75 research outputs found
Aging behavior of the localization length in a colloidal glass
No full textThe localization length rloc associated with a fast secondary relaxation in glassy Laponite is determined by X-ray photon correlation spectroscopy (XPCS) through a Debye-Waller fit of the non-ergodicity parameter. Quantitative differences are observed between the time dependence (aging) of rloc in spontaneously aged and rejuvenated samples. This behavior is also reflected in the calculated shear modulus which matches well with data obtained by rheological measurements. © 2014 Elsevier B.V
Slow dynamics in an azopolymer molecular layer studied by x-ray photon correlation spectroscopy
No full textDichotomic aging behaviour in a colloidal glass
No full textAn unexpected dichotomic long time aging behaviour is observed in a glassy colloidal clay suspension investigated by X-ray photon correlation spectroscopy and dynamic light scattering. In the long time aging regime the intensity autocorrelations are non-exponential, following the Kohlrausch-Williams-Watts functional form with an exponent beta(Q). We show that for spontaneously aged samples a stretched behaviour (beta(Q) 1) appears only when the system is rejuvenated by application of a shear field. In both cases the relaxation times scale as Q(-1). These observations shed light on the origin of compressed exponential behaviour and help in classifying previous results in the literature on anomalous dynamics
Glass-glass transition during aging of a colloidal clay
No full textColloidal suspensions are characterized by a variety of microscopic interactions, which generate unconventional phase diagrams encompassing fluid, gel and glassy states and offer the possibility to study new phase and/or state transitions. Among these, glass-glass transitions are rare to be found, especially at ambient conditions. Here, through a combination of dilution experiments, X-ray photon correlation spectroscopy, small angle X-ray scattering, rheological measurements and Monte Carlo simulations, we provide evidence of a spontaneous glass-glass transition in a colloidal clay. Two different glassy states are distinguished with evolving waiting time: a first one, dominated by long-range screened Coulombic repulsion (Wigner glass) and a second one, stabilized by orientational attractions (Disconnected House of Cards glass), occurring after a much longer time. These findings may have implications for heterogeneously charged systems out-of-equilibrium and for applications where a fine control of the local order and/or long term stability of the amorphous materials are required
Photon statistics and speckle visibility spectroscopy with partially coherent X-rays
No full textA new approach is proposed for measuring structural dynamics in materials from multi-speckle scattering patterns obtained with partially coherent X-rays. Coherent X-ray scattering is already widely used at high-brightness synchrotron lightsources to measure dynamics using X-ray photon correlation spectroscopy, but in many situations this experimental approach based on recording long series of images (i.e. movies) is either not adequate or not practical. Following the development of visible-light speckle visibility spectroscopy, the dynamic information is obtained instead by analyzing the photon statistics and calculating the speckle contrast in single scattering patterns. This quantity, also referred to as the speckle visibility, is determined by the properties of the partially coherent beam and other experimental parameters, as well as the internal motions in the sample (dynamics). As a case study, Brownian dynamics in a low-density colloidal suspension is measured and an excellent agreement is found between correlation functions measured by X-ray photon correlation spectroscopy and the decay in speckle visibility with integration time obtained from the analysis presented her
Dynamics in dense hard-sphere colloidal suspensions
No full textThe dynamic behavior of a hard-sphere colloidal suspension was studied by x-ray photon correlation spectroscopy and small-angle x-ray scattering over a wide range of particle volume fractions. The short-time mobility of the particles was found to be smaller than that of free particles even at relatively low concentrations, showing the importance of indirect hydrodynamic interactions. Hydrodynamic functions were derived from the data, and for moderate particle volume fractions (Φ≤ 0.40) there is good agreement with earlier many-body theory calculations by Beenakker and Mazur. Important discrepancies appear at higher concentrations, above Φ 0.40, where the hydrodynamic effects are overestimated by the Beenakker-Mazur theory, but predicted accurately by an accelerated Stokesian dynamics algorithm developed by Banchio and Brady. For the relaxation rates, good agreement was also found between the experimental data and a scaling form predicted by the mode coupling theory. In the high concentration range, with the fluid suspensions approaching the glass transition, the long-time diffusion coefficient was compared with the short-time collective diffusion coefficient to verify a scaling relation previously proposed by Segrè and Pusey. We discuss our results in view of previous experimental attempts to validate this scaling la
