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Machine technology frequently puts magnetic or electrostatic repulsive forces to practical use, as in maglev trains, vehicle suspensions or non-contact bearings (1,2). In contrast, materials design overwhelmingly focuses on attractive interactions, such as in the many advanced polymer-based composites, where inorganic fillers interact with a polymer matrix to improve mechanical properties. However, articular cartilage strikingly illustrates how electrostatic repulsion can be harnessed to achieve unparalleled functional efficiency: it permits virtually frictionless mechanical motion within joints, even under high compression (3,4). Here we describe a composite hydrogel with anisotropic mechanical properties dominated by electrostatic repulsion between negatively charged unilamellar titanate nanosheets (5) embedded within it. Crucial to the behaviour of this hydrogel is the serendipitous discovery of cofacial nanosheet alignment in aqueous colloidal dispersions subjected to a strong magnetic field, which maximizes electrostatic repulsion (6) and thereby induces a quasi-crystalline structural ordering (7,8) over macroscopic length scales and with uniformly large face-to-face nanosheet separation. We fix this transiently induced structural order by transforming the dispersion into a hydrogel (9,10) using light-triggered insituvinyl polymerization (11). The resultant hydrogel, containing charged inorganic structures that align cofacially in a magnetic flux (12-19), deforms easily under shear forces applied parallel to the embedded nanosheets yet resists compressive forces applied orthogonally. We anticipate that the concept of embedding anisotropic repulsive electrostatics within a composite material, inspired by articular cartilage, will open up new possibilities for developing soft materials with unusual functions., In an extension of our work on water-rich hydrogels supported by a three-dimensional supramolecular network composed of clay nanosheets (20,21), here we replace clays with metal-oxide nanometre-scale sheets. We focused [...]

Abstract The potential of mean force (PMF) is the total free energy of a many-body colloidal system, and consequently it includes all the interactions the colloids experience due to collisions [...]

The phenomenon of supercooling in metals--that is, the preservation of a disordered, fluid phase in a metastable state well below the melting point (1)--has led to speculation that local atomic structure configurations of dense, symmetric, but non-periodic packing act as the main barrier for crystal nucleation (2,3). For liquids in contact with solids, crystalline surfaces induce layering of the adjacent atoms in the liquid (4,5) and may prevent or lower supercooling (6). This seed effect is supposed to depend on the local lateral order adopted in the last atomic layers of the liquid in contact with the crystal. Although it has been suggested that there might be a direct coupling between surface-induced lateral order and supercooling (6), no experimental observation of such lateral ordering at interfaces is available (6). Here we report supercooling in gold-silicon (AuSi) eutectic droplets, enhanced by a Au-induced (6 X 6) reconstruction of the Si(111) substrate. In situ X-ray scattering and ab initio molecular dynamics reveal that pentagonal atomic arrangements of Au atoms at this interface favour a lateral-ordering stabilization process of the liquid phase. This interface-enhanced stabilization of the liquid state shows the importance of the solid-liquid interaction for the structure of the adjacent liquid layers. Such processes are important for present and future technologies, as fluidity and crystallization play a key part in soldering and casting, as well as in processing and controlling chemical reactions for microfluidic devices or during the vapour-liquid-solid growth of semiconductor nanowires., Clusters with icosahedral short-range order are now widely considered as basic structural elements of liquid metals and glasses (3,7-9). Their presence has been proven experimentally (4,7,10) and by ab initio [...]

