Density-functional study of defects in two-dimensional circular nematic nanocavities

We use density--functional theory to study the structure of two-dimensional defects inside a circular nematic nanocavity. The density, nematic order parameter, and director fields, as well as the defect core energy and core radius, are obtained in a thermodynamically consistent way for defects with topological charge $k=+1$ (with radial and tangential symmetries) and $k=+1/2$. An independent calculation of the fluid elastic constants, within the same theory, allows us to connect with the local free--energy density predicted by elastic theory, which in turn provides a criterion to define a defect core boundary and a defect core free energy for the two types of defects. The radial and tangential defects turn out to have very different properties, a feature that a previous Maier--Saupe theory could not account for due to the simplified nature of the interactions --which caused all elastic constants to be equal. In the case with two $k=+1/2$ defects in the cavity, the elastic r\'egime cannot be reached due to the small radii of the cavities considered, but some trends can already be obtained.Comment: 9 figures. Accepted for publication in liquid crystal

The Genetic Signature of Perineuronal Oligodendrocytes

Oligodendrocytes in the central nervous system can be categorized as precursors, myelin-forming, and non-myelinating perineuronal cells. The function of perineuronal oligodendrocytes is unknown; it was proposed that following injury, they may remyelinate denuded axons. We investigated these cells' potential. A combination of cell-specific tags, microarray technology and bioinformatics tools to identify gene expression differences between these subpopulations allowed us to capture the genetic signature of perineuronal oligodendrocytes. Here we report that perineuronal oligodendrocytes are configured for a dual role. As cells that embrace neuronal somata, they integrate a repertoire of transcripts designed to create their own code for communicating with neurons. But they maintain a reservoir of untranslated transcripts encoding the major myelin proteins for - we speculate - a demyelinating episode. We posit that the signature molecules, PDGFR-[alpha][beta] cytokine PDGF-CC, and transcription factor Pea3, used - among others - to define the non-myelinating phenotype, may be critical for mounting a myelinating programme during demyelination. Harnessing this capability is of therapeutic value for diseases such as multiple sclerosis. This is the first molecular characterization of an elusive neural cell

Demixing and orientational ordering in mixtures of rectangular particles

Using scaled-particle theory for binary mixtures of two-dimensional hard particles with rotational freedom, we analyse the stability of nematic phases and the demixing phase behaviour of a variety of mixtures, focussing on cases where at least one of the components consists of hard rectangles or hard squares. A pure fluid of hard rectangles may exhibit, aside from the usual uniaxial nematic phase, an additional (tetratic) oriented phase, possessing two directors, which is the analogue of the biaxial or cubatic phases in three- dimensional fluids. There is computer simulation evidence that the tetratic phase might be stable with respect to phases with spatial order for rectangles with low aspect ratios. As hard rectangles are mixed with other particles not possessing stable tetratic order by themselves, the tetratic phase is destabilised, via a first- or second-order phase transition, to uniaxial nematic or isotropic phases; for hard rectangles of low aspect ratio tetratic order persists in a relatively large range of volume fractions. The order of these transitions depends on the particle geometry, dimensions and thermodynamic conditions of the mixture. The second component of the mixture has been chosen to be hard discs or disco-rectangles, the geometry of which is different from that of rectangles, leading to packing frustration and demixing behaviour, or simply rectangles of different aspect ratio. These mixtures may be good candidates for observing thermodynamically stable tetratic phases in monolayers of hard particles. Finally, demixing between fluid (isotropic--tetratic or tetratic--tetratic) phases is seen to occur in mixtures of hard squares of different sizes when the size ratio is sufficiently large.Comment: 27 pages, 9 figure

Capillary and anchoring effects in thin hybrid nematic films and connection with bulk behavior

By means of a molecular model, we examine hybrid nematic films with antagonistic anchoring angles where one of the surfaces is in the strong anchoring regime. If anchoring at the other surface is weak, and in the absence of wetting by the isotropic phase, the anchoring transition may interact with the capillary isotropic-nematic transition in interesting ways. For general anchoring conditions on this surface we confirm the existence of the step-tilt, biaxial phase and the associated transition to the linear, constant-tilt-rotation, configuration. The step-like phase is connected with the bulk isotropic phase for increasing film thickness so that the latter transition is to be interpreted as the capillary isotropic-nematic transition. Finally, we suggest possible global surface phase diagrams.Comment: 7 pages, 5 figure

Some thoughts about nonequilibrium temperature

The main objective of this paper is to show that, within the present framework of the kinetic theoretical approach to irreversible thermodynamics, there is no evidence that provides a basis to modify the ordinary Fourier equation relating the heat flux in a non-equilibrium steady state to the gradient of the local equilibrium temperature. This fact is supported, among other arguments, through the kinetic foundations of generalized hydrodynamics. Some attempts have been recently proposed asserting that, in the presence of non-linearities of the state variables, such a temperature should be replaced by the non-equilibrium temperature as defined in Extended Irreversible Thermodynamics. In the approximations used for such a temperature there is so far no evidence that sustains this proposal.Comment: 13 pages, TeX, no figures, to appear in Mol. Phy

The effect of additions of anticorrosive pigments on the cathodic delamitation and wear resistance of an epoxy powder coating

The cathodic delamination and wear resistance of epoxy powder coatings were evaluated after adding 3 % (by wt.) of calcium ion exchanged micropigments from amorphous synthetic silica. The materials were manufactured through the innovative and economical hot mixing method, and three different coatings were considered: commercial epoxy, epoxy without micropigments submitted to the hot mixing treatment, and epoxy with micropigments. The curing kinetics of the powder coatings was studied in order to evaluate the possible effects of the micropigments on the epoxy, using differential scanning calorimetry (DSC). In addition, mechanical properties of coatings (hardness and scratch resistance) and their wear resistance (reciprocal tribometer tests) were assessed. After provoking a controlled mechanical failure in the coatings, their delamination resistance was analyzed by scanning Kelvin probe (SKP). The delamination front was calculated after adding a drop of 3.5 % NaCl solution and taking measurements for 26 days. The results show that the corrosion attack progresses through a cathodic delamination mechanism. The addition of corrosion inhibitors in epoxy powder coatings has not only allowed a considerable improvement in delamination resistance, but has also led to greater mechanical and wear resistance. At the same time, it has simultaneously reduced the chances for mechanical failure of the coating and decreased the progression rate of damage, if it occurs. The study has also been completed with electrochemical impedance spectroscopy and polarization measured of fully-immersed defective coatings in 3.5 % NaCl.The authors acknowledge financial support from the European Union's Horizon 2020 research and innovation program through grant agreement No 766437 (ESSIAL project)