27 research outputs found
Fluctuations in fluids in thermal nonequilibrium states below the convective Rayleigh-Benard instability
Starting from the linearized fluctuating Boussinesq equations we derive an
expression for the structure factor of fluids in stationary convection-free
thermal nonequilibrium states, taking into account both gravity and finite-size
effects. It is demonstrated how the combined effects of gravity and finite size
causes the structure factor to go through a maximum value as a function of the
wave number . The appearance of this maximum is associated with a crossover
from a dependence for larger to a dependence for very small
. The relevance of this theoretical result for the interpretation of light
scattering and shadowgraph experiments is elucidated. The relationship with
studies on various aspects of the problem by other investigators is discussed.
The paper thus provides a unified treatment for dealing with fluctuations in
fluid layers subjected to a stationary temperature gradient regardless of the
sign of the Rayleigh number , provided that is smaller than the critical
value associated with the appearance of Rayleigh-B\'{e}nard
convection.Comment: 33 pages, 6 figures, accepted for publication: Physica
Light scattering from self-affine fractal silver surfaces with nanoscale cutoff: Far-field and near-field calculations
We study the light scattered from randomly rough, one-dimensional self-affine
fractal silver surfaces with nanoscale lower cutoff, illuminated by s- or
p-polarized Gaussian beams a few microns wide. By means of rigorous numerical
calculations based on the Green theorem integral equation formulation, we
obtain both the far- and near-field scattered intensities. The influence of
diminishing the fractal lower scale cutoff (from below a hundred, down to a few
nanometers) is analyzed in the case of both single realizations and ensemble
average magnitudes. For s polarization, variations are small in the far field,
being only significant in the higher spatial frequency components of evanescent
character in the near field. In the case of p polarization, however, the
nanoscale cutoff has remarkable effects stemming from the roughness-induced
excitation of surface-plasmon polaritons. In the far field, the effect is
noticed both in the speckle pattern variation and in the decrease of the total
reflected energy upon ensemble averaging, due to increased absorption. In the
near field, more efficient excitation of localized optical modes is achieved
with smaller cutoff, which in turn leads to huge surface electric field
enhancements.Comment: REVTeX 4, 10 page