Driven Diffusion of Particles, First-Passage Front, and Interface Growth

We study a computer-simulation model for driven particles on a discrete lattice where a fraction p of the lattice sites is randomly occupied by frozen impurities (barriers), and an imposed bias governs the particles’ hopping through the lattice. These particles (the carriers) are initially released from a source of wetting fluid from one end of the lattice in order to wet and the dry lattice on their trails. We study the transport of particles, frontier of their trail, and the growth of the interface between the wet and dry regions as a function of the biased field and the number of carriers. The rms displacements of carriers (Rtr) and that of their center of mass (Rc.m.) show power-law behaviors with time t, with exponents depending on the biased field. At the impurity concentration p=0.30 in two dimensions, we find that the mean wetting front position Rf moves with a power law Rf∼t2/3 at low values of the biased field, whereas it becomes pinned at higher values. The interface width grows with time to a maximum value before relaxing to a saturation value

Evolution of the microstructure of cobalt during diffusionless transformation cycles

Differential scanning calorimetry and transmission electron microscopy have been used to study thermal fatigue due to diffusionless phase transformation cycling in pure cobalt. Thermal cycling through the allotropic (hcp ↔ fcc) transformation results in a temperature shift of the calorimetric peaks, which means a delay of the transformation. In addition, the transformation enthalpy, which is greater on heating than on cooling, diminishes when the number of transformation cycles increases. This is interpreted as being due to an evolution of the microstructure. Transmission electron microscopy shows the appearance of transformation-induced defects, which are mainly sessile dislocations. We can interpret the calorimetry results (enthalpy evolution and transformation delay) as due to the interactions between interface dislocations and these sessile dislocation

Reconnaissance de certains caractères chimiques des eaux souterraines des séries carbonatées karstiques dévono--carbonifères de Chine méridoniale

L'étude porte sur un magasin aquifère calcaréo-dolomitique très karstifié (karst à pitons ou tourelles) d'une région à climat tropical humide de la vallée de Li-Jiang.L'échantillonnage hydrochimique concerne les zones saturée et non saturée de différents secteurs du karst.La détermination des pressions de CO2 et des coefficients de saturation par rapport à la calcite ont fait l'objet d'une attention particulière. Les concentrations en calcium et bicarbonates (éléments prépondérants) varient respectivement dans les intervalles 3,1 à 4,7 et 2,5 à 4,8 mé.l-1.La plus grande partie des échantillons de la zone saturée est en équilibre avec la calcite et la dolomite, sous des pressions de CO2 de l'ordre de 1 à 2.10-2 atm.Les eaux de la zone non saturée sont celles de différents suintements recueillis au plafond d'une galerie naturelle. Elles se distinguent par une inégalité entre les teneurs en bicarbonates et en calcium, interprétée comme étant due à des échanges de bases avec les résidus de colmatage des fissures.Les résultats hydrochimiques font supposer l'existence, dans le karsts de plusieurs domaines aquifères liés à la morphologie, interconnectés, avec drainage général par le Li-Jiang.This paper deals with a research program aimed at studying the hydrogeologic properties of the water-rich carbonate rocks in Southern China. We focus our attention on the Guilin area (Guangxi province). Our results concern the hydrochemistry and we show how a study of time- and space-variations in the chemical composition of groundwater (saturated and unsaturated zone) may help in understanding the hydrodynamic conditions in a karstic aquifer.The Guilin karstic area is mainly composed of a limestone and dolomitic limestone series extending in age from Mid-Devonian to early Triassic, reaching 3 000 m in thickness. In the Li-Jiang valley, the quaternary caver (alluvial deposits) is 10 to 20 meters thick. Geomorphology is named "tower karst", as it may be observed in various tropical regions. In fact, two different types of landform appear : the "fonglin" (peak forest) and the "fongchong" (peak cluster). In the former, isolated peaks (mean altitude : 250-300 m) rise in the plain (altitude around 150 m). In the latter, the peaks join together and delimit closed depressions (attitude up to 600 m in the Guilin area). Deep karstification is as important as surface karstification, which is related to the tectonic context (rocks are intensely fractured), to the subtropical wet climate (mean annual rainfall : 1 900 mm) and also to intense groundwater circulation.Our hydrochemical study has been carried out on various types of water points (springs, sinkholes, large diameter wells, boreholes, an underground river, oozing points from the unsaturated zone). They are located in both the fonglin and the fongchong area, but the highest density is close to the contact fine between the two landforms. The chemical analyses are interpreted in three steps :1) calculation (if possible) of the saturation coefficient relative to calcite and dolomite and of the CO2 equilibrium pressure,2) study of time variations in the composition of water sampled at oozing points from the unsaturated zone and,3) statistic study of space variations in the composition of water sampled at alt points.Hydrochemistry is dominated by Ca2+ and HCO3- ions. Except for two aberrant points (probably resulting from a mixture with surface water), Ca2+ and HCO3- concentration varies in a fairly narrow range (respectively 3.12-4.72 meq.l-1 and 2.53-4.80 mea.l-1). Most of samples from the saturated zone are close to equilibrium with calcite and often with dolomite, under CO2 pressures about 0.01-0.02 atm. Small scale variations in lithology (presence of dolomite) and in CO2 pressure may therefore explain space variations in hydrochemistry and enable waters of the same origin to be identified.The study of the unsaturated zone consists in sampling the water at three oozing points located in three fractures of the roof of a natural cave. Althrough the points are quite near to each other, water composition differs clearly between them. The most interesting result though is provided by the following experiment. One of the three points was initially dry. The flow occurred when breaking the concretion obstructing it. For six hours we studied the discharge and the water-composition variations at that point. The initial value of the discharge (6 ml.mn-1) rapidly decreases and reaches its stable value (4 ml.mn-1) after 1 h 30 min. Ca2+, Mg2+, Na+, K+ and HCO3- concentration show erratic variations, but the fact that Na+ and K+ concentration are linked suggests that these fluctuations are related to adsorption-desorption phenomena. It should be noticed that SO42- concentration is not erratic : it decreases from 0.44 meq.l-1 and reaches a stable value (0.08 meq.l-1) after four hours, i.e. much later than the discharge. This proves that the oozing point yields water from two different origins : a small reservoir and a network of connected fractures reaching a given potential boundary, both being initially static.The statistic study of space variations identifies a mountain aquifer, in the Fongchong zone, and proves that relationships do exist between points which may be located quite far apart. Such relationships should be discussed within the tectonic setting : it has been shown that fracturation develops mainly in three directions (N000-020, N060-080, N120-130); the fracture opening is maximum in the N060-080 direction which corresponds to anormal and strike-slip faults. An open fracture in that direction, joining two points, may explain the relation between them. It also appears that the water composition of springs in the plain, far from the contact fine with the fongchong zone, may be similar to the fongchong one. This proves that water from the fongchong does not only yield springs on the contact fine between fonglin and fongchong. Thus, a groundwater flow occurs from fongchong to fonglin and the hydraulic boundary conditions between the two landforms way be very complex.Finally, we propose the most probable flow pattern in the area studied. The various elements (unsaturated zone, mountain karst aquifer, fongchong aquifer, deep plain aquifer, surface plain aquifer) appear to be closely linked. Further work is underway, in order to estimate the depth of karstification, to identify the groundwater flow more precisely and to quantify if

