Local observations of phase singularities in optical fields in waveguide structures

A linear copolymer made of two reciprocally attracting N-monomer blocks collapses to a compact phase through a novel transition, whose exponents are determined with extensive Monte Carlo simulations in two and three dimensions. In the former case, an identification with the statistical geometry of suitable percolation paths allows one to predict that the number of contacts between the blocks grows like N9/16. In the compact phase the blocks are mixed and, in two dimensions, also zipped, in such a way to form a spiral, double chain structure.     

Influencing the angular emission of a single molecule

We present the first experimental proof for the influence of a nearby nanosized metal object on the angular photon emission by a single molecule. Using a novel angular sensitive detection scheme, we directly quantify the redirection of angular emission for different molecular dipole orientations as an object is scanned laterally over the molecule at different heights. An excellent agreement between experiments and 2D-numerical simulations is found for molecules oriented perpendicular to the sample, whereas, for parallel orientations, the observed behavior contradicts the calculated behavior.     

Flavonoids in Lemon and Grapefruit IntegroPectin**

Following the analysis of terpenes present in new lemon and grapefruit “IntegroPectin” pectins obtained via the hydrodynamic cavitation of industrial lemon and grapefruit processing waste, the HPLC-MS analysis of flavonoid and other phenolic compounds reveals the presence of eriocitrin, naringin, hesperidin and kaempferol typical of the respective citrus fruits. The pectic fibers rich in rhamnogalacturonan-I regions act as chemical sponges adsorbing and concentrating at their outer surface highly bioactive citrus flavonoids and terpenes. These findings, together with the unique molecular structure of these new whole citrus pectins, provide preliminary insight into the broad-scope biological activity of these new biomaterials. Numerous new biomedical applications are anticipated, including likely use in the prevention and treatment of microbial infections and neurodegenerative disease.     

Dynamics of uniaxial hard ellipsoids

We study the dynamics of monodisperse hard ellipsoids via a new event-driven molecular dynamics algorithm as a function of volume fraction phi and aspect ratio X0. We evaluate the translational D(trans) and the rotational D(rot) diffusion coefficients and the associated isodiffusivity lines in the phi-X0 plane. We observe a decoupling of the translational and rotational dynamics which generates an almost perpendicular crossing of the D(trans) and D(rot) isodiffusivity lines. While the self-intermediate scattering function exhibits stretched relaxation, i.e., glassy dynamics, only for large phi and X(0) approximately 1, the second order orientational correlator C2(t) shows stretching only for large and small X0 values. We discuss these findings in the context of a possible prenematic order driven glass transition.     

Self-assembling DNA dendrimers: a numerical study

DNA is increasingly used as a specific linker to template nanostructured materials. We present a molecular dynamics simulation study of a simple DNA-dendrimer model designed to capture the basic characteristics of the biological interactions, where selectivity and strong cooperativity play an important role. Exploring a large set of densities and temperatures, we follow the progressive formation of a percolating large-scale network whose connectivity can be described by random percolation theory. We identify the relative regions of network formation and kinetic arrest versus phase separation and show that the location of the two-phase region can be interpreted in the same framework as reduced valency models. This correspondence provides guidelines for designing stable, equilibrium self-assembled low-density networks. Finally, we demonstrate a relation between bonding and dynamics, by showing that the temperature dependence of the diffusion constant is controlled by the number of fully unbonded dendrimers.     

Diastereoselective multicomponent synthesis and anti-HSV-1 evaluation of dihydrofuran-fused derivatives

Enolizable 6-membered cyclic 1,3-dicarbonyls undergo an efficient and diastereoselective domino condensation/addition/heterocyclization reaction with arylaldehydes and phenacyl chloride, producing highly substituted dihydrofuran-fused derivatives. Ring size of the cyclic 1,3-dicarbonyls and the presence of at least one keto group are crucial to the reaction's success. The new compounds were evaluated in vitro for antiviral activity against herpes simplex virus type-1 (HSV-1). Interestingly, some of them appeared able to interfere with HSV-1 replication, without detection of cytotoxic effects.     

Quantitative description of the self-assembly of patchy particles into chains and rings

We numerically study a simple fluid composed of particles having a hard-core repulsion complemented by two patchy attractive sites on the particle poles. An appropriate choice of the patch angular width allows for the formation of ring structures which, at low temperatures and low densities, compete with the growth of linear aggregates. The simplicity of the model makes it possible to compare simulation results and theoretical predictions based on the Wertheim perturbation theory, specialized to the case in which ring formation is allowed. Such a comparison offers a unique framework for establishing the quality of the analytic predictions. We find that the Wertheim theory describes remarkably well the simulation results.     

Development of a lattice Boltzmann method for two‐layered shallow‐water flow

In this paper the dynamics of a two‐layered liquid, made of two immiscible shallow‐layers of different density, has been investigated within the framework of the lattice Boltzmann method (LBM). The LBM developed in this paper for the two‐layered, shallow‐water flow has been obtained considering two separate sets of LBM equations, one for each layer. The coupling terms between the two sets have been defined as external forces, acted on one layer by the other. Results obtained from the LBM developed in this paper are compared with numerical results obtained solving the two‐layered, shallow‐water equations, with experimental and other numerical results published in literature. The results are interesting. First, the numerical results obtained by the LBM and by the shallow‐water model can be considered as equivalent. Second, the LBM developed in this paper is able to simulate motion conditions on nonflat topography. Third, the agreement between the LBM (and also shallow‐water model) numerical results and the experimental results is good when the evolution of the flow does not depend on the viscosity, that is, during the initial phase of the flow, dominated by gravity and inertia forces. When the viscous forces dominate the evolution of the flow the agreement between numerical and experimental results depends strongly on the viscosity; it is good if the numerical LBM viscosity has the same order of magnitude of the liquid's kinematic viscosity. Copyright © 2012 John Wiley & Sons, Ltd.     

Simulation and theory of a model for tetrahedral colloidal particles

We study the thermodynamic and structural properties of a five-site tetrahedral molecular model by means of different Monte Carlo simulation techniques, and the reference interaction site model (RISM) theory of molecular fluids. Simulations and theory signal the onset, at sufficiently low temperatures, of two different tetrahedral molecular arrangements, with a more open topology progressively giving place to a fully bonded one, as the temperature decreases. The RISM theory reproduces the splitting of the static structure factor at low temperatures, a feature intimately related to the onset of the tetrahedral ordering. Less accurate predictions are obtained for the liquid-vapor coexistence and the short-range correlations.     

Hierarchical Spin-Crossover Cooperativity in Hybrid 1D Chains of FeII-1,2,4-Triazole Trimers Linked by [Au(CN)2]− Bridges

Foremost, practical applications of spin-crossover (SCO) materials require control of the nature of the spin-state coupling. In existing SCO materials, there is a single, well-defined dimensionality relevant to the switching behavior. A new material, consisting of 1,2,4-triazole-based trimers coordinated into 1D chains by [Au(CN)₂]⁻ and spaced by anions and exchangeable guests, underwent SCO defined by elastic coupling across multiple dimensional hierarchies. Detailed structural, vibrational, and theoretical studies conclusively confirmed that intra-trimer coupling was an order of magnitude greater than the intramolecular coupling, which was an order of magnitude greater than intermolecular coupling. As such, a clear hierarchy on the nature of elastic coupling in SCO materials was ascertained for the first time, which is a necessary step for the technological development of molecular switching materials.