More on phase diagram of Laponite

The phase diagram of a charged colloidal system (Laponite) has been investigated by dynamic light scattering in a previously unexplored range of salt and clay concentrations. Specifically the clay weight and salt molar concentrations have been varied in the ranges Cw=0.004- 0.025, Cs=(1x 10^-3- 5x 10^-3) M respectively. As in the case of free salt water samples (Cs= 1x 10^-4 M) an aging dynamics towards two different arrested phases is found in the whole examined Cw and Cs range. Moreover a transition between these two different regimes is found for each investigated salt concentration. It is clear from these measurements that a revision of the phase diagram is necessary and a new "transition" line between two different arrested states is drawn.Comment: 16 pages, 5 figures, submitted to Langmui

Routes to gelation in a clay suspension

The gelation of water suspension of a synthetic clay (Laponite) has been studied by dynamic light scattering in a wide range of clay weight concentration (Cw = 0.003-0.031). At variance with previous determination, indicating a stable liquid phase for Cw < Cw*=0.015-0.018, we find that the gelation takes actually place in the whole examined Cw range. More importantly, we find that Cw* marks the transition between two different routes to gelation. We hypothesize that at low concentration Laponite suspension behaves as an attractive colloid and that the slowing down of the dynamics is attained by the formation of larger and larger clusters while at high concentration the basic units of the gel could be the Debye Huckel spheres associated to single Laponite plates.Comment: 5 pages, 4 figure

Influence of an adsorbing polymer in the aging dynamics of Laponite clay suspensions

Clay-polymer dispersions in aqueous solutions have attracted a great interest in recent years due to their industrial applications and intriguing physical properties. Aqueous solutions of bare Laponite particles are known to age spontaneously from an ergodic state to a non ergodic state in a time varying from hours to months depending on Laponite concentration. When a polymer species like Polyethylene Oxide (PEO) is added to the solution, it weakly adsorbs on clay particle surfaces modifying the effective interaction potential between Laponite particles. A dynamic light scattering study, varying polymer concentration at fixed polymer molecular weight (Mw=200.000 g/mol), has been performed in order to understand the effect of polymer on the aging dynamics of the system. The results obtained show that arresting phenomena between clay particles are hindered if PEO is added and consequently the aging dynamics slows down with increasing PEO concentration. This process is possibly due to the progressive coverage of the clay surface by polymers that grow with increasing PEO concentration and may lead to steric stabilization.Comment: 13 pages, 6 figures, manuscript accepted for publication on Philosophical Magazin

Fluctuations of entropy production in the isokinetic ensemble

We discuss the microscopic definition of entropy production rate in a model of a dissipative system: a sheared fluid in which the kinetic energy is kept constant via a Gaussian thermostat. The total phase space contraction rate is the sum of two statistically independent contributions: the first one is due to the work of the conservative forces, is independent of the driving force and does not vanish at zero drive, making the system non-conservative also in equilibrium. The second is due to the work of the dissipative forces, and is responsible for the average entropy production; the distribution of its fluctuations is found to verify the Fluctuation Relation of Gallavotti and Cohen. The distribution of the fluctuations of the total phase space contraction rate also verify the Fluctuation Relation. It is compared with the same quantity calculated in the isoenergetic ensemble: we find that the two ensembles are equivalent, as conjectured by Gallavotti. Finally, we discuss the implication of our results for experiments trying to verify the validity of the FR.Comment: 8 pages, 4 figure

Phase diagram and complexity of mode-locked lasers: from order to disorder

We investigate mode-locking processes in lasers displaying a variable degree of structural randomness, from standard optical cavities to multiple-scattering media. By employing methods mutuated from spin-glass theory, we analyze the mean-field Hamiltonian and derive a phase-diagram in terms of the pumping rate and the degree of disorder. Three phases are found: i) paramagnetic, corresponding to a noisy continuous wave emission, ii) ferromagnetic, that describes the standard passive mode-locking, and iii) the spin-glass in which the phases of the electromagnetic field are frozen in a exponentially large number of configurations. The way the mode-locking threshold is affected by the amount of disorder is quantified. The results are also relevant for other physical systems displaying a random Hamiltonian, like Bose-Einstein condensates and nonlinear optical beams.Comment: 4 pages, 2 figure

