Nature of crystalline particle assembly in ring shaped colloidal stains from concentrated dispersions

The drying of colloidal droplet suspensions is important in many realms of practical application and has sustained the interest of researchers over two decades. The arrangements of polystyrene and silica beads, both of diameter 1 μm, 10% by volume of solid deposited on normal glass (hydrophilic), and silicone (hydrophobic) surfaces evaporated from a suspension volume of 3 μL, were investigated. Doughnut shape depositions were found, imputing the influence of strong central circulation flows that resulted in three general regions. In the central region which had strong particle build-up, the top most layers of particle arrangement was confirmed to be disordered using power spectrum and radial distribution function analysis. On closer examination, this appeared more like frustrated attempts to crystallize into larger grains rather than beads arranging in a disordered fashion throughout the piling process. With an adapted micro-bulldozing operation to progressively remove layers of particles from the heap, we found that the later efforts to crystallize through lateral capillary inter-particle forces were liable to be undone once the particles contacted the disorganized particles underneath, which were formed out of the jamming of fast particles arriving at the surface. © 2014 Elsevier B.V.

Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide: Towards Multifunctional Textiles

Key points in the formation of liquid crystalline (LC) dispersions of graphene oxide (GO) and their processability via wet-spinning to produce long lengths of micrometer-dimensional fibers and yarns are addressed. Based on rheological and polarized optical microscopy investigations, a rational relation between GO sheet size and polydispersity, concentration, liquid crystallinity, and spinnability is proposed, leading to an understanding of lyotropic LC behavior and fiber spinnability. The knowledge gained from the straightforward formulation of LC GO “inks” in a range of processable concentrations enables the spinning of continuous conducting, strong, and robust fibers at concentrations as low as 0.075 wt%, eliminating the need for relatively concentrated spinning dope dispersions. The dilute LC GO dispersion is proven to be suitable for fiber spinning using a number of coagulation strategies, including non-solvent precipitation, dispersion destabilization, ionic cross-linking, and polyelectrolyte complexation. One-step continuous spinning of graphene fibers and yarns is introduced for the first time by in situ spinning of LC GO in basic coagulation baths (i.e., NaOH or KOH), eliminating the need for post-treatment processes. The thermal conductivity of these graphene fibers is found to be much higher than polycrystalline graphite and other types of 3D carbon based materials.

Better journalism or better profits? A key convergence issue in an age of concentrated ownership

Convergence has become an accepted form of journalism at media organisations around the world. These organisations are adopting a range of business models to find ways to pay for these innovations. The main drivers behind this radical change in media production are consumers' changing media habits, cheaper digital technology, and the disruptive forces that these two drivers generate. Technology also makes possible new forms of storytelling, which potentially allows journalists the chance to do better journalism through convergence. This article focuses on the key issue of whether editorial managers and journalists are embracing convergence to save money, or to do better journalism. It begins by defining convergence (while accepting the wide variety of definitions) and describing two main models of implementation. It then considers the factors that lead to easy introduction of convergence followed by the factors that hinder its introduction. Examples are provided of converged media around the world. This article ends with a warning about the dangers for democracy of misapplied convergence in an era of increasing concentration of ownership.

A study of hydrogen bonding in concentrated diol/water solutions by proton NMR correlations with glass formation

Solute/water interactions in a series of diol solutions have been investigated by ¹H NMR. Strong hydrogen bonding between water and alcohols that are more basic than water is thought to result in lower chemical shifts of water protons compared to the case of pure water. This is attributed to a greater degree of covalent character in the hydrogen bonding between water and the more basic diols. The inductive effect of the methyl group and longer chain alkyls is observed to increase basicity in ethylene glycol, propylene glycol, and 2,3-butanediol solutions. A correlation between the glass-forming ability of the diol solutions and the stronger hydrogen-bonding solutes (i.e., stronger bases) is developed, with 2,3-butanediol best promoting glass formation at the lowest concentrations.

Molecular dynamics simulations of highly concentrated salt solutions : structural and transport effects in polymer electrolytes

Structural, thermodynamic and transport properties have been calculated in concentrated non-aqueous NaI solutions using molecular dynamics simulations. Although the solvent has been represented by a simplistic Stockmayer fluid (spherical particles with point dipoles), the general trends observed are still a useful indication of the behavior of real non-aqueous electrolyte systems. Results indicate that in low dielectric media, significant ion pairing and clustering occurs. Contact ion pairs become more prominent at higher temperatures, independent of the dielectric strength of the solvent. Thermodynamic analysis shows that this temperature behavior is predominantly entropically driven. Calculation of ionic diffusivities and conductivities in the NaI/ether system confirms the clustered nature of the salt, with the conductivities significantly lower than those predicted from the Nernst-Einstein relation. In systems where the solvent-ion interactions increase relative to ion-ion interactions (lower charge or higher solvent dipole moment), less clustering is observed and the transport properties indicate independent motion of the ions, with higher calculated conductivities. The solvent in this system is the most mobile species, in comparison with the polymer electrolytes where the solvent is practically immobile.

