An easy sol–gel route for deposition of oriented Ln2Ti2O7 (Ln=La,Nd) films on SrTiO3 substrates

The effect of the orientation of SrTiO₃ (STO) substrates, (1 0 0) or (1 1 0), on the growth of La₂Ti₂O₇ (LTO) and Nd₂Ti₂O₇ (NTO) thin films was investigated. The films were deposited via a sol–gel process coupled to the spin-coating technique. Depending on the substrate orientation, a similar effect was observed on the structural properties for both the LTO and NTO thin films. For (1 1 0)-oriented STO substrate, the films are preferentially (0 0 1) oriented while for (1 0 0)-oriented STO substrates, the orientation is mainly (0 1 2). Those matching appear to be in good agreement with the compatibility between the film/substrate crystal lattices, in relation with the existence of perovskite slabs in Ln₂Ti₂O₇ (Ln=rare earth) compounds. In these latter, a slight disorientation of the (0 1 2) planes with respect to the surface of the substrate was measured. This tilting is even more marked in the case of NTO. The surface morphology was studied by atomic force microscopy.     

Rainwater rivulets running on a stay cable subject to wind

Stay cables are likely to vibrate under the combined effect of rain and wind in the so-called phenomenon rain–wind-induced vibrations (RWIVs). Rain takes part in the phenomenon in the shape of water rivulets that run along the cables. In previous articles, the authors investigated the conditions under which such rivulets can be formed. Using a lubrication model, it was shown for a particular wind–cable configuration that rivulets can only be exhibited above a critical wind speed for which gravity is overcome. The rivulets’ position was also predicted with the model. The results were validated by experiments.In this paper, the wind speed at which rivulets appear and their position are expressed for an arbitrary wind–cable orientation. A maximum wind speed for the rivulets’ existence is then estimated as the result of a balance between the drag force and the surface tension. A wind speed interval is consequently obtained for the rivulets’ appearance and maintenance on a cylinder. The boundaries of this interval are expressed in term of Weber numbers, comparing the surface tension and wind effect. These predictions are successfully compared with all the measurements that have been published in the literature.     

Crystallization mechanisms of acicular crystals

In this contribution, we present an experimental investigation of the growth of four different organic molecules produced at industrial scale with a view to understand the crystallization mechanism of acicular or needle-like crystals. For all organic crystals studied in this article, layer-by-layer growth of the lateral faces is very slow and clear, as soon as the supersaturation is high enough, there is competition between growth and surface-activated secondary nucleation. This gives rise to pseudo-twinned crystals composed of several needle individuals aligned along a crystallographic axis; this is explained by regular over- and inter-growths as in the case of twinning. And when supersaturation is even higher, nucleation is fast and random.In an industrial continuous crystallization, the rapid growth of needle-like crystals is to be avoided as it leads to fragile crystals or needles, which can be partly broken or totally detached from the parent crystals especially along structural anisotropic axis corresponding to weaker chemical bonds, thus leading to slower growing faces. When an activated mechanism is involved such as a secondary surface nucleation, it is no longer possible to obtain a steady state. Therefore, the crystal number, size and habit vary significantly with time, leading to troubles in the downstream processing operations and to modifications of the final solid-specific properties.These results provide valuable information on the unique crystallization mechanisms of acicular crystals, and show that it is important to know these threshold and critical values when running a crystallizer in order to obtain easy-to-handle crystals.     

Stochastic complexity integral image based technique for fast video tracking

We propose a new method based on the minimization of the stochastic complexity for fast and efficient tracking adapted to video images with a static camera. The obtained criterion combines the advantages of background-subtraction-based techniques and those of using measures of similarities to a target model without requiring any tuning of a weighting parameter. It is then demonstrated that this approach can be implemented with a fast integral image technique to estimate the location and the rectangular shape of the target in a few milliseconds.     

Synchronization between optically injected semiconductor lasers on undamped relaxation oscillations

Seeding light from a laser into the cavity of a second one is a simple experiment largely encountered in optical domain (Stover, H.L., Steier, W.H.: Appl. Phys. Lett. 8, 91–93 (1966); Lang, R.: IEEE J. Quant. Elect. 18(6), 976–983 (1982)). With an unidirectional coupling, optical injection is a basic tool to study synchronization process between oscillators. The dynamics induced in this experiment have been extensively described, theoretically and experimentally (Simpson, T.B. et al.: Quant. Semiclassical Opt. 9(5), 765–784 (1997); Wieczorek S.: Opt. Commun. 172, 279–295 (1999); Blin, S.: Comptes Rendus de la Phys. 4(6), 687–699 (2003)) but mainly when the seeded light is a continuous wave (CW). In this paper, we describe the synchronization on an undamped relaxation regime. In order to study the degree of correlation between the two synchronized lasers (TL and RL), we use a cascade of two optical injections (Guignard, C.: Conference on Lasers and Electro Optics—the Europeen Quant. Electronics Conference (CLEO-EQEC), Munich (Germany), EC1M (2003)) thus three lasers: Master-transmitter laser (TL)-receiver laser (RL). The seeding by a field showing undamped relaxations can be directly compared with the optical injection by a CW signal, especially when the injected power and the detuning between the TL and RL frequencies are varied. Synchronization on undamped relaxation are shown to occur at values of the optical injected power and detuning, for which the RL is in the locked or wave-mixing regime when it is seeded by a CW. However, additional tongues of synchronization also exist at detuning equal to the relaxation oscillation frequency (ROF).     