Supercooling on contact The local atomic arrangement in liquids differs substantially from the order in crystals. But when in contact with a crystal, the neighbouring atoms in the liquid can mimic the crystalline order, often triggering solidification of the liquid. This 'seeding' effect is a familiar one, seen in water as it nears its freezing point and in crystal nucleation. More surprising is the discovery, presented in this issue, of an opposite effect: if the atomic structure of a surface is similar to that of the contacting liquid phase, instead of crystallizing, the liquid remains liquid far below its melting point and supercooling is achieved. The phenomenon was observed in gold-silicon eutectic droplets in contact with gold atoms in a pentagonal arrangement layered onto a silicon substrate. This finding has implications for fundamental studies of freezing and also for the practical control of phase transitions. For instance, the containerless techniques often used to obtain supercooling might conceivably be avoided by coating containers with an icosahedrally ordered surface. Supercooling is a phenomenon by which a liquid remains in its fluid phase well below its melting point. Supercooling can be inhibited by the presence of a solid surface, whereby crystalline surfaces cause adjacent atoms in the liquid to become ordered, inducing crystal nucleation of the melt. Here it is shown that a particular surface ordering of gold atoms on top of a silicon substrate can stabilize the liquid phase of a gold-silicon eutectic droplet, and thus enhance supercooling. The phenomenon of supercooling in metals--that is, the preservation of a disordered, fluid phase in a metastable state well below the melting point.sup.1--has led to speculation that local atomic structure configurations of dense, symmetric, but non-periodic packing act as the main barrier for crystal nucleation.sup.2,3. For liquids in contact with solids, crystalline surfaces induce layering of the adjacent atoms in the liquid.sup.4,5 and may prevent or lower supercooling.sup.6. This seed effect is supposed to depend on the local lateral order adopted in the last atomic layers of the liquid in contact with the crystal. Although it has been suggested that there might be a direct coupling between surface-induced lateral order and supercooling.sup.6, no experimental observation of such lateral ordering at interfaces is available.sup.6. Here we report supercooling in gold-silicon (AuSi) eutectic droplets, enhanced by a Au-induced (6 × 6) reconstruction of the Si(111) substrate. In situ X-ray scattering and abinitio molecular dynamics reveal that pentagonal atomic arrangements of Au atoms at this interface favour a lateral-ordering stabilization process of the liquid phase. This interface-enhanced stabilization of the liquid state shows the importance of the solid-liquid interaction for the structure of the adjacent liquid layers. Such processes are important for present and future technologies, as fluidity and crystallization play a key part in soldering and casting, as well as in processing and controlling chemical reactions for microfluidic devices or during the vapour-liquid-solid growth of semiconductor nanowires., Author(s): T. U. Schülli [sup.1] [sup.2] , R. Daudin [sup.1] , G. Renaud [sup.1] , A. Vaysset [sup.1] , O. Geaymond [sup.3] , A. Pasturel [sup.4] Author Affiliations: (1) CEA, [...]

Geometric frustration arises when lattice structure prevents simultaneous minimization of local interaction energies. It leads to highly degenerate ground states and, subsequently, to complex phases of matter, such as water [...]

Author(s): Mirjam E. Leunissen (corresponding author) [1, 4]; Christina G. Christova [1, 4]; Antti-Pekka Hynninen [1]; C. Patrick Royall [1, 3]; Andrew I. Campbell [1]; Arnout Imhof [1]; Marjolein Dijkstra [...]

Premelting is the localized toss of crystalline order at surfaces and defects at temperatures below the bulk melting transition. It can be thought of as the nucleation of the melting process. Premelting has been observed at the surfaces of crystals but not within. We report observations of premelting at grain boundaries and dislocations within bulk colloidal crystals using real-time video microscopy. The crystals are equilibrium close-packed, three-dimensional colloidal structures made from thermally responsive microgel spheres. Particle tracking reveals increased disorder in crystalline regions bordering defects, the amount of which depends on the type of defect, distance from the defect, and particle volume fraction. Our observations suggest that interfacial free energy is the crucial parameter for premelting in colloidal and atomic-scale crystals., The importance of melting in nature can hardly be overestimated, and yet a detailed understanding of the mechanisms that drive this transformation is still evolving. Scientists have speculated for more [...]

Author(s): Mircea Raşa [1]; Albert P. Philipse (corresponding author) [1] The determination of molecular masses from barometric sedimentation profiles, a main topic in ultracentrifugal analysis, is thought to be quantitatively [...]

The British generals of World War Two have not been made to carry the same obloquy borne by their predecessors of the First World War. Contemporaries and historians have generally [...]

Crystallization of concentrated colloidal suspensions was studied in real space with laser scanning confocal microscopy. Direct imaging in three dimensions allowed identification and observation of both nucleation and growth of crystalline regions, providing an experimental measure of properties of the nucleating crystallites. By following their evolution, we identified critical nuclei, determined nucleation rates, and measured the average surface tension of the crystal-liquid interface. The structure of the nuclei was the same as the bulk solid phase, random hexagonal close-packed, and their average shape was rather nonspherical, with rough rather than faceted surfaces., The study of the structure, growth, and properties of crystals is one of the most important areas of solid state physics (1). Although a great deal is known about the [...]