14 research outputs found
Electro-osmotic flow in coated nanocapillaries: a theoretical investigation
Motivated by recent experiments, we present a theoretical investigation of
how the electro-osmotic flow occurring in a capillary is modified when its
charged surfaces are coated by charged polymers. The theoretical treatment is
based on a three dimensional model consisting of a ternary fluid-mixture,
representing the solvent and two species for the ions, confined between two
parallel charged plates decorated by a fixed array of scatterers representing
the polymer coating. The electro-osmotic flow, generated by a constant electric
field applied in a direction parallel to the plates, is studied numerically by
means of Lattice Boltzmann simulations. In order to gain further understanding
we performed a simple theoretical analysis by extending the Stokes-Smoluchowski
equation to take into account the porosity induced by the polymers in the
region adjacent the walls. We discuss the nature of the velocity profiles by
focusing on the competing effects of the polymer charges and the frictional
forces they exert. We show evidence of the flow reduction and of the flow
inversion phenomenon when the polymer charge is opposite to the surface charge.
By using the density of polymers and the surface charge as control variables,
we propose a phase diagram that discriminates the direct and the reversed flow
regimes and determine its dependence on the ionic concentration.Comment: 15 pages, 6 figures in Physical Chemistry Chemical Physics, 201
Lattice Boltzmann Method for mixtures at variable Schmidt number
When simulating multicomponent mixtures via the Lattice Boltzmann Method, it
is desirable to control the mutual diffusivity between species while
maintaining the viscosity of the solution fixed. This goal is herein achieved
by a modification of the multicomponent Bhatnagar-Gross-Krook (BGK) evolution
equations by introducing two different timescales for mass and momentum
diffusion. Diffusivity is thus controlled by an effective drag force acting
between species. Numerical simulations confirm the accuracy of the method for
neutral binary and charged ternary mixtures in bulk conditions. The simulation
of a charged mixture in a charged slit channel show that the conductivity and
electro-osmotic mobility exhibit a departure from the Helmholtz-Smoluchowski
prediction at high diffusivity.Comment: 18 pages, 6 figure
A DFT study of Cr on graphene, with additional material on molecular dynamics
The aim of this work is to have an understanding of the mechanical properties
of MWCNTs under repeated twisting cycles. In particular we concentrated
on the interaction between Chromium and graphene and on how this
interaction can produce links between MWCNT’s wall
Linearized symmetrized quantum time correlation functions calculation via phase preaveraging
International audienceWe recently introduced an iterative method to compute quantum time correlation functions [Bonella et al. J. Chem.Phys 133 (16) 164105 (2010)]. There, the thermal part of the correlation function is treated exactly and, similar to the linearization techniques, at zero order of iteration only classical dynamics is required. In this work, we propose a new scheme for the zero order iteration of the method which significantly improves the efficiency of the calculations for high dimensional model systems
Short range hydrogen diffusion in Na3AlH6
Ab initio free energy and rate calculations are performed to investigate two activated mobility processes observed, respectively, in neutron scattering and anelastic spectroscopy experiments on sodium alanates. The system is modeled as a Na3AlH6 crystal hosting one hydrogen vacancy. We identify the process observed via neutron scattering with a positively charged hydrogen vacancy diffusing from the AlH52- to one of the AlH63- groups. As for the anelastic spectroscopy experiments, our calculations negate the current hypothesis on the process, i.e. local rearrangement of the H vacancy around the pentacoordinated Al group
Quantum dynamical structure factor of liquid neon via a quasiclassical symmetrized method
We apply the phase integration method for quasiclassical quantum time correlation functions [M. Monteferrante, S. Bonella, and G. Ciccotti, Mol. Phys. 109, 3015 (2011)] to compute the dynamic structure factor of liquid neon. So far the method had been tested only on model systems. By comparing our results for neon with experiments and previous calculations, we demonstrate that the scheme is accurate and efficient also for a realistic model of a condensed phase system showing quantum behavior. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4789760
The quantum free energy barrier for hydrogen vacancy diffusion in Na3AlH6.
The path integral single sweep method is used to assess quantum effects on the free energy barrier for hydrogen vacancy diffusion in a defective Na(3)AlH(6) crystal. This process has been investigated via experiments and simulations due to its potential relevance in the H release mechanism in sodium alanates, prototypical materials for solid state hydrogen storage. Previous computational studies, which used density functional methods for the electronic structure, were restricted to a classical treatment of the nuclear degrees of freedom. We show that, although they do not change the qualitative picture of the process, nuclear quantum effects reduce the free energy barrier height by about 18% with respect to the classical calculation improving agreement with available neutron scattering data
Simulating Polymerization by Boltzmann Inversion Force Field Approach and Dynamical Nonequilibrium Reactive Molecular Dynamics
The radical polymerization process of acrylate compounds is, nowadays, numerically investigated using classical force fields and reactive molecular dynamics, with the aim to probe the gel-point transition as a function of the initial radical concentration. In the present paper, the gel-point transition of the 1,6-hexanediol dimethacrylate (HDDMA) is investigated by a coarser force field which grants a reduction in the computational costs, thereby allowing the simulation of larger system sizes and smaller radical concentrations. Hence, the polymerization is investigated using reactive classical molecular dynamics combined with a dynamical approach of the nonequilibrium molecular dynamics (D-NEMD). The network structures in the polymerization process are probed by cluster analysis tools, and the results are critically compared with the similar all-atom system, showing a good agreement
Path integral based calculations of symmetrized time correlation functions. I.
In this paper, we examine how and when quantum evolution can be approximated in terms of (generalized) classical dynamics in calculations of correlation functions, with a focus on the symmetrized time correlation function introduced by Schofield. To that end, this function is expressed as a path integral in complex time and written in terms of sum and difference path variables. Taylor series expansion of the path integral's exponent to first and second order in the difference variables leads to two original developments. The first order expansion is used to obtain a simple, path integral based, derivation of the so-called Schofield's quantum correction factor. The second order result is employed to show how quantum mechanical delocalization manifests itself in the approximation of the correlation function and hinders, even in the semiclassical limit, the interpretation of the propagators in terms of sets of guiding classical trajectories dressed with appropriate weights