How Lewis Acids Catalyze Diels–Alder Reactions

The Lewis acid(LA)‐catalyzed Diels–Alder reaction between isoprene and methyl acrylate was investigated quantum chemically using a combined density functional theory and coupled‐cluster theory approach. Computed activation energies systematically decrease as the strength of the LA increases along the series I₂<SnCl₄<TiCl₄<ZnCl₂<BF₃<AlCl₃. Emerging from our activation strain and Kohn–Sham molecular orbital bonding analysis was an unprecedented finding, namely that the LAs accelerate the Diels–Alder reaction by a diminished Pauli repulsion between the π‐electron systems of the diene and dienophile. Our results oppose the widely accepted view that LAs catalyze the Diels–Alder reaction by enhancing the donor–acceptor [HOMO_diene–LUMO_dienophile] interaction and constitute a novel physical mechanism for this indispensable textbook organic reaction.     

Improving anharmonic infrared spectra using semiclassically prepared molecular dynamics simulations

Classical molecular dynamics is a convenient method for computing anharmonic infrared spectra of polyatomic molecules and condensed phase systems. However it does not perform well for predicting accurate intensities and it lacks nuclear quantization, two deficiencies that are usually accounted for by empirical scaling factors. In this paper we show on the examples of the trans isomer of nitrous acid and naphthalene that both issues can be alleviated by preparing the initial conditions according to semiclassical quantization based on a normal mode representation. The method correctly reproduces fundamental frequencies obtained with quantum mechanical methods. At increasing temperatures, the effective frequencies are found to follow the same trends as path-integral based methods. In the low-temperature limit, the band intensities predicted by the method are also found to agree with quantum mechanical considerations.     

Directed Supramolecular Surface Assembly of SNAP‐tag Fusion Proteins

Supramolecular assembly of proteins on surfaces and vesicles was investigated by site‐selective incorporation of a supramolecular guest element on proteins. Fluorescent proteins were site‐selectively labeled with bisadamantane by SNAP‐tag technology. The assembly of the bisadamantane functionalized SNAP‐fusion proteins on cyclodextrin‐coated surfaces yielded stable monolayers. The binding of the fusion proteins is specific and occurs with an affinity in the order of 10⁶ M ^?1 as determined by surface plasmon resonance. Reversible micropatterns of the fusion proteins on micropatterned cyclodextrin surfaces were visualized by using fluorescence microscopy. Furthermore, the guest‐functionalized proteins could be assembled out of solution specifically onto the surface of cyclodextrin vesicles. The SNAP‐tag labeling of proteins thus allows for assembly of modified proteins through a host–guest interaction on different surfaces. This provides a new strategy in fabricating protein patterns on surfaces and takes advantage of the high labeling efficiency of the SNAP‐tag with designed supramolecular elements.     

White Emitters by Tuning the Excited‐State Intramolecular Proton‐Transfer Fluorescence Emission in 2‐(2′‐Hydroxybenzofuran)benzoxazole Dyes

The synthesis, structural, and photophysical properties of a new series of original dyes based on 2‐(2′‐hydroxybenzofuran)benzoxazole (HBBO) is reported. Upon photoexcitation, these dyes exhibit intense dual fluorescence with contribution from the enol (E*) and the keto (K*) emission, with K* being formed through excited‐state intramolecular proton transfer (ESIPT). We show that the ratio of emission intensity E*/K* can be fine‐tuned by judiciously decorating the molecular core with electron‐donating or ‐attracting substituents. Push–pull dyes 9 and 10 functionalized by a strong donor (nNBu₂) and a strong acceptor group (CF₃ and CN, respectively) exhibit intense dual emission, particularly in apolar solvents such as cyclohexane in which the maximum wavelength of the two bands is the more strongly separated. Moreover, all dyes exhibit strong solid‐state dual emission in a KBr matrix and polymer films with enhanced quantum yields reaching up to 54 %. A wise selection of substituents led to white emission both in solution and in the solid state. Finally, these experimental results were analyzed by time‐dependent density functional theory (TD‐DFT) calculations, which confirm that, on the one hand, only E* and K* emission are present (no rotamer) and, on the other hand, the relative free energies of the two tautomers in the excited state guide the ratio of the E*/K* emission intensities.     

On viscoelastic effects in non-Newtonian steady flows through porous media

This paper presents a mathematical model for describing approximately the viscoelastic effects in non-Newtonian steady flows through a porous medium. The rheological behaviour of power law fluids is considered in the Maxwell model of elastic behaviour of the fluids. The equations governing the steady flow through porous media are derived and an analytical solution of these equations in the case of a simple flow system is obtained. The conditions for which the viscoelastic effects may become observable from the pressure distribution measurements are shown and expressed in terms of some dimensionless groups. These have been found to be relevant in the evaluation of viscoelastic effects in the steady flow through porous media.