398 research outputs found
Influence of Disorder Strength on Phase Field Models of Interfacial Growth
We study the influence of disorder strength on the interface roughening
process in a phase-field model with locally conserved dynamics. We consider two
cases where the mobility coefficient multiplying the locally conserved current
is either constant throughout the system (the two-sided model) or becomes zero
in the phase into which the interface advances (one-sided model). In the limit
of weak disorder, both models are completely equivalent and can reproduce the
physical process of a fluid diffusively invading a porous media, where
super-rough scaling of the interface fluctuations occurs. On the other hand,
increasing disorder causes the scaling properties to change to intrinsic
anomalous scaling. In the limit of strong disorder this behavior prevails for
the one-sided model, whereas for the two-sided case, nucleation of domains in
front of the invading front are observed.Comment: Accepted for publication in PR
Ionic current inversion in pressure-driven polymer translocation through nanopores
We predict streaming current inversion with multivalent counterions in
hydrodynamically driven polymer translocation events from a
correlation-corrected charge transport theory including charge fluctuations
around mean-field electrostatics. In the presence of multivalent counterions,
electrostatic many-body effects result in the reversal of the DNA charge. The
attraction of anions to the charge-inverted DNA molecule reverses the sign of
the ionic current through the pore. Our theory allows for a comprehensive
understanding of the complex features of the resulting streaming currents. The
underlying mechanism is an efficient way to detect DNA charge reversal in
pressure-driven translocation experiments with multivalent cations.Comment: This version is accepted for publication in Physical Review Letter
Bit Level Correlations in Some Pseudorandom Number Generators
We present results of extensive bit level tests on some pseudorandom number
generators which are commonly used in physics applications. The generators have
first been tested with an extended version of the -tuple test. Second, we
have developed a novel {\it cluster test} where a physical analogy of the
binary numbers with the two dimensional Ising model has been utilized. We
demonstrate that the new test is rather powerful in finding periodic
correlations on bit level. Results of both test methods are presented for each
bit of the output of the generators. Some generators exhibit clear bit level
correlations but we find no evidence of discernible correlations for
generators, which have recently produced systematic errors in Monte Carlo
simulations.Comment: University of Helsinki preprint HU-TFT-93-4
Instability and wavelength selection during step flow growth of metal surfaces vicinal to fcc(001)
We study the onset and development of ledge instabilities during growth of
vicinal metal surfaces using kinetic Monte Carlo simulations. We observe the
formation of periodic patterns at [110] close packed step edges on surfaces
vicinal to fcc(001) under realistic molecular beam epitaxy conditions. The
corresponding wavelength and its temperature dependence are studied by
monitoring the autocorrelation function for step edge position. Simulations
suggest that the ledge instability on fcc(1,1,m) vicinal surfaces is controlled
by the strong kink Ehrlich-Schwoebel barrier, with the wavelength determined by
dimer nucleation at the step edge. Our results are in agreement with recent
continuum theoretical predictions, and experiments on Cu(1,1,17) vicinal
surfaces.Comment: 4 pages, 4 figures, RevTe
Long wavelength properties of phase field crystal models with second order dynamics
The phase field crystal (PFC) approach extends the notion of phase field
models by describing the topology of the microscopic structure of a crystalline
material. One of the consequences is that local variation of the interatomic
distance creates an elastic excitation. The dynamics of these excitations poses
a challenge: pure diffusive dynamics cannot describe relaxation of elastic
stresses that happen through phonon emission. To this end, several different
models with fast dynamics have been proposed. In this article we use the
amplitude expansion of the PFC model to compare the recently proposed
hydrodynamic PFC amplitude model with two simpler models with fast dynamics. We
compare these different models analytically and numerically. The results
suggest that in order to have proper relaxation of elastic excitations, the
full hydrodynamical description of the PFC amplitudes is required.Comment: 10 pages, 7 figure
Dynamics of Chainlike Molecules on Surfaces
We consider the diffusion and spreading of chainlike molecules on solid
surfaces. We first show that the steep spherical cap shape density profiles,
observed in some submonolayer experiments on spreading polymer films, imply
that the collective diffusion coefficient must be an increasing
function of the surface coverage for small and intermediate coverages.
Through simulations of a discrete model of interacting chainlike molecules, we
demonstrate that this is caused by an entropy-induced repulsive interaction.
Excellent agreement is found between experimental and numerically obtained
density profiles in this case, demonstrating that steep submonolayer film edges
naturally arise due to the diffusive properties of chainlike molecules. When
the entropic repulsion dominates over interchain attractions,
first increases as a function of but then eventually approaches zero
for . The maximum value of decreases for increasing
attractive interactions, leading to density profiles that are in between
spherical cap and Gaussian shapes. We also develop an analytic mean field
approach to explain the diffusive behavior of chainlike molecules. The
thermodynamic factor in is evaluated using effective free energy
arguments, and the chain mobility is calculated numerically using the recently
developed dynamic mean field theory. Good agreement is obtained between theory
and simulations.Comment: 16 pages, 13 Postscript figure
Interface Equations for Capillary Rise in Random Environment
We consider the influence of quenched noise upon interface dynamics in 2D and
3D capillary rise with rough walls by using phase-field approach, where the
local conservation of mass in the bulk is explicitly included. In the 2D case
the disorder is assumed to be in the effective mobility coefficient, while in
the 3D case we explicitly consider the influence of locally fluctuating
geometry along a solid wall using a generalized curvilinear coordinate
transformation. To obtain the equations of motion for meniscus and contact
lines, we develop a systematic projection formalism which allows inclusion of
disorder. Using this formalism, we derive linearized equations of motion for
the meniscus and contact line variables, which become local in the Fourier
space representation. These dispersion relations contain effective noise that
is linearly proportional to the velocity. The deterministic parts of our
dispersion relations agree with results obtained from other similar studies in
the proper limits. However, the forms of the noise terms derived here are
quantitatively different from the other studies
Dipolar depletion effect on the differential capacitance of carbon based materials
The remarkably low experimental values of the capacitance data of carbon
based materials in contact with water solvent needs to be explained from a
microscopic theory in order to optimize the efficiency of these materials. We
show that this experimental result can be explained by the dielectric screening
deficiency of the electrostatic potential, which in turn results from the
interfacial solvent depletion effect driven by image dipole interactions. We
show this by deriving from the microscopic system Hamiltonian a non-mean-field
dipolar Poisson-Boltzmann equation. This can account for the interaction of
solvent molecules with their electrostatic image resulting from the dielectric
discontinuity between the solvent medium and the substrate. The predictions of
the extended dipolar Poisson-Boltzmann equation for the differential
capacitance are compared with experimental data and good agreement is found
without any fitting parameters
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