Rheology and dynamics of a solvent segregation driven gel (SeedGel)
No full textBicontinuous structures promise applications in a broad range of research fields, such as energy storage, membrane science, and biomaterials. Kinetically arrested spinodal decomposition is found responsible for stabilizing such structures in different types of materials. A recently developed solvent segregation driven gel (SeedGel) is demonstrated to realize bicontinuous channels thermoreversibly with tunable domain sizes by trapping nanoparticles in a particle domain. As the mechanical properties of SeedGel are very important for its future applications, a model system is characterized by temperature-dependent rheology. The storage modulus shows excellent thermo-reproducibility and interesting temperature dependence with the maximum storage modulus observed at an intermediate temperature range (around 28 degrees C). SANS measurements are conducted at different temperatures to identify the macroscopic solvent phase separation during the gelation transition, and solvent exchange between solvent and particle domains that is responsible for this behavior. The long-time dynamics of the gel is further studied by X-ray Photon Correlation Spectroscopy (XPCS). The results indicate that particles in the particle domain are in a glassy state and their long-time dynamics are strongly correlated with the temperature dependence of the storage modulus.
Le roumain influencé par le turc : les emprunts
No full textThe Romanian language is a Latin language spoken continuously in the eastern part of the Roman Empire (Dacia, Pannonia, Dardania, Mesia) since the adoption of the Latin language in these provinces until today. Of course, Romanian is more than Latin. This language has undergone constant transformations, due both to its natural evolution and to the influences of neighbouring languages. Of all these influences, our work focuses on only one: the Turkish influence. The Ottoman Empire, at the height of its expansion, extended from north western Africa to the Persian Gulf and the Caspian Sea, and into the heart of Europe. As a result, the Turkish language has influenced all the idioms spoken in this vast space. These are countries that were part of the Ottoman Empire or countries found under its political and military control. The objectives of this research are: - to specify how, when and in what way Romanians and Turks interacted; - to analyse what kinds of cultural and linguistic imprints imposed by the Turks in the Romanised Danube provinces. The work also highlights the obvious linguistic similarities between the Romanian language and other languages that have been in contact with Turkish. Almost half of Turkish lexical borrowings in Romanian are, in addition to Romanian, in at least two other Balkan languages and/or the Hungarian language. This paper invites interested parties to take a look at the Romanian etymology, which is based on the linguistic and cultural influence of a colonial empire: the Ottoman Empire
Surface phenomena on the Tin-dioxide polycrystalline layers
No full textPolycrystalline tin-dioxide is widely used in the detection of reducing gases (such as H2, CO, CH4, C2H5OH,...) in air by measuring its conductivity changes. The advantage of gas sensors based on such sensing devices is low cost and high sensitivity. In contrast to their widespread applications and to their successful empirical research and development work, the present understanding of the chemical sensing mechanisms is still immature.
In this thesis, for gas sensors based on thick and porous tin-dioxide layers, a study of the response function upon variation of the partial pressure of ethanol vapors in 100% humidified air has been carried out. The influence of the working temperature and of the water vapors on the conductivity of the sensor was particularly emphasized.
Based on our experimental data, a theoretical model of the sensing mechanism in thick and porous tin-dioxide layers is presented. The model accepts the conduction mechanism as being governed by the Schottky potential barriers at the junction between grains. For describing the adsorption of gas molecules on the solid surface a method of conditioned adsorption was developed. The central idea was to assume that the reducing gas molecules are adsorbed (i.e. react) only on pre-adsorbed oxygen. The predictions made in the frame of our theoretical model are in good agreement with the experimental data
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