41 research outputs found
Quantitative two-dimensional shadowgraphic set-up for high-sensitivity measurement of low-density laser-plasmas
We present a quantitative shadowgraphic method which can measure the density
of a laser-generated plasma in air with sensitivity and resolution comparable
or better than traditional interferometric techniques. Simultaneous comparison
of both shadowgraphy and interferometry has been carried out allowing the
experimental evaluation of the reliability of the shadowgraphic method
Wave-number Selection by Target Patterns and Side Walls in Rayleigh-Benard Convection
We present experimental results for Rayleigh-Benard convection patterns in a
cylindrical container with static side-wall forcing induced by a heater. This
forcing stabilized a pattern of concentric rolls (a target pattern) with the
central roll (the umbilicus) at the center of the cell after a jump from the
conduction to the convection state. A quasi-static increase of the control
parameter (epsilon) beyond 0.8 caused the umbilicus of the pattern to move off
center. As observed by others, a further quasi-static increase of epsilon up to
15.6 caused a sequence of transitions. Each transition began with the
displacement of the umbilicus and then proceeded with the loss of one
convection roll at the umbilicus and the return of the umbilicus to a location
near the center of the cell. Alternatively, with decreasing epsilon new rolls
formed at the umbilicus but large umbilicus displacements did not occur. In
addition to quantitative measurements of the umbilicus displacement, we
determined and analyzed the entire wave-director field of each image. The wave
numbers varied in the axial direction, with minima at the umbilicus and at the
cell wall and a maximum at a radial position close to 2/3 Gamma. The wave
numbers at the maximum showed hysteretic jumps at the transitions, but on
average agreed well with the theoretical predictions for the wave numbers
selected in the far field of an infinitely extended target pattern.Comment: ReVTeX, 11 pages, 16 eps figures include
The Domain Chaos Puzzle and the Calculation of the Structure Factor and Its Half-Width
The disagreement of the scaling of the correlation length xi between
experiment and the Ginzburg-Landau (GL) model for domain chaos was resolved.
The Swift-Hohenberg (SH) domain-chaos model was integrated numerically to
acquire test images to study the effect of a finite image-size on the
extraction of xi from the structure factor (SF). The finite image size had a
significant effect on the SF determined with the Fourier-transform (FT) method.
The maximum entropy method (MEM) was able to overcome this finite image-size
problem and produced fairly accurate SFs for the relatively small image sizes
provided by experiments.
Correlation lengths often have been determined from the second moment of the
SF of chaotic patterns because the functional form of the SF is not known.
Integration of several test functions provided analytic results indicating that
this may not be a reliable method of extracting xi. For both a Gaussian and a
squared SH form, the correlation length xibar=1/sigma, determined from the
variance sigma^2 of the SF, has the same dependence on the control parameter
epsilon as the length xi contained explicitly in the functional forms. However,
for the SH and the Lorentzian forms we find xibar ~ xi^1/2.
Results for xi determined from new experimental data by fitting the
functional forms directly to the experimental SF yielded xi ~ epsilon^-nu} with
nu ~= 1/4 for all four functions in the case of the FT method, but nu ~= 1/2,
in agreement with the GL prediction, in the the case of the MEM. Over a wide
range of epsilon and wave number k, the experimental SFs collapsed onto a
unique curve when appropriately scaled by xi.Comment: 15 pages, 26 figures, 1 tabl
Nano-particle characterization by using Exposure Time Dependent Spectrum and scattering in the near field methods: how to get fast dynamics with low-speed CCD camera
Light scattering detection in the near field, a rapidly expanding family of
scattering techniques, has recently proved to be an appropriate procedure for
performing dynamic measurements. Here we report an innovative algorithm, based
on the evaluation of the Exposure Time Dependent Spectrum (ETDS), which makes
it possible to measure the fast dynamics of a colloidal suspension with the aid
of a simple near field scattering apparatus and a CCD camera. Our algorithm
consists in acquiring static spectra in the near field at different exposure
times, so that the measured decay times are limited only by the exposure time
of the camera and not by its frame rate. The experimental set-up is based on a
modified microscope, where the light scattered in the near field is collected
by a commercial objective, but (unlike in standard microscopes) the light
source is a He-Ne laser which increases the instrument sensitivity. The
apparatus and the algorithm have been validated by considering model systems of
standard spherical nano-particle
Time-resolved refractive index and absorption mapping of light-plasma filaments in water
By means of a quantitative shadowgraphic method, we performed a space-time
characterization of the refractive index variation and transient absorption
induced by a light-plasma filament generated by a 100 fs laser pulse in water.
