arXiv subjects

J. M. Rubi

Publications and source records attributed to J. M. Rubi.

At least 73 records · Page 4Linked to original sources

arXiv2000

Breaking of scale-invariance symmetry in adsorption processes

Standard models of sequential adsorption are implicitly formulated in a {\em scale invariant} form, by assuming adsorption on an infinite surface, with no characteristic length scales. In real situations, however, involving complex surfaces, intrinsic length scales may be relevant. We present an analytic model of continuous random sequential adsorption, in which the scale invariance symmetry is explicitly broken. The characteristic length is imposed by a set of scattered obstacles, previously adsorbed onto the surface. We show, by means of analytic solutions and numerical simulations, the profound effects of the symmetry breaking on both the jamming limit and the correlation function of the adsorbed layer.

cond-mat.soft↗

arXiv2000

Chaining in Magnetic Colloids in the Presence of Flow

We discuss the effect of an homogeneous flow in the aggregation process of colloidal magnetic particles at moderate concentration. Situations in which the presence of flow acts in favor of the chaining process: particles assemble into chains larger than the ones emerging in the absence of flow, under the only influence of an externnaly imposed field, have been analyzed. The results we obtain follow from the analysis of the pair correlation function which, owing to the potencial character of the flow we consider, can be interpreted in terms of a Boltzmann-like stationary distribution function. To render the influence of the flow on the resulting structures explicit, we study the particular cases of axisymmetric and planar elongational flows.

cond-mat.stat-mech↗

arXiv2000

Dipolar interactions induced order in assemblies of magnetic particles

We discuss the appareance of ordered structures in assemblies of magnetic particles. The phenomenon occurs when dipolar interactions and the thermal motion of the particles compete, and is mediated by screening and excluded volume effects. It is observed irrespective of the dimensionality of the system and the resulting structures, which may be regular or fractal, indicate that new ordered phases may emerge in these system when dipolar interactions play a significant role.

cond-mat.soft↗

arXiv2000

Nonequilibrium Thermodynamics of Colloids

The common rules of classical nonequilibrium thermodynamics do not allow an Onsager coefficient like the viscosity to depend on the shear rate. Such a dependence is experimentally well documented, however. In this paper it is shown, using nonequilibrium thermodynamics alone, how it is possible that the effective viscosity coefficient of an isotropic colloidal system depends on both the shear rate and the frequency.

cond-mat.soft↗

arXiv1999

Non-equilibrium Thermodynamics and Hydrodynamic Fluctuations

The reformulation of nonequilibirum thermodynamics, to include the treatment of thermodynamic fluctuations, is applied to the hydrodynamic fluctuations of a simple fluid. It is shown that the nonequilibrium thermodynamic scheme leads to the explicit form of the Fokker-Planck equation which describes the time behaviour of the probability distribution function of these hydrodynamic fluctuations as well as the irreversible processes which are connected with this behaviour.

cond-mat.stat-mech↗

arXiv1999

Electron spectroscopy of the shot noise reduction effect

A general formula for current noise in a two-terminal ballistic nondegenerate conductor under the action of long-range Coulomb correlations has been derived. The noise reduction factor (in respect to the uncorrelated value) is obtained for biases ranging from thermal to shot noise limits, and it is related to spatial variations in transport characteristics. The contributions of different energy groups of carriers to the noise are found, that leads us to suggest an electron energy spectroscopy experiment to probe the Coulomb correlations in ballistic conductors.

cond-mat.mes-hall↗

arXiv1999

A Mesoscopic Approach to the ``Negative'' Viscosity Effect in Ferrofluids

We present a mesoscopic approach to analyze the dynamics of a single magnetic dipole under the influence of an oscillating magnetic field, based on the formulation of a Fokker-Planck equation. The dissipated power and the viscosity of a suspension of such magnetic dipoles are calculated from non-equilibrium thermodynamics of magnetized systems. By means of this method we have found a non-monotonous behaviour of the viscosity as a function of the frequency of the field which has been referred to as the ``negative'' viscosity effect. Moreover, we have shown that the viscosity depends on the vorticity field thus exhibiting non-Newtonian behaviour. Our analysis is complemented with numerical simulations which reproduce the behaviour of the viscosity we have found and extend the scope of our analytical approach to higher values of the magnetic field.

