“Integrative sol–gel chemistry”: a nanofoundry for materials science

Integrative sol–gel chemistry based strategies allow, through the strong coupling between materials chemistry and advanced processing, the fabrication of functional inorganic and hybrid materials. The following article will highlight some of the main accomplishments performed during the last years in the design of nano- and multi-scale structured materials shaped as thin films, powders and monoliths with additional functionalities and outstanding properties in several fields of application such as optics, catalysis and nanomedicine. In particular we discuss the key role played by the adapted liquid processing of sol–gel based solution. We will describe some technologies (including dip coating, spray drying, droplet-microfluidics, ink-jet and foaming) in which a high degree of control in term of liquid shaping/evaporation/manipulation is required in order to achieve specific functionalities.     

A single parameter determines mesophase transitions in Swollen Liquid Crystals

We report how the control of a single parameter, the co-surfactant, determines the phase transitions of oil-in-water swollen liquid crystals (SLCs) prepared with cetyltrimethylammonium bromide (CTAB), from cubic to hexagonal, lamellar, and finally sponge-like structures. SLCs are complex mixtures (surfactant + co-surfactant + water + salt + oil) usually prepared to form hexagonal mesophases, with cell parameters tunable between 3 and 30 nm. These hexagonal mesophases were successfully used as nanoreactors to prepare a broad range of nanostructured materials. Because the potential of these mesophases as adaptive nanoreactors has not been extended to other liquid crystal geometries than the hexagonal, we studied in a first step the structure evolution of SLCs made with CTAB, cyclohexane, pentanol-1, water and different stabilising salts. We used small-angle X-ray scattering (SAXS), polarised light microscopy and Freeze-Fracture TEM to provide a partial phase diagram and list the different mesophases obtained as a function of composition. We report that the adjustment of a single parameter, the co-surfactant (pentanol-1), determines the phase transition between cubic, hexagonal, lamellar, and sponge-like structures, all other parameters such as the nature and concentration of salt, or amount of oil being constant.     

Spherically symmetric inhomogeneous dust collapse in higher dimensional space-time and cosmic censorship hypothesis

We consider a collapsing spherically symmetric inhomogeneous dust cloud in higher dimensional space-time. We show that the central singularity of collapse can be a strong curvature or a weak curvature naked singularity depending on the initial density distribution.     

Dramatic variation of magnetic exchange through double end-on azide bridges in a series of ladder-like copper(II) coordination polymers

Three ladder-like coordination polymers, [Cu2(phprpy)2-mu-(N3)2(N3)2], 1; [Cu2(terpy)2-mu-(N3)4Cu2-mu-(N3)2(N3)2], 2; and[Cu2(terpy)2-mu-(N3)2(N3)2Cu3-mu-(N3)4(N3)2], 3, consisting of Cu2+ ions with double end-on azide bridges were synthesized, their crystal structures and magnetic properties were determined, and spin dimer analysis was performed to explain the signs and strengths of their strong spin exchange interactions [phprpy is 4-(3-phenylpropyl)pyridine and terpy is 2,2':6,2'-terpyridine]. Although these compounds have ladder-like arrangements of Cu2+ ions, their magnetic structures are described as isolated dimers for 1 and 2 and as isolated trimers for 3. The predominant spin exchange paths in 1-3 have double end-on azide bridges linking adjacent Cu2+ ions, and the geometrical parameters of these bridging structures are similar. However, the spin dimer of 1 exhibits a strong ferromagnetic coupling; that of 2, a strong antiferromagnetic coupling; and that of 3, a weak ferromagnetic coupling. These findings are well explained by the present spin dimer analysis and show that the nature and geometry of the nonbridging ligands can have a strong influence on the sign and strength of the spin exchange interaction between Cu2+ ions connected by double end-on azide bridges.     

