2,241 research outputs found
Liquid friction on charged surfaces: from hydrodynamic slippage to electrokinetics
Hydrodynamic behavior at the vicinity of a confining wall is closely related
to the friction properties of the liquid/solid interface. Here we consider,
using Molecular Dynamics simulations, the electric contribution to friction for
charged surfaces, and the induced modification of the hydrodynamic boundary
condition at the confining boundary. The consequences of liquid slippage for
electrokinetic phenomena, through the coupling between hydrodynamics and
electrostatics within the electric double layer, are explored. Strong
amplification of electro-osmotic effects is revealed, and the non-trivial
effect of surface charge is discussed. This work allows to reconsider existing
experimental data, concerning Zeta potentials of hydrophobic surfaces and
suggest the possibility to generate ``giant'' electro-osmotic and
electrophoretic effects, with direct applications in microfluidics
Substellar fragmentation in self-gravitating fluids with a major phase transition
The existence of substellar cold H2 globules in planetary nebulae and the
mere existence of comets suggest that the physics of cold interstellar gas
might be much richer than usually envisioned.
We study the case of a cold gaseous medium in ISM conditions which is subject
to a gas-liquid/solid phase transition.
First the equilibrium of general non-ideal fluids is studied using the virial
theorem and linear stability analysis. Then the non-linear dynamics is studied
by using simulations to characterize the expected formation of solid bodies
analogous to comets. The simulations are run with a state of the art molecular
dynamics code (LAMMPS). The long-range gravitational forces can be taken into
account with short-range molecular forces with finite limited computational
resources by using super-molecules, provided the right scaling is followed.
The concept of super-molecule is tested with simulations, allowing us to
correctly satisfy the Jeans instability criterion for one-phase fluids. The
simulations show that fluids presenting a phase transition are gravitationally
unstable as well, independent of the strength of the gravitational potential,
producing two distinct kinds of sub-stellar bodies, those dominated by gravity
("planetoids") and those dominated by molecular attractive force ("comets").
Observations, formal analysis and computer simulations suggest the
possibility of the formation of substellar H2 clumps in cold molecular clouds
due to the combination of phase transition and gravity. Fluids presenting a
phase transition are gravitationally unstable, independent of the strength of
the gravitational potential. Arbitrarily small H2 clumps may form even at
relatively high temperatures up to 400 - 600K, according to virial analysis.
The combination of phase transition and gravity may be relevant for a wider
range of astrophysical situations, such as proto-planetary disks.Comment: 24 pages, 44 figures. accepted for publication in A&
Free Energy Evaluation in Polymer Translocation via Jarzynski Equality
We perform, with the help of cloud computing resources, extensive Langevin
simulations which provide free energy estimates for unbiased three dimensional
polymer translocation. We employ the Jarzynski equality in its rigorous
setting, to compute the variation of the free energy in single monomer
translocation events. In our three-dimensional Langevin simulations, the
excluded-volume and van der Waals interactions between beads (monomers and
membrane atoms) are modeled through a repulsive Lennard-Jones (LJ) potential
and consecutive monomers are subject to the Finite-Extension Nonlinear Elastic
(FENE) potential. Analysing data for polymers with different lengths, the free
energy profile is noted to have interesting finite size scaling properties.Comment: 14 pages, 5 figures, Accepted for publication in Physics Letters
Monte Carlo cluster algorithm for fluid phase transitions in highly size-asymmetrical binary mixtures
Highly size-asymmetrical fluid mixtures arise in a variety of physical
contexts, notably in suspensions of colloidal particles to which much smaller
particles have been added in the form of polymers or nanoparticles.