Finite-Size Scaling and Damage Spreading in Ising Systems with Multispin Interactions

We investigate two-dimensional Ising systems with multisping interactions of three- (m=3) and four-body terms (m=4). The application of a new type of finite-size algorithm of de Oliveira allow us to clearly distinguish a first-order transition (in the m=4 case) from a continuous one (in the m=3 one). We also study the damage spreading in these systems. In this study, a dynamical phenomenon is observed to occur at a critical point separating a chaotic phase from a frozen one. However, the width of the interval where this transition happens does not yield a conclusive evidence about the order of the phase transition.Comment: 18 pages, 6 figure

Equilibrium onions?

We demonstrate the possibility of a stable equilibrium multi-lamellar ("onion") phase in pure lamellar systems (no excess solvent) due to a sufficiently negative Gaussian curvature modulus. The onion phase is stabilized by non-linear elastic moduli coupled to a polydisperse size distribution (Apollonian packing) to allow space-filling without appreciable elastic distortion. This model is compared to experiments on copolymer-decorated lamellar surfactant systems, with reasonable qualitative agreement

Investigation of the Chromatic Dispersion in Two-Section InAs/GaAs Quantum-Dot Lasers

We present the measurements of the dispersion of InAs/GaAs quantum-dot lasers emitting at 1230 nm (ground state) and 1160 nm (excited state) from the analysis of their subthreshold emission spectra. Measurements from devices with various lengths allow us to deduce that the group velocity dispersion is as high as 2270 fs2mm−1 and is mainly due to the dispersion of bulk GaAs. The gain-induced dispersion varies with the injected current at a rate of ≃−2 fs2 mA−1mm−1, whereas the effect of a saturable absorber on the dispersion is found to be negligible. These results suggest that the implementation of integrated dispersion compensation could significantly reduce the pulse duration of these lasers in mode-locked regime and lead to an enhancement of the formation of optical frequency combs in these devices

Series expansions of the percolation probability on the directed triangular lattice

We have derived long series expansions of the percolation probability for site, bond and site-bond percolation on the directed triangular lattice. For the bond problem we have extended the series from order 12 to 51 and for the site problem from order 12 to 35. For the site-bond problem, which has not been studied before, we have derived the series to order 32. Our estimates of the critical exponent $\beta$ are in full agreement with results for similar problems on the square lattice, confirming expectations of universality. For the critical probability and exponent we find in the site case: $q_c = 0.4043528 \pm 0.0000010$ and $\beta = 0.27645 \pm 0.00010$; in the bond case: $q_c = 0.52198\pm 0.00001$ and $\beta = 0.2769\pm 0.0010$; and in the site-bond case: $q_c = 0.264173 \pm 0.000003$ and $\beta = 0.2766 \pm 0.0003$. In addition we have obtained accurate estimates for the critical amplitudes. In all cases we find that the leading correction to scaling term is analytic, i.e., the confluent exponent $\Delta = 1$.Comment: 26 pages, LaTeX. To appear in J. Phys.