Effective interactions and phase behaviour for a model clay suspension in an electrolyte

Since the early observation of nematic phases of disc-like clay colloids by Langmuir in 1938, the phase behaviour of such systems has resisted theoretical understanding. The main reason is that there is no satisfactory generalization for charged discs of the isotropic DLVO potential describing the effective interactions between a pair of spherical colloids in an electrolyte. In this contribution, we show how to construct such a pair potential, incorporating approximately both the non-linear effects of counter-ion condensation (charge renormalization) and the anisotropy of the charged platelets. The consequences on the phase behaviour of Laponite dispersions (thin discs of 30 nm diameter and 1 nm thickness) are discussed, and investigation into the mesostructure via Monte Carlo simulations are presented.Comment: LaTeX, 12 pages, 11 figure

Slow flows of yield stress fluids: complex spatio-temporal behaviour within a simple elasto-plastic model

A minimal athermal model for the flow of dense disordered materials is proposed, based on two generic ingredients: local plastic events occuring above a microscopic yield stress, and the non-local elastic release of the stress these events induce in the material. A complex spatio-temporal rheological behaviour results, with features in line with recent experimental observations. At low shear rates, macroscopic flow actually originates from collective correlated bursts of plastic events, taking place in dynamically generated fragile zones. The related correlation length diverges algebraically at small shear rates. In confined geometries bursts occur preferentially close to the walls yielding an intermittent form of flow localization.Comment: 4 pages, 4 figure

Ion specificity and anomalous electrokinetic effects in hydrophobic nanochannels

We demonstrate with computer simulations that anomalous electrokinetic effects, such as ion specificity and non-zero zeta potentials for uncharged surfaces, are generic features of electro-osmotic flow in hydrophobic channels. This behavior is due to the stronger attraction of larger ions to the ``vapour--liquid-like'' interface induced by a hydrophobic surface. An analytical model involving a modified Poisson--Boltzmann description for the ion density distributions is proposed, which allows the anomalous flow profiles to be predicted quantitatively. This description incorporates as a crucial component an ion-size-dependent hydrophobic solvation energy. These results provide an effective framework for predicting specific ion effects, with important implications for the modeling of biological problems

Degenerate Kalman filter error covariances and their convergence onto the unstable subspace

The characteristics of the model dynamics are critical in the performance of (ensemble) Kalman filters. In particular, as emphasized in the seminal work of Anna Trevisan and coauthors, the error covariance matrix is asymptotically supported by the unstable-neutral subspace only, i.e., it is spanned by the backward Lyapunov vectors with nonnegative exponents. This behavior is at the core of algorithms known as assimilation in the unstable subspace, although a formal proof was still missing. This paper provides the analytical proof of the convergence of the Kalman filter covariance matrix onto the unstable-neutral subspace when the dynamics and the observation operator are linear and when the dynamical model is error free, for any, possibly rank-deficient, initial error covariance matrix. The rate of convergence is provided as well. The derivation is based on an expression that explicitly relates the error covariances at an arbitrary time to the initial ones. It is also shown that if the unstable and neutral directions of the model are sufficiently observed and if the column space of the initial covariance matrix has a nonzero projection onto all of the forward Lyapunov vectors associated with the unstable and neutral directions of the dynamics, the covariance matrix of the Kalman filter collapses onto an asymptotic sequence which is independent of the initial covariances. Numerical results are also shown to illustrate and support the theoretical findings

Jordan-Wigner Approach to Dynamic Correlations in 2D Spin-1/2 Models

We discuss the dynamic properties of the square-lattice spin-1/2 XY model obtained using the two-dimensional Jordan-Wigner fermionization approach. We argue the relevancy of the fermionic picture for interpreting the neutron scattering measurements in the two-dimensional frustrated quantum magnet Cs_2CuCl_4.Comment: Presented at 12-th Czech and Slovak Conference on Magnetism, Ko\v{s}ice, 12-15 July 200

Thermally Activated Dynamics of the Capillary Condensation

This paper is devoted to the thermally activated dynamics of the capillary condensation. We present a simple model which enables us to identify the critical nucleus involved in the transition mechanism. This simple model is then applied to calculate the nucleation barrier from which we can obtain informations on the nucleation time. We present a simple estimation of the nucleation barrier in slab geometry both in the two dimensional case and in the three dimensional case. We extend the model in the case of rough surfaces which is closer to the experimental case and allows comparison with experimental datas.Comment: 6 pages, 3 figures, Submitted to J. Phys. : Condens. Matter, Proceedings of the IV Liquid Matter Conference - Grenada(Spain) july 199

