112 research outputs found
Kinetics of growth of non-equilibrium fluctuations during thermodiffusion in a polymer solution
Abstract.: A thermal diffusion process occurring in a binary liquid mixture is accompanied by long ranged non-equilibrium concentration fluctuations. The amplitude of these fluctuations at large length scales can be orders of magnitude larger than that of equilibrium ones. So far non-equilibrium fluctuations have been mainly investigated under stationary or quasi-stationary conditions, a situation that allows to achieve a detailed statistical characterization of their static and dynamic properties. In this work we investigate the kinetics of growth of non-equilibrium concentration fluctuations during a transient thermodiffusion process, starting from a configuration where the concentration of the sample is uniform. The use of a large molecular weight polymer solution allows to attain a slow dynamics of growth of the macroscopic concentration profile. We focus on the development of fluctuations at small wave vectors, where their amplitude is strongly limited by the presence of gravity. We show that the growth rate of non-equilibrium fluctuations follows a power law Rf(q,t) 1d1t as a function of time, without any typical time scale and independently of the wave vector. We formulate a phenomenological model that allows to relate the rate of growth of non-equilibrium fluctuations to the growth of the macroscopic concentration profile in the absence of arbitrary parameters
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
Impact of thermodiffusion on the initial vertical distribution of species in hydrocarbon reservoirs
Concentration dependent refractive index of a binary mixture at high pressure
In the present work binary mixtures of varying concentrations of two miscible hydrocarbons, 1,2,3,4-tetrahydronaphtalene (THN) and n-dodecane (C12), are subjected to increasing pressure up to 50 MPa in order to investigate the dependence of the so-called concentration contrast factor (CF), i.e., (∂n/∂c)p, T, on pressure level. The refractive index is measured by means of a Mach-Zehnder interferometer. The setup and experimental procedure are validated with different pure fluids in the same pressure range. The refractive index of the THN-C12 mixture is found to vary both over pressure and concentration, and the concentration CF is found to exponentially decrease as the pressure is increased. The measured values of the refractive index and the concentration CFs are compared with values obtained by two different theoretical predictions, the well-known Lorentz-Lorenz formula and an alternative one proposed by Looyenga. While the measured refractive indices agree very well with predictions given by Looyenga, the measured concentration CFs show deviations from the latter of the order of 6% and more than the double from the Lorentz-Lorenz prediction
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
Fatigue response of additively manufactured Maraging Stainless Steel CX and effects of heat treatment and surface finishing
This paper deals with the novel topic of the fatigue response of additively manufactured Maraging Stainless Steel CX. A two-by-two factorial plan was arranged, to experimentally assess the effects of heat treatment and machining on the fatigue strength in both finite and infinite life domains. The two factors were regarded as on–off, taking the untreated unmachined condition as a reference for comparisons. Cylindrical specimens with vertical build orientation were involved in the fatigue campaign under four-point rotating bending. The results indicate that the fatigue strength may be remarkably incremented (up to five times) with respect to the as received conditions, especially thanks to surface smoothing and taking advantage of a very low porosity level. Heat treatment strengthening mechanisms were also interpreted in the light of optical and electron microscope observations. Fatigue enhancement arises from precipitate size increment throughout the conducted heat treatment, although the fracture mode turns to be more brittle
Fatigue response of additively manufactured as-built 15-5 PH stainless steel and effects of machining and thermal and surface treatments
Additively produced 15-5 PH stainless steel has wide industrial applications, but the combined effects of heat treatment, machining, and shot-peening and their order have not been deeply investigated. This topic is addressed here by a 2-by-3 experimental plan that has involved S–N curve and fatigue limit determination, using vertically built cylindrical samples, tested under rotating bending. The obtained responses have been analyzed by an ANOVA-based statistical approach for comparison of fatigue trends. Results indicate that heat treatment without machining may be even detrimental for fatigue due to embrittlement. Conversely, machining with subsequent shot-peening, even without heat treatment, has a remarkable impact and leads to a doubled fatigue strength with respect to as-built material. This strength is also quite close to that achievable for wrought material. The study has been completed by micrography and fractography, to reveal the dependence of microstructure, crack initiation sites, and failure mode on the performed treatments
Confinement effect on the dynamics of non-equilibrium concentration fluctuations far from the onset of convection
Photon correlation spectroscopy with incoherent light
Photon correlation spectroscopy (PCS) is based on measuring the temporal
correlation of the light intensity scattered by the investigated sample. A
typical setup requires a temporally coherent light source. Here, we show that a
short-coherence light source can be used as well, provided that its coherence
properties are suitably modified. This results in a "skewed-coherence" light
beam allowing that restores the coherence requirements. This approach overcomes
the usual need for beam filtering, which would reduce the total brightness of
the beam.Comment: 4 pages, 4 figure
Inclined layer convection in a colloidal suspension with negative Soret coefficient at large solutal Rayleigh numbers
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