cond-mat↗

arXiv1998

A field-oriented chain of dipolar particles in elongational flow

We study the behavior of an isolated field-oriented chain of dipolar particles in elongational fluid flow. Our main goal is to emphasize the effect of dipolar interactions on the chain's contribution to the pressure tensor and to the viscosities of a dilute suspension of these linear aggregates. In our model, despite the overall rigid appearance of the chain at rest, the constituent beads may move slightly relative to one another, conferring a certain degree of flexibility to the chain. This flexibility is quantified in terms of a dimensionless parameter, $λ^{-1}$, comparing thermal and dipolar energies. We perform an expansion in $λ^{-1}$, and obtain the first correction to the rigid chain contribution to the Kramers' pressure tensor for different flow geometries. The interplay of the elongational flow field and the field-induced chain orientation gives rise to a rich variety of scenarios. We compute the elongational, shear, and rotational viscosities in some representative situations.

cond-mat.soft↗

arXiv1998

Inertial Effects in Non-Equilibrium Thermodynamics

We discuss inertial effects in systems outside equilibrium within the framework of non-equilibrium thermodynamics. By introducing a Gibbs equation in which the entropy depends on the probability density, we are able to describe a system of Brownian particles immersed in a heat bath in both inertial and diffusion regimes. In the former, a relaxation equation for the diffusion current is obtained whereas in the latter we recover Fick's law. Our approach, which uses the elements of the theory of internal degrees of freedom, constitutes the mesoscopic version of a previous analysis which takes into account the kinetic energy of diffusion.

cond-mat.stat-mech↗

arXiv1998

Kramers-like Picture for Crystal Nucleation

We introduce a new scheme to analyze the kinetics of homogeneous nucleation in terms of a global order parameter. Our approach is based on the application of the internal degrees of freedom formalism to derive a kinetic equation of the Kramers type formulated for a global reaction coordinate. We provide explicit expressions for the quantities and coefficients involved in the process, suitable for simulation. In addition, our picture recovers in the quasi-stationary case the transition rate obtained from the method of reactive flux. The equation we present may provide a link between theoretical approaches to homogeneous nucleation (generally formulated in terms of a kinetic equation of the Fokker-Planck type) and simulations (which mostly employ linear response theory). In this context, our scheme provides a theoretical framework to interpret and extend the results obtained in recent simulations.

cond-mat.stat-mech↗

arXiv1998

Long-range-interactions induced ordered structures in deposition processes

We present a new model of sequential adsorption in which the adsorbing particles experience dipolar interactions. We show that in the presence of these long-range interactions, highly ordered structures in the adsorbed layer may be induced at low temperatures. The new phenomenology manifests through significant variations of the pair correlation function and the jamming limit, with respect to the case of noninteracting particles. Our study could be relevant in understanding the adsorption of magnetic colloidal particles in presence of a magnetic field.

cond-mat.soft↗

arXiv1998

Universality of the 1/3 shot-noise suppression factor in nondegenerate diffusive conductors

Shot-noise suppression is investigated in nondegenerate diffusive conductors by means of an ensemble Monte Carlo simulator. The universal 1/3 suppression value is obtained when transport occurs under elastic collision regime provided the following conditions are satisfied: (i) The applied voltage is much larger than the thermal value; (ii) the length of the device is much longer than both the elastic mean free path and the Debye length. By fully suppressing carrier-number fluctuations, long range Coulomb interaction is essential to obtain the 1/3 value in the low-frequency limit.