Conventional schemes for simulating models of such systems are hamstrung by the
difficulty of relaxing the large species in the presence of the small one. Here
we describe how the rejection-free geometrical cluster algorithm (GCA) of Liu
and Luijten [Phys. Rev. Lett 92, 035504 (2004)] can be embedded within a
restricted Gibbs ensemble to facilitate efficient and accurate studies of fluid
phase behavior of highly size-asymmetrical mixtures. After providing a detailed
description of the algorithm, we summarize the bespoke analysis techniques of
Ashton et al. [J. Chem. Phys. 132, 074111 (2010)] that permit accurate
estimates of coexisting densities and critical-point parameters. We apply our
methods to study the liquid--vapor phase diagram of a particular mixture of
Lennard-Jones particles having a 10:1 size ratio. As the reservoir volume
fraction of small particles is increased in the range 0--5%, the critical
temperature decreases by approximately 50%, while the critical density drops by
some 30%. These trends imply that in our system, adding small particles
decreases the net attraction between large particles, a situation that
contrasts with hard-sphere mixtures where an attractive depletion force occurs.Comment: 11 pages, 10 figure
Neutron scattering and molecular correlations in a supercooled liquid
We show that the intermediate scattering function for neutron
scattering (ns) can be expanded naturely with respect to a set of molecular
correlation functions that give a complete description of the translational and
orientational two-point correlations in the liquid. The general properties of
this expansion are discussed with special focus on the -dependence and hints
for a (partial) determination of the molecular correlation functions from
neutron scattering results are given. The resulting representation of the
static structure factor is studied in detail for a model system using
data from a molecular dynamics simulation of a supercooled liquid of rigid
diatomic molecules. The comparison between the exact result for and
different approximations that result from a truncation of the series
representation demonstrates its good convergence for the given model system. On
the other hand it shows explicitly that the coupling between translational
(TDOF) and orientational degrees of freedom (ODOF) of each molecule and
rotational motion of different molecules can not be neglected in the
supercooled regime.Further we report the existence of a prepeak in the
ns-static structure factor of the examined fragile glassformer, demonstrating
that prepeaks can occur even in the most simple molecular liquids. Besides
examining the dependence of the prepeak on the scattering length and the
temperature we use the expansion of into molecular correlation
functions to point out intermediate range orientational order as its principle
origin.Comment: 13 pages, 7 figure
Systematic Coarse-Graining in Nucleation Theory
In this work we show that the standard method to obtain nucleation
rate-predictions with the aid of atomistic Monte-Carlo simulations leads to
nucleation rate predictions that deviate orders of magnitude from the
recent brute-force molecular dynamics simulations [J. Diemand, R. Ang\'{e}lil,
K. K. Tanaka, and H. Tanaka, J. Chem. Phys. \textbf{139}, 074309 (2013)]
conducted in the experimental accessible supersaturation regime for
Lennard-Jones argon. We argue that this is due to the truncated state space
literature mostly relies on, where the number of atoms in a nucleus is
considered the only relevant order parameter. We here formulate the
nonequilibrium statistical mechanics of nucleation in an extended state space,
where the internal energy and momentum of the nuclei is additionally
incorporated. We show that the extended model explains the lack in agreement
between the molecular dynamics simulations by Diemand et al.\ and the truncated
state space. We demonstrate additional benefits of using the extended state
space; in particular, the definition of a nucleus temperature arrises very
naturally and can be shown without further approximation to obey the
fluctuation law of McGraw and Laviolette. In addition, we illustrate that our
theory conveniently allows to extend existing theories to richer sets of order
parameters
Range separation: The divide between local structures and field theories
This work presents parallel histories of the development of two modern
theories of condensed matter: the theory of electron structure in quantum
mechanics, and the theory of liquid structure in statistical mechanics.
Comparison shows that key revelations in both are not only remarkably similar,
but even follow along a common thread of controversy that marks progress from
antiquity through to the present. This theme appears as a creative tension
between two competing philosophies, that of short range structure (atomistic
models) on the one hand, and long range structure (continuum or density
functional models) on the other. The timeline and technical content are
designed to build up a set of key relations as guideposts for using density
functional theories together with atomistic simulation.Comment: Expanded version of a 30 minute talk delivered at the 2018 TSRC
workshop on Ions in Solution, to appear in the March, 2019 issue of
Substantia (https://riviste.fupress.net/index.php/subs/index
Polyelectrolytes in Solution - Recent Computer Simulations
We present a short overview over recent MD simulations of systems of fully
flexible polyelectrolyte chains with explicitly treated counter ions using the
full Coulomb potential. The main emphasis is given on the conformational
properties of the polymers, with a short discussion on counter ion
condensation.Comment: 10 pages, including 5 figures, to appear in the proceedings of the
50th Yamada Conference on Polyelectrolytes, Inuyama, Japan (1998
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