Statistical characterization of the forces on spheres in an upflow of air

The dynamics of a sphere fluidized in a nearly-levitating upflow of air were previously found to be identical to those of a Brownian particle in a two-dimensional harmonic trap, consistent with a Langevin equation [Ojha {\it et al.}, Nature {\bf 427}, 521 (2004)]. The random forcing, the drag, and the trapping potential represent different aspects of the interaction of the sphere with the air flow. In this paper we vary the experimental conditions for a single sphere, and report on how the force terms in the Langevin equation scale with air flow speed, sphere radius, sphere density, and system size. We also report on the effective interaction potential between two spheres in an upflow of air.Comment: 7 pages, experimen

Adsorption kinetics in open nanopores as a source of low-frequency noise

Ionic current measurements through solid-state nanopores consistently show a power spectral density that scales as 1/f α at low frequency f, with an exponent α ∼ 0.5–1.5, but strikingly, the physical origin of this behavior remains elusive. Here, we perform simulations of particles reversibly adsorbing at the surface of a nanopore and show that the fluctuations in the number of adsorbed particles exhibit low-frequency pink noise. We furthermore propose theoretical modeling for the time-dependent adsorption of particles on the nanopore surface for various geometries, which predicts a frequency spectrum in very good agreement with the simulation results. Altogether, our results highlight that the low-frequency noise takes its origin in the reversible adsorption of ions at the pore surface combined with the long-lasting excursions of the ions in the reservoirs. The scaling regime of the power spectrum extends down to a cutoff frequency which is far smaller than simple diffusion estimates. Using realistic values for the pore dimensions and the adsorption–desorption kinetics, this predicts the observation of pink noise for frequencies down to the hertz for a typical solid-state nanopore, in good agreement with experiments

Stellar Mass to Halo Mass Scaling Relation for X-ray Selected Low Mass Galaxy Clusters and Groups out to Redshift z≈1z\approx1

We present the stellar mass-halo mass scaling relation for 46 X-ray selected low-mass clusters or groups detected in the XMM-BCS survey with masses $2\times10^{13}M_{\odot}\lesssim M_{500}\lesssim2.5\times10^{14}M_{\odot}$ at redshift $0.1\le z \le1.02$. The cluster binding masses $M_{500}$ are inferred from the measured X-ray luminosities \Lx, while the stellar masses $M_{\star}$ of the galaxy populations are estimated using near-infrared imaging from the SSDF survey and optical imaging from the BCS survey. With the measured \Lx\ and stellar mass $M_{\star}$, we determine the best fit stellar mass-halo mass relation, accounting for selection effects, measurement uncertainties and the intrinsic scatter in the scaling relation. The resulting mass trend is $M_{\star}\propto M_{500}^{0.69\pm0.15}$, the intrinsic (log-normal) scatter is $\sigma_{\ln M_{\star}|M_{500}}=0.36^{+0.07}_{-0.06}$, and there is no significant redshift trend $M_{\star}\propto (1+z)^{-0.04\pm0.47}$, although the uncertainties are still large. We also examine $M_{\star}$ within a fixed projected radius of $0.5$~Mpc, showing that it provides a cluster binding mass proxy with intrinsic scatter of $\approx93\%$ (1$\sigma$ in $M_{500}$). We compare our $M_{\star}=M_{\star}(M_{500}, z)$ scaling relation from the XMM-BCS clusters with samples of massive, SZE-selected clusters ($M_{500}\approx6\times10^{14}M_{\odot}$) and low mass NIR-selected clusters ($M_{500}\approx10^{14}M_{\odot}$) at redshift $0.6\lesssim z \lesssim1.3$. After correcting for the known mass measurement systematics in the compared samples, we find that the scaling relation is in good agreement with the high redshift samples, suggesting that for both groups and clusters the stellar content of the galaxy populations within $R_{500}$ depends strongly on mass but only weakly on redshift out to $z\approx1$.Comment: 15 pages, 10 figures. Accepted for publication in MNRA