The formation and evolution of the plasma channel in the proximity of the
nonlinear focus were observed with a 23 fs time resolution.Comment: 3 pages, 3 picture
Characterization of anisotropic nano-particles by using depolarized dynamic light scattering in the near field
Light scattering techniques are widely used in many fields of condensed and
sof t matter physics. Usually these methods are based on the study of the
scattered light in the far field. Recently, a new family of near field
detection schemes has been developed, mainly for the study of small angle light
scattering. These techniques are based on the detection of the light intensity
near to the sample, where light scattered at different directions overlaps but
can be distinguished by Fourier transform analysis. Here we report for the
first time data obtained with a dynamic near field scattering instrument,
measuring both polarized and depolarized scattered light. Advantages of this
procedure over the traditional far field detection include the immunity to
stray light problems and the possibility to obtain a large number of
statistical samples for many different wave vectors in a single instantaneous
measurement. By using the proposed technique we have measured the translational
and rotational diffusion coefficients of rod-like colloidal particles. The
obtained data are in very good agreement with the data acquired with a
traditional light scattering apparatus.Comment: Published in Optics Express. This version has changes in bibliograph
Nanoparticle characterization by using Tilted Laser Microscopy: back scattering measurement in near field
By using scattering in near field techniques, a microscope can be easily
turned into a device measuring static and dynamic light scattering, very useful
for the characterization of nanoparticle dispersions. Up to now, microscopy
based techniques have been limited to forward scattering, up to a maximum of 30
degrees. In this paper we present a novel optical scheme that overcomes this
limitation, extending the detection range to angles larger than 90 degrees
(back-scattering). Our optical scheme is based on a microscope, a wide
numerical aperture objective, and a laser illumination, with the collimated
beam positioned at a large angle with respect to the optical axis of the
objective (Tilted Laser Microscopy, TLM). We present here an extension of the
theory for near field scattering, which usually applies only to paraxial
scattering, to our strongly out-of-axis s ituation. We tested our instrument
and our calculations with calibrated spherical nanoparticles of several
different diameters, performing static and dynamic scattering measurements up
to 110 degrees. The measured static spectra and decay times are compatible with
the Mie theory and the diffusion coefficients provided by the Stokes-Einstein
equation. The ability of performing backscattering measurements with this
modified microscope opens the way to new applications of scattering in near
field techniques to the measurement of systems with strongly angle dependent
scattering.Comment: 18 pages, 10 figures. Accepted for publication in Optics Express,
vol. 17, no. 17 (08/17/2009
Dynamics of fluctuations in a fluid below the onset of Rayleigh-B\'enard convection
We present experimental data and their theoretical interpretation for the
decay rates of temperature fluctuations in a thin layer of a fluid heated from
below and confined between parallel horizontal plates. The measurements were
made with the mean temperature of the layer corresponding to the critical
isochore of sulfur hexafluoride above but near the critical point where
fluctuations are exceptionally strong. They cover a wide range of temperature
gradients below the onset of Rayleigh-B\'enard convection, and span wave
numbers on both sides of the critical value for this onset. The decay rates
were determined from experimental shadowgraph images of the fluctuations at
several camera exposure times. We present a theoretical expression for an
exposure-time-dependent structure factor which is needed for the data analysis.
As the onset of convection is approached, the data reveal the critical
slowing-down associated with the bifurcation. Theoretical predictions for the
decay rates as a function of the wave number and temperature gradient are
presented and compared with the experimental data. Quantitative agreement is
obtained if allowance is made for some uncertainty in the small spacing between
the plates, and when an empirical estimate is employed for the influence of
symmetric deviations from the Oberbeck-Boussinesq approximation which are to be
expected in a fluid with its density at the mean temperature located on the
critical isochore.Comment: 13 pages, 10 figures, 52 reference
Fractal fronts of diffusion in microgravity
Spatial scale invariance represents a remarkable feature of natural phenomena. A ubiquitous example is represented by miscible liquid phases undergoing diffusion. Theory and simulations predict that in the absence of gravity diffusion is characterized by long-ranged algebraic correlations. Experimental evidence of scale invariance generated by diffusion has been limited, because on Earth the development of long-range correlations is suppressed by gravity. Here we report experimental results obtained in microgravity during the flight of the FOTON M3 satellite. We find that during a diffusion process a dilute polymer solution exhibits scale-invariant concentration fluctuations with sizes ranging up to millimetres, and relaxation times as large as 1,000 s. The scale invariance is limited only by the finite size of the sample, in agreement with recent theoretical predictions. The presence of such fluctuations could possibly impact the growth of materials in microgravity