Saddles and softness in simple model liquids

We report a numerical study of saddles properties of the potential energy landscape for soft spheres with different softness, i.e. different power n of the interparticle repulsive potential. We find that saddle-based quantities rescale into master curves once energies and temperatures are scaled by mode-coupling temperature T_MCT, confirming and generalizing previous findings obtained for Lennard-Jones like models.Comment: 2 pages, 2 figure

Light diffusion and localization in 3D nonlinear disordered media

Using a 3D Finite-Difference Time-Domain parallel code, we report on the linear and nonlinear propagation of light pulses in a disordered assembly of scatterers, whose spatial distribution is generated by a Molecular Dynamics code; refractive index dispersion is also taken into account. We calculate the static and dynamical diffusion constant of light, while considering a pulsed excitation. Our results are in quantitative agreement with reported experiments, also furnishing evidence of a non-exponential decay of the transmitted pulse in the linear regime and in the presence of localized modes. By using an high power excitation, we numerically demonstrate the ``modulational instability random laser'': at high peak input powers energy is transferred to localized states from the input pulse, via third-order nonlinearity and optical parametric amplification, and this process is signed by a power-dependent non-exponential time-decay of the transmitted pulse.Comment: 5 pages, 4 figures. Revised version with new figure 4 with localized state

Quasi-saddles as relevant points of the potential energy surface in the dynamics of supercooled liquids

The supercooled dynamics of a Lennard-Jones model liquid is numerically investigated studying relevant points of the potential energy surface, i.e. the minima of the square gradient of total potential energy $V$. The main findings are: ({\it i}) the number of negative curvatures $n$ of these sampled points appears to extrapolate to zero at the mode coupling critical temperature $T_c$; ({\it ii}) the temperature behavior of $n(T)$ has a close relationship with the temperature behavior of the diffusivity; ({\it iii}) the potential energy landscape shows an high regularity in the distances among the relevant points and in their energy location. Finally we discuss a model of the landscape, previously introduced by Madan and Keyes [J. Chem. Phys. {\bf 98}, 3342 (1993)], able to reproduce the previous findings.Comment: To be published in J. Chem. Phy

General features of the energy landscape in Lennard-Jones like model liquids

Features of the energy landscape sampled by supercooled liquids are numerically analyzed for several Lennard-Jones like model systems. The properties of quasisaddles (minima of the square gradient of potential energy W=|grad V|^2), are shown to have a direct relationship with the dynamical behavior, confirming that the quasisaddle order extrapolates to zero at the mode-coupling temperature T_MCT. The same result is obtained either analyzing all the minima of W or the saddles (absolute minima of W), supporting the conjectured similarity between quasisaddles and saddles, as far as the temperature dependence of the properties influencing the slow dynamics is concerned. We find evidence of universality in the shape of the landscape: plots for different systems superimpose into master curves, once energies and temperatures are scaled by T_MCT. This allows to establish a quantitative relationship between T_MCT and potential energy barriers for LJ-like systems, and suggests a possible generalization to different model liquids.Comment: 8 pages, 5 figure

Generalized fluctuation relation and effective temperatures in a driven fluid

By numerical simulation of a Lennard-Jones like liquid driven by a velocity gradient \gamma we test the fluctuation relation (FR) below the (numerical) glass transition temperature T_g. We show that, in this region, the FR deserves to be generalized introducing a numerical factor X(T,\gamma)<1 that defines an ``effective temperature'' T_{FR}=T/X. On the same system we also measure the effective temperature T_{eff}, as defined from the generalized fluctuation-dissipation relation, and find a qualitative agreement between the two different nonequilibrium temperatures.Comment: Version accepted for publication on Phys.Rev.E; major changes, 1 figure adde