Three-dimensional opal-like silica foams

The synthesis of novel meso-/macroporous SiO2 monoliths by combining a nano-building-blocks-based approach with the confined geometry of a tailored air-liquid foam structure is described. The resulting macrostructure in which ordered close-packed colloidal silica nanoparticles constitute the monolith's scaffolds very closely resembles the tailored periodic air-liquid foam template. The void spaces between adjacent particles create textural mesoporosity; therefore, the as-prepared silica networks are characterized by hierarchical porosity at the macroscopic and mesoscopic length scales. The fine-tuning of both the liquid foam's fraction and the bubble size allows a rational design over the macroscopic cell morphologies (shape, Plateau border's length, and width). Striking results of this approach are the weak shrinkage of the as-synthesized opal-like scaffolds during the thermally induced sintering process and, in contrast with previous studies, the formation of closed-cell structures. Particle organization and the foam film surface roughness are investigated by atomic force microscopy (AFM), showing the influence of the liquid flow, within the foams' Plateau borders and films, on the final assemblies.     

Scattering by small defects in the neighbourhood of a fluid-solid interface

The scattering of incident plane elastic waves by a varietyof different defects that lie upon a fluid-solid interface isconsidered here using matched asymptotic expansions. The expansionscheme is developed in terms of a parameter , the ratio of typicaldefect length scale to a typical wavelength of the incidentfield, taken to be small. Three different canonical situations occur and these are illustratedvia three specific examples treated here: a rigid strut, anedge crack, and a rigid strip. In each case the leading-ordermatching is performed to identify the leading-order contributionof the defect to the acoustic field in the far field. In particular,each defect is identified with a source of dipole response ininterfacial stress of displacement. It is shown in the limit as s<<s1 that in the inner problemsthe fluid and solid pieces uncouple in a particularly convenientmanner allowing analytical solutions to be deduced. These arethen matched with appropriate outer solutions.     

[Ni(terpy)(H2O)]‐trans‐[Ni‐(μ‐CN)2‐(CN)2]n,a one‐dimensional linear tetra­cyano­nickelate chain

The one‐dimensional chain catena‐poly­[[aqua(2,2′:6′,2′′‐terpyridyl‐κ³N)­nickel(II)]‐μ‐cyano‐κ²N:C‐[bis­(cyano‐κC)nickelate(II)]‐μ‐cyano‐κ²C:N], [Ni(terpy)(H₂O)]‐trans‐[Ni‐μ‐(CN)₂‐(CN)₂]_n or [Ni₂­(CN)₄­(C₁₅H₁₁N₃)(H₂O)], consists of infinite linear chains along the crystallographic [10] direction. The chains are composed of two distinct types of nickel ions, paramagnetic octahedral [Ni(terpy)(H₂O)]²⁺ cations (with twofold crystallographic symmetry) and diamagnetic planar [Ni(CN)₄]^2? anions (with the Ni atom on an inversion center). The [Ni(CN)₄]^2? units act as bidentate ligands bridging through two trans cyano groups thus giving rise to a new example of a trans–trans chain among planar tetra­cyano­nickelate complexes. The coordination geometry of the planar nickel unit is typical of slightly distorted octahedral nickel(II) complexes, but for the [Ni(CN)₄]^2? units, the geometry deviates from a planar configuration due to steric interactions with the ter­pyridine ligands.     

Magnetic spin ladder (C5H12N)2CuBr4: high-field magnetization and scaling near quantum criticality

The magnetization, M(H< or =30 T,0.7< or =T< or =300 K), of (C5H12N)2CuBr4 has been used to identify this system as an S = 1/2 Heisenberg two-leg ladder in the strong-coupling limit, J( perpendicular) = 13.3 K and J( parallel) = 3.8 K, with H(c1) = 6.6 T and H(c2) = 14.6 T. An inflection point in M(H,T = 0.7 K) at half saturation, M(s)/2, is described by an effective XXZ chain. The data exhibit universal scaling behavior in the vicinity of H(c1) and H(c2), indicating that the system is near a quantum critical point.     

Controlled production of emulsions and particles by milli- and microfluidic techniques

The recent developments of soft lithography and microfluidic techniques now permit the manipulation of small quantities of fluids with very good control and reproducibility. These advances open a new “bottom-up” route to emulsification that paves the way to the fabrication of calibrated hierarchically organized emulsions and particles. In this article, we describe the microfluidic techniques elaborated for engineering emulsions and new dispersed materials and discuss their advantages over “top-down” approaches. We review and comment the high potentialities these techniques offer to emulsion and colloid science, to the development of high-throughput set-ups for chemistry, physics and biology. We illustrate them through a few examples taken from the current literature.