Numerical Simulations of Sintering, Application to Partially Densified Aerogels

Following recent scaling theories, the sintering of silica aerogels in modeled starting from computer generated gel structures obtained by diffusion-limited cluster-cluster aggregation on a cubic lattice. Two steps compose the sintering process model: a dressing step in which all particles are replaced by overlapping larger particles and a contraction step in which an adequate length rescaling is performed in order to conserve the total mass. Several quantities are computed during sintering as a function of the aerogel density such as the two characteristic cut-off lengths (lower and upper limits of the fractal regime) and the specific interface area. Comparison is made with results of similar off-lattice calculations when available. Some of the numerical results are compared with experiments on partially densified base-catalysed aerogels.     

Reactant concentrations from fluorescence correlation spectroscopy with tailored fluorescent probes. An example of local calibration-free pH measurement

The present account is concerned with the measurement of local reactant concentrations by observing specific fluorescent probes in fluorescence correlation spectroscopy (FCS). The Theoretical Analysis section revisits the photophysical, thermodynamic, and kinetic information that is contained in the corresponding FCS correlation curves. In particular, we examine the conditions under which FCS is revealed as a superior tool to measure concentrations of reactive species. Careful molecular engineering of the specific fluorescent probes that simultaneously integrates photophysical, thermodynamic, and kinetic constraints will be required to benefit most from FCS. We illustrate the FCS titration approach with a series of fluorescent probes that we tailored to measure pH at around 4-6 by FCS after two-photon excitation. We show that an optimal design allows one to access pH without any preliminary calibrations such as the determination of the protonation constant or the photophysical properties of the fluorescent probe.     

Examen de l'état de fatigue statique et dynamique d'un matériau par tomodensitométrie

Résumé L'observation par tomodensitométrie de la section transversale d'une poutre soumise à un moment de flexion met en évidence d'une part une zône isodensitométrique stable correspondant à la concentration des contraintes, d'autre part un décalage très net du maximum de la répartition densitométrique par histogramme pouvant être interprêté comme une certaine hétérogénéité. Ces phénomènes existent pour tous les matériaux que nous avons testés: plexiglass, nylon, rilsan, delrin. En fatigue dynamique, après 20 × 10⁶ cycles, l'histogramme comparatif révèle un décalage encore plus significatif que dans le cas statique; tout ceci apporte ainsi un élément positif à la nouvelle utilisation de cet appareil médical.

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Summary Observation by scanner of the cross section of a beam submitted to a bending moment gives evidence of a stable isodensitometric area in agreement with the concentration of stresses, as well as of a very sharp displacement of the maximum of the densitometric distribution by histogram which can be expounded as a certain heterogeneity. These phenomena exist for all materials we have studied: plexiglass, nylon, rilsan, delrin. In the case of dynamic fatigue, after 20 × 10⁶ cycles, the comparative histogram shows a displacement more significant than in the static case and gives in this way a positive argumentation to the new use of this medical apparatus.

Avec 11 figures     

2‐Hydroxyazobenzenes to Tailor pH Pulses and Oscillations with Light

This paper evaluates the 2‐hydroxyazobenzene platform for tailoring proton concentration pulses and oscillations with monochromatic light. The easily prepared 2‐hydroxyazobenzenes exhibit large absorptions in the near‐UV range. Photoisomerization was investigated by UV/Vis absorption, ¹H NMR spectroscopy, and steady‐state fluorescence emission. In the whole investigated series, the trans stereoisomer of the 2‐hydroxyazobenzene motif provides the corresponding cis derivative with an action cross section in the 10³ M ^?1 cm^?1 range. At the same time, photoisomerization is accompanied by a significant pK drop of the phenol group. According to the phenyl‐substituent pattern, cis‐to‐trans thermal back‐isomerization can be tuned in the 10 ms–100 s range. Up to 2 units of reversible pH drops or pH oscillations on the 10 s timescale have been obtained by appropriately tailoring single‐wavelength illumination of 2‐hydroxyazobenzene solutions.     

Preface

Journal of Sol-Gel Science and Technology -     

Sur un traitement unifie des deplacements chimiques en spectrometrie photoelectronique de rayons—X (ESCA), des moments dipolaires et des polarisabilites

The simplest and most general model for interpreting ESCA chemical shifts, the physical meaning of which is very clear, also allows the discussion of dipole moments and polarisabilities. Sixty-eight ESCA chemical shifts of C_1s, N_1s, O_1s and F_1s in 41 differently-substituted compounds (?F, ?Cl, ?Br, ?OH, ?NH₂), saturated or unsaturated (aldehydes, ketones, acids), have been calculated with this model from electric charge distribution; the mean quadratic deviations are respectively ±1.1 eV for C, ±1.6 eV for N, ±0.8 eV for 0, ±0.7 eV for F.With this model, electric charge repartitions can be deduced from ESCA data, even for aromatic molecules. The calculated electric dipole moments for 12 fluorinated aromatic molecules agree very well with experimental results. Other data have been calculated for molecules for which experimental data are not yet available, including examples where heavy atoms are present.