arXiv subjects

Enrique Abad

Publications and source records attributed to Enrique Abad.

7 recordsLinked to original sources

arXiv2023

Diffusion of intruders in granular suspensions: Enskog theory and random walk interpretation

The Enskog kinetic theory is applied to compute the mean square displacement of intruders immersed in a granular gas of smooth inelastic hard spheres (grains). Both species (intruders and grains) are surrounded by an interstitial molecular gas (background) that plays the role of a thermal bath. The influence of the latter on the motion of intruders and grains is modeled via a standard viscous drag force supplemented by a stochastic Langevin-like force proportional to the background temperature. We solve the corresponding Enskog--Lorentz kinetic equation by means of the Chapman--Enskog expansion truncated to first order in the gradient of the intruder number density. The integral equation for the diffusion coefficient is solved by considering the first two Sonine approximations. To test these results, we also compute the diffusion coefficient from the numerical solution of the inelastic Enskog equation by means of the direct simulation Monte Carlo method. We find that the first Sonine approximation generally agrees well with the simulation results, although significant discrepancies arise when the intruders become lighter than the grains. Such discrepancies are largely mitigated by the use of the second-Sonine approximation, in excellent agreement with computer simulations even for moderately strong inelasticities and/or dissimilar mass and diameter ratios. We invoke a random walk picture of the intruders' motion to shed light on the physics underlying the intricate dependence of the diffusion coefficient on the main system parameters. This approach, recently employed to study the case of an intruder immersed in a granular gas, also proves useful in the present case of a granular suspension. Finally, we discuss the applicability of our model to real systems in the self-diffusion case. We conclude that collisional effects may strongly impact the diffusion coefficient of the grains.

cond-mat.soft↗

arXiv2022

On the mean square displacement of intruders in freely cooling granular gases

We compute the mean square displacement (MSD) of intruders immersed in a freely cooling granular gas made up of smooth inelastic hard spheres. In general, intruders and particles of the granular gas are assumed to have different mechanical properties, implying that non-equipartition of energy must be accounted for in the computation of the diffusion coefficient $D$. In the hydrodynamic regime, the time decay of the granular temperature $T$ of the cooling granular gas is known to be dictated by Haff's law; the corresponding decay of the intruder's collision frequency entails a time decrease of the diffusion coefficient $D$. Explicit knowledge of this time dependence allows us to determine the MSD by integrating the corresponding diffusion equation. As in previous studies of self-diffusion (intruders mechanically equivalent to gas particles) and the Brownian limit (intruder's mass much larger than the grain's mass), we find a logarithmic time dependence of the MSD as a consequence of Haff's law. Beyond the logarithmic time growth, we find that the MSD depends on the mechanical system parameters in a highly complex way. To explain the observed behaviour, we analyze in detail the intruder's random walk, consisting of ballistic displacements interrupted by anisotropic deflections caused by the collisions with the hard spheres. We also show that the MSD can be thought of as arising from an equivalent random walk with isotropic, uncorrelated steps.

cond-mat.soft↗

arXiv2019

A game-based educational method relying on student-generated questions

Student-generated multiple-choice questions (MCQs) revised by the instructor were used to design an educational game and to set part of the exam in the framework of an elementary course in Photonics. An anonymous survey among the students revealed their general satisfaction with the method. The exam results of students who had been asked to author MCQs were compared to those of a control group subject to traditional teaching. The results of the former in the MCQ test of the exam were significantly better, without this being detrimental to their comparative performance in problem-solving.

physics.ed-ph↗

arXiv2014

Photo-induced reactions from efficient molecular dynamics with electronic transitions using the FIREBALL local-orbital density functional theory formalism

The computational simulation of photo-induced processes in large molecular systems is a very challenging problem. Here, we present a detailed description of our implementation of a molecular dynamics with electronic transitions algorithm within the local-orbital density functional theory code Fireball, suitable for the computational study of these problems. Our methodology enables simulating photo-induced reaction mechanisms over hundreds of trajectories; therefore, large statistically significant ensembles can be calculated to accurately represent a reaction profile. As an example of the application of this approach, we report results on the [2+2] cycloaddition of ethylene with maleic anhydride and on the [2+2] photo-induced polymerization reaction of two C60 molecules. We identify different deactivation channels of the initial electron excitation, depending on the time of the electronic transition from LUMO to HOMO, and the character of the HOMO after the transition.

physics.comp-ph↗

arXiv2013

Barrier Height Formation in Organic Blends / Metal Interfaces: Case of (TTF-TCNQ) / Au(111)

The interface between the tetrathiafulvalene / tetracyanoquinodimethane (TTFTCNQ) organic blend and the Au(111) metal surface is analyzed by Density Functional Theory calculations, including the effect of the charging energies on the molecule transport gaps. Given the strong donor and acceptor characters of the TTF and TCNQ molecules, respectively, there is a strong intermolecular interaction, with a relatively high charge transfer between the two organic materials. We find that the TCNQ LUMO peak is very close to the Fermi level; due to the interaction with the metal surface, the organic blend molecular levels are broadened, creating an important induced density of interface states. We show that the interface energy level alignment is controlled by the charge transfer between TTF, TCNQ and Au, and by the molecular dipoles created in the molecules because of their deformations when adsorbed on Au(111); in particular the TCNQ molecules present a bent adsorption geometry with the N atoms bonded to the Au surface. A generalization of the Unified-IDIS model, to explain how the interface energy levels alignment is achieved for the case of this blended organic layer, is presented by introducing matrix equations associated with the Charge Neutrality Levels of both organic materials and with their intermixed screening properties.

cond-mat.mtrl-sci↗

arXiv2011

On a novel iterative method to compute polynomial approximations to Bessel functions of the first kind and its connection to the solution of fractional diffusion/diffusion-wave problems

We present an iterative method to obtain approximations to Bessel functions of the first kind $J_p(x)$ ($p>-1$) via the repeated application of an integral operator to an initial seed function $f_0(x)$. The class of seed functions $f_0(x)$ leading to sets of increasingly accurate approximations $f_n(x)$ is considerably large and includes any polynomial. When the operator is applied once to a polynomial of degree $s$, it yields a polynomial of degree $s+2$, and so the iteration of this operator generates sets of increasingly better polynomial approximations of increasing degree. We focus on the set of polynomial approximations generated from the seed function $f_0(x)=1$. This set of polynomials is not only useful for the computation of $J_p(x)$, but also from a physical point of view, as it describes the long-time decay modes of certain fractional diffusion and diffusion-wave problems.

math-ph↗

arXiv1998

Brownian motion in fluctuating periodic potentials

This work deals with the overdamped motion of a particle in a fluctuating one-dimensional periodic potential. If the potential has no inversion symmetry and its fluctuations are asymmetric and correlated in time, a net flow can be generated at finite temperatures. We present results for the stationary current for the case of a piecewise linear potential, especially for potentials being close to the case with inversion symmetry. The aim is to study the stationary current as a function of the potential. Depending on the form of the potential, the current changes sign once or even twice as a function of the correlation time of the potential fluctuations. To explain these current reversals, several mechanisms are proposed. Finally, we discuss to what extent the model is useful to understand the motion of biomolecular motors.

cond-mat↗