A challenging goal in materials chemistry and physics is spontaneously to form intended superstructures from designed building blocks. In fields such as crystal engineering and the design of porous materials, this typically involves building blocks of organic molecules, sometimes operating together with metallic ions or clusters. The translation of such ideas to nanoparticles and colloidal-sized building blocks would potentially open doors to new materials and new properties, but the pathways to achieve this goal are still undetermined. Here we show how colloidal spheres can be induced to self-assemble into a complex predetermined colloidal crystal-in this case a colloidal kagome lattice-through decoration of their surfaces with a simple pattern of hydrophobic domains. The building blocks are simple micrometre-sized spheres with interactions (electrostatic repulsion in the middle, hydrophobic attraction at the poles, which we call 'triblock Janus') that are also simple, but the self-assembly of the spheres into an open kagome structure contrasts with previously known close-packed periodic arrangements of spheres. This open network is of interest for several theoretical reasons. With a view to possible enhanced functionality, the resulting lattice structure possesses two families of pores, one that is hydrophobic on the rims of the pores and another that is hydrophilic. This strategy of 'convergent' self-assembly from easily fabricated colloidal building blocks encodes the target supracolloidal architecture, not in localized attractive spots but instead in large redundantly attractive regions, and can be extended to form other supracolloidal networks.

When a liquid is confined in a narrow gap (as near a cell membrane, in a lubricated contact between solids, or in a porous medium), new dynamic behavior emerges. The effective shear viscosity is enhanced compared to the bulk, relaxation times are prolonged, and nonlinear responses set in at lower shear rates. These effects are more prominent, the thinner the liquid film. They appear to be the manifestation of collective motions. The flow of liquids under extreme confinement cannot be understood simply by intuitive extrapolation of bulk properties. Practical consequences are possible in areas from tribology and materials processing to membrane physics.

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Robust multilayers can be formed on solid surfaces, and subsequently destroyed by changing the environmental conditions, by the layer-by-layer sequential assembly of monomolecular films of a polyacid and polybase from aqueous solution. Interlayer hydrogen bonding produces stable multilayers up to the point where altered pH or other environmental stimulus introduces an unacceptably large electrical charge within them. This is demonstrated for the polyacids poly(acrylic acid), PAA, and poly(methacrylic acid), PMAA, and for the polybases poly(vinylpyrrolidone), PVPON, and poly(ethylene oxide), PEO, in D2O. The adsorption was quantified by Fourier transform infrared spectroscopy in attenuated total reflection (FTIR-ATR). The ratio between suppressed ionization of the carboxylic groups within the film and their ionization in solution, as directly measured by FTIR-ATR, was used to estimate the fraction of hydrogen-bonded carboxylic groups; this was ∼0.5 in PVPON/PMAA but only ∼0.1 in the PEO/PMAA system, though the dielectric environments appeared to be similar. The critical pH, at which these hydrogen-bonded layers disintegrated, was controlled by a balance of internal ionization and a fraction of carboxylic groups that formed hydrogen bonds with either PVPON or PEO. The critical point was at pH = 6.9 for the PVPON/PMAA films (relatively strong hydrogen bonding), but pH = 4.6 for the PEO/PMAA films (in which a smaller fraction of segments participated in hydrogen bonding). It was even less, pH = 3.6, in the PEO/PAA system, which contained a larger proportion of ionized groups at a given pH owing to the higher acidity of PAA. As a second avenue to control the stability of these multilayer films, ionic strength was varied systematically. In the PEO/PMAA system, the multilayers were stable up to pH = 4.6 in the environment of 10 mM ions (this ionic strength resulted from the buffer solution to control pH), but the multilayers were stable up to higher pH, pH = 5.15, when 0.4 M NaCl was added. The reason is that a higher ionic strength reduced the intensity of electrostatic repulsion between a given number of ionized groups within the multilayer assembly. A slight weakening of stability with decrease of molecular weight was observed (these experiments concerned the PVPON system) as expected from fewer hydrogen bonds per molecule. Finally, experiments with added rhodamine 6G dye showed that dye or drug molecules can be incorporated into such multilayers and then released as needed at preselected conditionsa feature that may be used in drug release devices.

Janus particles, colloid-sized particles with two regions of different surface chemical composition, possess energetic interactions that depend not only on their separation but also on their orientation. Research on Janus and colloidal particles that are chemically patchy in even more complicated fashion has opened a new chapter in the colloid research field. This article highlights recent progress in both experiment and theory regarding synthesis and self-assembly of Janus particles, and tentatively outlines some areas of future opportunity.