Highly concentrated salt solutions : molecular dynamics simulations of structure and transport

Molecular dynamics (MD) simulations in NaI solutions, where the solvent has been represented by the Stockmayer fluid, were performed as a function of temperature, salt concentration, and solvent dipole strength. At higher temperatures contact ion pairs become more prevalent, regardless of solvent strength. An examination of the temperature dependence of the potential of mean force demonstrates the entropic nature of this effect. The transport properties calculated in the simulations are dependent on the balance between solvent dielectric constant and ion charge. In systems with a large solvent dipole moment, the ions appear to be independently mobile, and deviations from Nernst–Einstein behavior are small. In systems of smaller solvent dipole moment or greater ion charge, the ions form clusters, and large deviations from Nernst–Einstein behavior are observed.

Aggregation, ageing and transport properties of surface modified fumed silica dispersions

We have investigated the aggregation, ageing and transport properties of surface modified silica dispersions in DMSO by photon correlation spectroscopy and conductivity measurements. The surface modification introduces Li+-ions that dissociate in the dispersion creating a single Li+-ion conducting electrolyte. We show that the surface modification changes the aggregation and ageing properties of the material. There is a pronounced ageing observed for the modified silica dispersions. At high concentrations of fumed silica a gel state is found, which in the case of the surface modified silica is a very weak gel that can be rejuvenated by ultrasonic treatment. The key parameter controlling the aggregation in this system is hydrogen bonding and the surface modification results in a very low number of sites for hydrogen bonding. In addition there is a contribution from repulsive electrostatic interactions in the surface modified silica dispersions due to the highly charged surfaces of these particles. Furthermore, the Li+-ion diffusion, at low silica concentration, is three orders of magnitude faster than that of the silica particles and in the gel state the silica particles are immobile. We also find that the Li+-ion diffusion is virtually independent of the silica concentration in the dispersions.

Colloidal interaction between a rigid solid and a fluid drop

We present results of a theoretical study of the effect of surface deformation on a macroscopic system composed of a solid surface interacting with a fluid drop through electrostatic double-layer forces. The analysis involves numerically solving a Laplace equation suitably modified to describe the shape of a liquid drop subjected to a repulsive double-layer force. The latter is evaluated in nonlinear mean-field theory. Some analytical results are also given. The results indicate that although deformation need not be significant on the macroscopic scale, its effect on the interaction is significant and modifies the picture usually presented in DLVO theory. The decay length of the exponential repulsion deviates marginally from the Debye length, dependent on the interfacial tension of the drop. More significantly, at separations where the double-layer force becomes comparable to the internal pressure of the drop, the net force between the two bodies, the local radius of curvature of the drop, and the amount of deformation grow abruptly. The results of this work are relevant to emulsion stability, micelle, vesicle, and cell interactions, and recent experiments on bubble-particle interaction.

Thinning of a vertical free draining aqueous film incorporating colloidal particles

The drainage under gravity of a vertical foam film formed on a wire frame has been investigated. Dual-wavelength optical interferometry was used so that unambiguous fringe order assignments could be made, enabling absolute film thicknesses to be calculated with confidence. Films were stabilized by nonionic polypropylene glycol surfactant. Halfmicrometer silica particles with varying degrees of hydrophobicity were added to the film-forming liquid to investigate their effect on film drainage rate and stability. Hydrophilic particles had little or no effect, while hydrophobic particles slowed the drainage of the film and caused a minor increase in film lifetime, from ∼10 to ∼30 s. In both the hydrophilic and hydrophobic cases the films ruptured when they reached a thickness of ∼2 particle diameters. Particles of intermediate hydrophobicity had the most significant effect, increasing film lifetime by an order of magnitude over that for hydrophilic particles. The intermediate particles allowed films to thin down to a thickness less than the particle diameter, indicating that particles bridge across the entire film. This did not occur with more hydrophobic particles even though they were embedded in each of the two film surfaces. These results correlate well with previous literature on particle-laden foams. The film thickness and drainage measurements allow drainage mechanisms for the different particles to be identified, thus providing a mechanistic explanation for the observation by several previous authors that foams formed in the presence of particles, for example during mineral processing, have the greatest stability when the particles are of intermediate hydrophobicity.