cond-mat.mes-hall↗

arXiv1998

Fokker-Planck Equations for Nucleation Processes Revisited

We present a new approach to analyze homogeneous nucleation based on non-equilibrium thermodynamics. The starting point is the formulation of a Gibbs equation for the variations of the entropy of the system, whose state is characterized by an internal coordinate or degree of freedom. By applying the method of non-equilibrium thermodynamics we then obtain the entropy production corresponding to a diffusion process in the internal space. The linear laws together with the continuity equation lead to a kinetic equation of the Fokker-Planck type. By choosing properly the degree of freedom we are able to obtain a new kinetic equation for a global crystallization order parameter (used in recent simulations), and also we recover some of the existing equations. The consistency of the scheme we propose is proved in the quasi-stationary case. Finally, we also outline the way in which our formalism could be extended to more general situations.

cond-mat↗

arXiv1998

Periodic Modulation Induced Increase of Reaction Rates in Autocatalytic Systems

We propose a new mechanism to increase the reactions ratesin multistable autocatalytic systems. The mechanism is based upon the possibility for the enhancement of the response of the system due to the cooperative behavior between the noise and an external periodic modulation. In order to illustrate this feature we compute the reaction velocities for the particular case of the Sel'Kov model, showing that they increase significantly when the periodic modulation is introduced. This behavior originates from the existence of a minimum in the mean first passage time, one of the signatures of stochastic resonance.

cond-mat↗

arXiv1998

Stochastic Resonance in Nonpotential Systems

We propose a method to analytically show the possibility for the appearance of a maximum in the signal-to-noise ratio in nonpotential systems. We apply our results to the FitzHugh-Nagumo model under a periodic external forcing, showing that the model exhibits stochastic resonance. The procedure that we follow is based on the reduction to a one-dimensional dynamics in the adiabatic limit, and in the topology of the phase space of the systems under study. Its application to other nonpotential systems is also discussed.

cond-mat↗

arXiv1997

Electron-number statistics and shot-noise suppression by Coulomb correlation in nondegenerate ballistic transport

Within a Monte Carlo simulation we investigate the statistical properties of an electron flow injected with a Poissonian distribution and transmitted under ballistic regime in the presence of long-range Coulomb interaction. Electrons are shown to exhibit a motional squeezing which tends to space them more regularly rather than strictly at random, and evidence a sub-Poissonian statistics with a substantially reduced Fano factor $F_n\ll 1$. The temporal (anti)correlation among carriers is demonstrated to be a collective effect which persists over the transit of several successive electrons and results in a considerable (more than one order of magnitude) shot noise suppression.

cond-mat.mes-hall↗

arXiv1997

Noise suppression due to long-range Coulomb interaction: Crossover between diffusive and ballistic transport regimes

We present a Monte Carlo analysis of shot-noise suppression due to long-range Coulomb interaction in semiconductor samples under a crossover between diffusive and ballistic transport regimes. By varying the mean time between collisions we find that the strong suppression observed under the ballistic regime persists under quasi-ballistic conditions, before being washed out when a complete diffusive regime is reached.

cond-mat.mes-hall↗

arXiv1997

Fractal properties of clusters of colloidal magnetic particles

We have studied the properties of clusters of colloidal magnetic particles generated from a 2D aggregation model with dipolar interparticle interactions. Particles diffuse off-lattice, experiencing dipolar interactions with the already attached particles until either they stick to the cluster or wander far away and are removed. Our results are interpreted in terms of a fractal dimension that is a monotonically decreasing function of the temperature, varying between a definite value close to 1 at T=0, and the limit $T\to\infty$, corresponding to free diffusion-limited aggregation. By analyzing orientational correlation functions, an ordered state is found at low temperatures; this state is destroyed by the fractal disorder generated at high $T$. Our study could be relevant in understanding aggregation of dipolar colloids and phase transitions in Langmuir monolayers.

cond-mat↗