Assembly of colloidal particles on fluid interfaces is a promising technique for synthesizing two-dimensional microcrystalline materials useful in fields as diverse as biomedicine, materials science, mineral flotation and food processing. Current approaches rely on bulk emulsification methods, require further chemical and thermal treatments, and are restrictive with respect to the materials used. The development of methods that exploit the great potential of interfacial assembly for producing tailored materials have been hampered by the lack of understanding of the assembly process. Here we report a microfluidic method that allows direct visualization and understanding of the dynamics of colloidal crystal growth on curved interfaces. The crystals are periodically ejected to form stable jammed shells, which we refer to as colloidal armour. We propose that the energetic barriers to interfacial crystal growth and organization can be overcome by targeted delivery of colloidal particles through hydrodynamic flows. Our method allows an unprecedented degree of control over armour composition, size and stability.

Surface tension gives gas bubbles their perfect spherical shape by minimizing the surface area for a given volume. Here we show that gas bubbles and liquid drops can exist in stable, non-spherical shapes if the surface is covered, or 'armoured', with a close-packed monolayer of particles. When two spherical armoured bubbles are fused, jamming of the particles on the interface supports the unequal stresses that are necessary to stabilize a non-spherical shape.

This paper describes the fabrication and properties of "fluoroalkylated paper" ("R F paper") by vapor-phase silanization of paper with fluoroalkyl trichlorosilanes. R F paper is both hydrophobic and oleophobic: it repels water (θ app H 2 O >140°), organic liquids with surface tensions as low as 28 mN/m, aqueous solutions containing ionic and non-ionic surfactants, and complex liquids such as blood (which contains salts, surfactants, and biological material such as cells, proteins, and lipids). The propensity of the paper to resist wetting by liquids with a wide range of surface tensions correlates (with a few exceptions) with the length and degree of fluorination of the organosilane, and with the roughness of the paper. R F paper 2 maintains the high permeability to gases, and the mechanical flexibility of the untreated paper, and can be folded into functional shapes (e.g. microtiter plates and liquid-filled gas sensors).When impregnated with a perfluorinated oil, R F paper forms a "slippery" surface (paper slippery liquid-infused porous surface, or "paper SLIPS") capable of repelling liquids with surface tensions as low as 15 mN/m. The foldability of the paper SLIPS allows the fabrication of channels and flow switches to guide the transport of liquid droplets.

Experiments show that bubbles covered with monodisperse polystyrene particles, with particle to bubble radius ratios of about 0.1, evolve to form faceted polyhedral shapes that are stable to dissolution in air-saturated water. We perform Surface Evolver simulations and find that the faceted particle-covered bubble represents a local minimum of energy. At the faceted state, the Laplace overpressure vanishes, which together with the positive slope of the bubble pressure-volume curve, ensures phase stability. The repulsive interactions between the particles cause a reduction of the curvature of the gas-liquid interface, which is the mechanism that arrests dissolution and stabilizes the bubbles.

This paper describes the fabrication of pressure-driven, open-channel microfluidic systems with lateral dimensions of 45-300 microns carved in omniphobic paper using a craft-cutting tool. Vapor phase silanization with a fluorinated alkyltrichlorosilane renders paper omniphobic, but preserves its high gas permeability and mechanical properties. When sealed with tape, the carved channels form conduits 10 capable of guiding liquid transport in the low-Reynolds number regime (i.e. laminar flow). These devices are compatible with complex fluids such as droplets of water in oil. The combination of omniphobic paper and a craft cutter enables the development of new types of valves and switches, such as "fold" valves and "porous switches," which provide new methods to control fluid flow.

We report the behavior of particle-stabilized bubbles (armored bubbles) when exposed to various classes and concentrations of surfactants. The bubbles are nonspherical, which is a signature of the jamming of the particles on the interface, and are stable to dissolution prior to the addition of surfactant. Armored bubbles exposed to surfactants, dissolve, and exhibit distinct morphological, microstructural, and lifetime changes, which correlate with the concentration of surfactant employed. For low concentrations of surfactant, an armored bubble remains nonspherical while dissolving, whereas for concentrations close to and above the surfactant cmc a bubble reverts to a spherical shape before dissolving. We propose a microstructural interpretation, supported by our experimental observations of particle dynamics on the bubble interface, that recognizes the role of interfacial jamming and stresses in particle-stabilization and surfactant-mediated destabilization of armored bubbles.

Recent experiments and simulations have demonstrated that particle-covered fluid/fluid interfaces can exist in stable nonspherical shapes as a result of the steric jamming of the interfacially trapped particles. The jamming confers the interface with solidlike properties. We provide an experimental and theoretical characterization of the mechanical properties of these armored objects, with attention given to the two-dimensional granular state of the interface. Small inhomogeneous stresses produce a plastic response, while homogeneous stresses produce a weak elastic response. Shear-driven particle-scale rearrangements explain the basic threshold needed to obtain the near-perfect plastic deformation that is observed. Furthermore, the inhomogeneous stress state of the interface is exhibited experimentally by using surfactants to destabilize the particles on the surface. Since the interfacially trapped particles retain their individual characteristics, armored interfaces can be recognized as a kind of composite material with distinct chemical, structural, and mechanical properties.

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