Sedimentation of Oblate Ellipsoids at low and Moderate Reynolds numbers

In many applications to biophysics and environmental engineering, sedimentation of non-spherical particles for example: ellipsoids, is an important problem. In our work, we simulate the dynamics of oblate ellipsoids under gravity. We study the settling velocity and the average orientation of the ellipsoids as a function of volume fraction. We see that the settling velocity shows a local maximum at the intermmediate densities unlike the spheres. The average orientation of the ellipsoids also shows a similar local maximum and we observe that this local maximum disappears as the Reynolds number is increased. Also, at small volume fractions, we observe that the oblate ellipsoids exhibit an orientational clustering effect in alignment with gravity accompanied by strong density fluctuations. The vertical and horizontal fluctuations of the oblate ellipsoids are small compared to that of the spheres

Hybrid inflation along waterfall trajectories

We identify a new inflationary regime for which more than 60 e-folds are generated classically during the waterfall phase occuring after the usual hybrid inflation. By performing a bayesian Monte-Carlo-Markov-Chain analysis, this scenario is shown to take place in a large part of the parameter space of the model. When this occurs, the observable perturbation modes leave the Hubble radius during waterfall inflation. The power spectrum of adiabatic perturbations is red, possibly in agreement with CMB constraints. A particular attention has been given to study only the regions for which quantum backreactions do not affect the classical dynamics. Implications concerning the preheating and the absence of topological defects in our universe are discussed.Comment: 10 pages, 5 figures, section III-A on quantum backreactions more detailed, comments on transverse field gradient contribution added, version accepted for publication in Phys.Rev.

The ion motion in self-modulated plasma wakefield accelerators

The effects of plasma ion motion in self-modulated plasma based accelerators is examined. An analytical model describing ion motion in the narrow beam limit is developed, and confirmed through multi-dimensional particle-in-cell simulations. It is shown that the ion motion can lead to the early saturation of the self-modulation instability, and to the suppression of the accelerating gradients. This can reduce the total energy that can be transformed into kinetic energy of accelerated particles. For the parameters of future proton-driven plasma accelerator experiments, the ion dynamics can have a strong impact. Possible methods to mitigate the effects of the ion motion in future experiments are demonstrated.Comment: 11 pages, 3 figures, accepted for publication in Phys. Rev. Let

Energetics of ion competition in the DEKA selectivity filter of neuronal sodium channels

The energetics of ionic selectivity in the neuronal sodium channels is studied. A simple model constructed for the selectivity filter of the channel is used. The selectivity filter of this channel type contains aspartate (D), glutamate (E), lysine (K), and alanine (A) residues (the DEKA locus). We use Grand Canonical Monte Carlo simulations to compute equilibrium binding selectivity in the selectivity filter and to obtain various terms of the excess chemical potential from a particle insertion procedure based on Widom's method. We show that K$^{+}$ ions in competition with Na$^{+}$ are efficiently excluded from the selectivity filter due to entropic hard sphere exclusion. The dielectric constant of protein has no effect on this selectivity. Ca$^{2+}$ ions, on the other hand, are excluded from the filter due to a free energetic penalty which is enhanced by the low dielectric constant of protein.Comment: 14 pages, 7 figure

Monitoring Corrosion Products on Metal Artefacts by Linear Sweep Voltammetry (LSV)

Metallic surfaces are highly sensitive to their surroundings, and prone to react with airborne pollutants to form complex layers. Electrochemical techniques have the possibility of simultaneously identifying different electroactive compounds [1]. The high sensitivity of Linear Sweep Voltammetry (LSV) allows the detection of extremely thin surface films before they became visible. LSV leads to reduction peaks, which can identify the compounds within the “tarnishing” layer and also its relative abundance [1, 2]. The potentialities of this and others electrochemical techniques has been well demonstrated in several recent works and they seem to be promising and non-invasive tools, even for in situ investigations on the metallic artifacts from cultural heritage. This communication presents data showing the application of the technique to silver and sterling silver coupons exposed, during periods of 1 and 12 months, in the Treasure Room in the Museum (inside showcases) and near the Holy Altar of the Chapel, of Porto Cathedral (Portugal). The influence of various conditions: the atmosphere (Museum and Chapel), exposure time (1 and 12 months) and season (spring, summer, autumn and winter) have been analysed. The LSV spectra of blank polished samples as well as samples covered with thin films of silver sulphide and of silver chloride, formed under controlled conditions, respectively: (i) H2S, 1000 ppm, RH= 53% ± 2, T= 21 ºC ± 1, over periods of 24, 48 and 72 h) and (ii) 3.5% of NaCl, RH=41% periods of 5 h, have been used as references. Concerning the nature of the products developed on the surface during the exposure, data has revealed that the tarnishing layers have a complex nature and are not composed only by silver sulphide. It happens even that silver sulphide is a relative minor component in the tarnish layer. Peaks corresponding to silver chloride and silver oxides were well visible. Small peaks assigned to silver sulphide were identified on the samples exposed in the Chapel. For the 12 months exposures both peaks have increased in size with the peak assigned to chloride being much higher in the case of the Chapel. The seasonal influence, even for the 1 month exposures, was particularly notable on those samples with exposures started in winter with well visible effects on the chloride peak on the tarnish layer of the samples exposed in the Chapel. Obviously, that to obtain analytical information electrochemical techniques can be complemented with spectroscopic techniques, namely, X- ray photon spectroscopy (XPS), laser induced breakdown spectroscopy (LIBS), x-rays fluorescence spectroscopy, among others. [1] A. Doménech-Carbó, M.T. Doménech-Carbó, V. Costa (2009), Electrochemical Methods in Archaeometry, Conservation and Restoration, Berlin: F. Scholz. [2] V. Costa, M. Dubus (2007), Impact of the environmental conditions on the conservation of metal artifacts: an evaluation using electrochemical techniques, in National Museum of Denmark, Museum Microclimates, Copenhagen: T. Padfield and K. Borchersen, 63-65