Colloidal aggregation induced by long range attractions

The structure of colloidal clusters formed by long-range attractive interactions under diluted conditions is studied by means of Monte Carlo simulations. For a not-too-long attraction range, clusters show self-similar internal structure with lower density than that typical for diffusive aggregation. For long-range interactions, low kappa, nonfractal clusters are formed (dense at short scales but open at long ones). The dependence on the volume fraction shows that more-compact clusters are grown the higher the colloidal density for diffusive aggregation and attraction-driven aggregation in the fractal regime. The whole trend is explained in terms of the interpenetration among aggregates. In attraction-driven aggregations, the interpenetration of clusters competes with aggregation in the tips of the clusters, causing low-density clusters.     

Dynamics of Si-H vibrations in an amorphous environment

We present results of the first vibrational photon-echo, transient-grating, and temperature dependent transient-bleaching experiments on a-Si:H. Using these techniques, and the infrared light of a free electron laser, the vibrational population decay and phase relaxation of the Si-H stretching mode were investigated. Careful analysis of the data indicates that the vibrational energy relaxes directly into Si-H bending modes and Si phonons, with a distribution of rates determined by the amorphous host. Conversely, the pure dephasing appears to be single exponential, and can be modeled by dephasing via two-phonon interactions.     

Des anomalies observées dans l'etude par effet Raman de La2(SO4)3. 9H2O monocristallin en fonction de la temperature

In this paper we present some new results obtained by Raman spectrometry in La₂(SO₄)₂. 9H₂O single crystal. Several particular structural and dynamic properties as well as the anomalies observed near 260 K are reported.     

A study on the structural phase transitions of ferroelastic [N(CH3)4]2CuCl4 from microhardness measurements

Measurements of Vickers microhardness (MHv) as a function of temperature has been carried out on [N(CH₃)₄]₂CuCl₄ single crystals. The suitability of this technique for the determination of transition temperatures in the paraelastic-incommensurate→ferroelastic (commensurate) phase transitions sequence is shown.     

Combined Raman spectrometer/laser-induced breakdown spectrometer for the next ESA mission to Mars

Among the different instruments that have been pre-selected to be on-board the Pasteur payload on ExoMars is the Raman/laser induced breakdown spectroscopy (LIBS) instrument. Raman spectroscopy and LIBS will be integrated into a single instrument sharing many hardware commonalities. An international team under the lead of TNO has been gathered to produce a design concept for a combined Raman spectrometer/LIBS elegant bread-board (EBB). The instrument is based on a specially designed, extremely compact, spectrometer with high resolution over a large wavelength range, suitable for both Raman spectroscopy and LIBS measurements. Low mass, size and power consumption are the main drivers of the instrument's design concept. In this paper, science objectives for the combined instrument are detailed. Background information on Raman spectroscopy and LIBS are presented, focussing on the synergy of these two techniques. In the last section, the instrument concept resulting from the assessment of the feasibility of the combined Raman/LIBS EBB is presented.     

Computer simulations of nematic drops: coupling between drop shape and nematic order

We perform Monte Carlo computer simulations of nematic drops in equilibrium with their vapor using a Gay-Berne interaction between the rod-like molecules. To generate the drops, we initially perform NPT simulations close to the nematic-vapor coexistence region, allow the system to equilibrate and subsequently induce a sudden volume expansion, followed with NVT simulations. The resultant drops coexist with their vapor and are generally not spherical but elongated, have the rod-like particles tangentially aligned at the surface and an overall nematic orientation along the main axis of the drop. We find that the drop eccentricity increases with increasing molecular elongation, κ. For small κ the nematic texture in the drop is bipolar with two surface defects, or boojums, maximizing their distance along this same axis. For sufficiently high κ, the shape of the drop becomes singular in the vicinity of the defects, and there is a crossover to an almost homogeneous texture; this reflects a transition from a spheroidal to a spindle-like drop.     

Effect of surface active sites on adsorption of associating chain molecules in pores: A Monte Carlo study

This work describes the adsorption behavior of associating and non-associating chains and their mixtures in pores with activated surfaces. The systems are studied using Gibbs ensemble Monte Carlo molecular simulations. Fluid molecules are modeled as freely jointed Lennard-Jones chains. Associating chains have, additionally, an associating square-well site placed in an end sphere. The pores are modeled as regular slit pores via an integrated Lennard-Jones potential (10-4-3); activation is achieved by placing specific association sites protruding from the surface. Two different solid-fluid interaction parameters are used, one of which corresponds roughly to alkanes on graphite, the other being a much weaker interaction. Adsorption isotherms are presented for several different cases: associating and non-associating chains confined within both neutral and activated walls. Mixtures of associating and non-associating chains are also considered. The effects of pore size, temperature and chain length are quantified. Selectivities obtained are in the range of those seen in adsorption experiments of alkane-alkanol mixtures.