6,241 research outputs found
On the corrections to Strong-Stretching Theory for end-confined, charged polymers in a uniform electric field
We investigate the properties of a system of semi-diluted polymers in the
presence of charged groups and counter-ions, by means of self-consistent field
theory. We study a system of polyelectrolyte chains grafted to a similarly, as
well as an oppositely charged surface, solving a set of saddle-point equations
that couple the modified diffusion equation for the polymer partition function
to the Poisson-Boltzmann equation describing the charge distribution in the
system. A numerical study of this set of equations is presented and comparison
is made with previous studies. We then consider the case of semi-diluted,
grafted polymer chains in the presence of charge-end-groups. We study the
problem with self-consistent field as well as strong-stretching theory. We
derive the corrections to the Milner-Witten-Cates (MWC) theory for weakly
charged chains and show that the monomer-density deviates from the parabolic
profile expected in the uncharged case. The corresponding corrections are shown
to be dictated by an Abel-Volterra integral equation of the second kind. The
validity of our theoretical findings is confirmed comparing the predictions
with the results obtained within numerical self-consistent field theory.Comment: 15 Pages, 12 figure
Motion of Contact Line of a Crystal Over the Edge of Solid Mask in Epitaxial Lateral Overgrowth
Mathematical model that allows for direct tracking of the homoepitaxial
crystal growth out of the window etched in the solid, pre-deposited layer on
the substrate is described. The growth is governed by the normal (to the
crystal-vapor interface) flux from the vapor phase and by the interface
diffusion. The model accounts for possibly inhomogeneous energy of the mask
surface and for strong anisotropies of crystal-vapor interfacial energy and
kinetic mobility. Results demonstrate that the motion of the crystal-mask
contact line slows down abruptly as radius of curvature of the mask edge
approaches zero. Numerical procedure is suggested to overcome difficulties
associated with ill-posedness of the evolution problem for the interface with
strong energy anisotropy.
Keywords: Thin films, epitaxy, MOCVD, surface diffusion, interface dynamics,
contact lines, rough surfaces, wetting, regularization of ill-posed evolution
problems.Comment: 21 pages, 11 figures; to appear in Computational Materials Scienc
Generation of interface for an Allen-Cahn equation with nonlinear diffusion
In this note, we consider a nonlinear diffusion equation with a bistable
reaction term arising in population dynamics. Given a rather general initial
data, we investigate its behavior for small times as the reaction coefficient
tends to infinity: we prove a generation of interface property
Spectral collocation methods
This review covers the theory and application of spectral collocation methods. Section 1 describes the fundamentals, and summarizes results pertaining to spectral approximations of functions. Some stability and convergence results are presented for simple elliptic, parabolic, and hyperbolic equations. Applications of these methods to fluid dynamics problems are discussed in Section 2
Statistical stability in time reversal
When a signal is emitted from a source, recorded by an array of transducers,
time reversed and re-emitted into the medium, it will refocus approximately on
the source location. We analyze the refocusing resolution in a high frequency,
remote sensing regime, and show that, because of multiple scattering, in an
inhomogeneous or random medium it can improve beyond the diffraction limit. We
also show that the back-propagated signal from a spatially localized
narrow-band source is self-averaging, or statistically stable, and relate this
to the self-averaging properties of functionals of the Wigner distribution in
phase space. Time reversal from spatially distributed sources is self-averaging
only for broad-band signals. The array of transducers operates in a
remote-sensing regime so we analyze time reversal with the parabolic or
paraxial wave equation
Maxwell-Drude-Bloch dissipative few-cycle optical solitons
We study the propagation of few-cycle pulses in two-component medium
consisting of nonlinear amplifying and absorbing two-level centers embedded
into a linear and conductive host material. First we present a linear theory of
propagation of short pulses in a purely conductive material, and demonstrate
the diffusive behavior for the evolution of the low-frequency components of the
magnetic field in the case of relatively strong conductivity. Then, numerical
simulations carried out in the frame of the full nonlinear theory involving the
Maxwell-Drude-Bloch model reveal the stable creation and propagation of
few-cycle dissipative solitons under excitation by incident femtosecond optical
pulses of relatively high energies. The broadband losses that are introduced by
the medium conductivity represent the main stabilization mechanism for the
dissipative few-cycle solitons.Comment: 38 pages, 10 figures. submitted to Physical Review
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