dynamics of drying show signatures of molecular-scale fluid effects.

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