2,234 research outputs found

### Discrete aqueous solvent effects and possible attractive forces

We study discrete solvent effects on the interaction of two parallel charged
surfaces in ionic aqueous solution. These effects are taken into account by
adding a bilinear non-local term to the free energy of Poisson-Boltzmann
theory. We study numerically the density profile of ions between the two
plates, and the resulting inter-plate pressure. At large plate separations the
two plates are decoupled and the ion distribution can be characterized by an
effective Poisson-Boltzmann charge that is smaller than the nominal charge. The
pressure is thus reduced relative to Poisson-Boltzmann predictions. At plate
separations below ~2 nm the pressure is modified considerably, due to the
solvent mediated short-range attraction between ions in the the system. For
high surface charges this contribution can overcome the mean-field repulsion
giving rise to a net attraction between the plates.Comment: 12 figures in 16 files. 19 pages. Submitted to J. Chem. Phys., July
200

### Improved real-space genetic algorithm for crystal structure and polymorph prediction

Existing genetic algorithms for crystal structure and polymorph prediction can suffer from stagnation during evolution, with a consequent loss of efficiency and accuracy. An improved genetic algorithm is introduced herein which penalizes similar structures and so enhances structural diversity in the population at each generation. This is shown to improve the quality of results found for the theoretical prediction of simple model crystal structures. In particular, this method is demonstrated to find three new zero-temperature phases of the Dzugutov potential that have not been previously reported

### Patterning of dielectric nanoparticles using dielectrophoretic forces generated by ferroelectric polydomain films

A theoretical study of a dielectrophoretic force, i.e. the force acting on an
electrically neutral particle in the inhomogeneous electric field, which is
produced by a ferroelectric domain pattern, is presented. It has been shown by
several researchers that artificially prepared domain patterns with given
geometry in ferroelectric single crystals represent an easy and flexible method
for patterning dielectric nanoobjects using dielectrophoretic forces. The
source of the dielectrophoretic force is a strong and highly inhomogeneous
(stray) electric field, which exists in the vicinity of the ferroelectric
domain walls at the surface of the ferroelectric film. We analyzed
dielectrophoretic forces in the model of a ferroelectric film of a given
thickness with a lamellar 180${}^\circ$ domain pattern. The analytical formula
for the spatial distribution of the stray field in the ionic liquid above the
top surface of the film is calculated including the effect of free charge
screening. The spatial distribution of the dielectrophoretic force produced by
the domain pattern is presented. The numerical simulations indicate that the
intersection of the ferroelectric domain wall and the surface of the
ferroelectric film represents a trap for dielectric nanoparticles in the case
of so called positive dielectrophoresis. The effects of electrical neutrality
of dielectric nanoparticles, free charge screening due to the ionic nature of
the liquid, domain pattern geometry, and the Brownian motion on the mechanism
of nanoparticle deposition and the stability of the deposited pattern are
discussed.Comment: Accepted in the Journal of Applied Physics, 10 pages, 5 figure

### Interfacial Tension of Electrolyte Solutions

A theory is presented to account for the increase in surface tension of water
in the presence of electrolyte. Unlike the original ``grand-canonical''
calculation of Onsager and Samaras, which relied on the Gibbs adsorption
isotherm and lead to a result which could only be expressed as an infinite
series, our approach is ``canonical'' and produces an analytic formula for the
excess surface tension. For small concentrations of electrolyte, our result
reduces to the Onsager-Samaras limiting law.Comment: contains two figures. Journal of Chemical Physics, in pres

### Electrostatic interactions between discrete helices of charge

We analytically examine the pair interaction for parallel, discrete helices
of charge. Symmetry arguments allow for the energy to be decomposed into a sum
of terms, each of which has an intuitive geometric interpretation. Truncated
Fourier expansions for these terms allow for accurate modeling of both the
axial and azimuthal terms in the interaction energy and these expressions are
shown to be insensitive to the form of the interaction. The energy is evaluated
numerically through application of an Ewald-like summation technique for the
particular case of unscreened Coulomb interactions between the charges of the
two helices. The mode structures and electrostatic energies of flexible helices
are also studied. Consequences of the resulting energy expressions are
considered for both F-actin and A-DNA aggregates

### Non-linear charge reduction effect in strongly-coupled plasmas

The charge reduction effect, produced by the nonlinear Debye screening of
high-Z charges occuring in strongly-coupled plasmas, is investigated. An
analytic asymptotic expression is obtained for the charge reduction factor
which determines the Debye-Hueckel potential generated by a charged test
particle. Its relevant parametric dependencies are analyzed and shown to
predict a strong charge reduction effect in strongly-coupled plasmas.Comment: 4 figure

### Dynamics in the Sherrington-Kirkpatrick Ising spin glass at and above Tg

A detailed numerical study is made of relaxation at equilibrium in the
Sherrington-Kirkpatrick Ising spin glass model, at and above the critical
temperature Tg. The data show a long time stretched exponential relaxation q(t)
~ exp[-(t/tau(T))^beta(T)] with an exponent beta(T) tending to ~ 1/3 at Tg. The
results are compared to those which were observed by Ogielski in the 3d ISG
model, and are discussed in terms of a phase space percolation transition
scenario.Comment: 6 pages, 7 figure

### Nonlinear dielectric effect of dipolar fluids

The nonlinear dielectric effect for dipolar fluids is studied within the
framework of the mean spherical approximation (MSA) of hard core dipolar Yukawa
fluids. Based on earlier results for the electric field dependence of the
polarization our analytical results show so-called normal saturation effects
which are in good agreement with corresponding NVT ensemble Monte Carlo
simulation data. The linear and the nonlinear dielectric permittivities
obtained from MC simulations are determined from the fluctuations of the total
dipole moment of the system in the absence of an applied electric field. We
compare the MSA based theoretical results with the corresponding Langevin and
Debye-Weiss behaviors.Comment: 10 pages including 4 figure

### All-Electron Path Integral Monte Carlo Simulations of Warm Dense Matter: Application to Water and Carbon Plasmas

We develop an all-electron path integral Monte Carlo (PIMC) method with
free-particle nodes for warm dense matter and apply it to water and carbon
plasmas. We thereby extend PIMC studies beyond hydrogen and helium to elements
with core electrons. PIMC pressures, internal energies, and pair-correlation
functions compare well with density functional theory molecular dynamics
(DFT-MD) at temperatures of (2.5-7.5)$\times10^5$ K and both methods together
form a coherent equation of state (EOS) over a density-temperature range of
3--12 g/cm$^3$ and 10$^4$--10$^9$ K

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