159 research outputs found
Geometry for a `penguin-albatross' rookery
We introduce a simple ecological model describing the spatial organization of
two interacting populations whose individuals are indifferent to conspecifics
and avoid the proximity to heterospecifics. At small population densities
a non-trivial structure is observed where clusters of individuals
arrange into a rhomboidal bipartite network with an average degree of four. For
the length scale, order parameter and susceptibility of the
network exhibit power-law divergences compatible with hyper-scaling, suggesting
the existence of a zero density - non-trivial - critical point. At larger
densities a critical threshold is identified above which the
evolution toward a partially ordered configuration is prevented and the system
becomes jammed in a fully mixed state
Heterodyne Near-Field Scattering
We describe an optical technique based on the statistical analysis of the
random intensity distribution due to the interference of the near-field
scattered light with the strong transmitted beam. It is shown that, from the
study of the two-dimensional power spectrum of the intensity, one derives the
scattered intensity as a function of the scattering wave vector. Near-field
conditions are specified and discussed. The substantial advantages over
traditional scattering technique are pointed out, and is indicated that the
technique could be of interest for wave lengths other than visible light.Comment: 3 pages, 2 figure
Dynamic scaling for the growth of non-equilibrium fluctuations during thermophoretic diffusion in microgravity
Diffusion processes are widespread in biological and chemical systems, where
they play a fundamental role in the exchange of substances at the cellular
level and in determining the rate of chemical reactions. Recently, the
classical picture that portrays diffusion as random uncorrelated motion of
molecules has been revised, when it was shown that giant non-equilibrium
fluctuations develop during diffusion processes. Under microgravity conditions
and at steady-state, non-equilibrium fluctuations exhibit scale invariance and
their size is only limited by the boundaries of the system. In this work, we
investigate the onset of non-equilibrium concentration fluctuations induced by
thermophoretic diffusion in microgravity, a regime not accessible to analytical
calculations but of great relevance for the understanding of several natural
and technological processes. A combination of state of the art simulations and
experiments allows us to attain a fully quantitative description of the
development of fluctuations during transient diffusion in microgravity. Both
experiments and simulations show that during the onset the fluctuations exhibit
scale invariance at large wave vectors. In a broader range of wave vectors
simulations predict a spinodal-like growth of fluctuations, where the amplitude
and length-scale of the dominant mode are determined by the thickness of the
diffuse layer.Comment: To appear in Scientific Report
Equilibrium and non-equilibrium concentration fluctuations in a critical binary mixture
When a macroscopic concentration gradient is present across a binary mixture,
long-ranged non-equilibrium concentration fluctuations (NCF) appear as a
consequence of the coupling between the gradient and spontaneous equilibrium
velocity fluctuations. Long-ranged equilibrium concentration fluctuations (ECF)
may be also observed when the mixture is close to a critical point. Here we
study the interplay between NCF and critical ECF in a near critical mixture
aniline/cyclohexane in the presence of a vertical concentration gradient. To
this aim, we exploit a commercial optical microscope and a simple, custom-made,
temperature-controlled cell to obtain simultaneous static and dynamic
scattering information on the fluctuations. We first characterise the critical
ECF at fixed temperature above the upper critical solution temperature
, in the wide temperature range C. In this
range, we observe the expected critical scaling behaviour for both the
scattering intensity and the mass diffusion coefficient and we determine the
critical exponents , and , which are found in agreement
with the 3D Ising values. We then study the system in the two-phase region
(). In particular, we characterise the interplay between ECF and NCF
when the mixture, initially at a temperature , is rapidly brought to a
temperature . During the transient, a vertical diffusive mass flux
is present that causes the onset of NCF, whose amplitude vanishes with time, as
the flux goes to zero. We also study the time dependence of the equilibrium
scattering intensity , of the crossover wave-vector and of the
diffusion coefficient during diffusion and find that all these quantities
exhibit an exponential relaxation enslaved to the diffusive kinetics.Comment: 11 pages, 4 figure
Equilibrium and nonequilibrium fluctuations at the interface between two fluid phases
We have performed small-angle light-scattering measurements of the static
structure factor of a critical binary mixture undergoing diffusive partial
remixing. An uncommon scattering geometry integrates the structure factor over
the sample thickness, allowing different regions of the concentration profile
to be probed simultaneously. Our experiment shows the existence of interface
capillary waves throughout the macroscopic evolution to an equilibrium
interface, and allows to derive the time evolution of surface tension.
Interfacial properties are shown to attain their equilibrium values quickly
compared to the system's macroscopic equilibration time.Comment: 10 pages, 5 figures, submitted to PR
An Ultrasound Study of Cerebral Venous Drainage after Internal Jugular Vein Catheterization
Objectives. It has been advocated that internal jugular vein (IJV) cannulation in patients at risk for intracranial hypertension could impair cerebral venous return. Aim of this study was to demonstrate that ultrasound-guided IJV cannulation in elective neurosurgical patients is safe and does not impair cerebral venous return. Methods. IJV cross-sectional diameter and flow were measured using two-dimensional ultrasound and Doppler function bilaterally before and after IJV cannulation with the head supine and elevated at 30°. Results. Fifty patients with intracranial lesions at risk for intracranial hypertension were enrolled in this observational prospective study. IJV diameters before and after ultrasound-guided cannulation were not statistically different during supine or head-up position and the absolute variation of the venous flow revealed an average reduction of the venous flow after cannulation without a significant reduction of the venous flow rate after cannulation. Conclusions. Ultrasound-guided IJV cannulation in neurosurgical patients at risk for intracranial hypertension does not impair significantly jugular venous flow and indirectly cerebral venous return
Real-Time Wavelet-transform spectrum analyzer for the investigation of 1/f^\alpha noise
A wavelet transform spectrum analyzer operating in real time within the
frequency range 3X10^(-5) - 1.3X10^5 Hz has been implemented on a low-cost
Digital Signal Processing board operating at 150MHz. The wavelet decomposition
of the signal allows to efficiently process non-stationary signals dominated by
large amplitude events fairly well localized in time, thus providing the
natural tool to analyze processes characterized by 1/f^alpha power spectrum.
The parallel architecture of the DSP allows the real-time processing of the
wavelet transform of the signal sampled at 0.3MHz. The bandwidth is about
220dB, almost ten decades. The power spectrum of the scattered intensity is
processed in real time from the mean square value of the wavelet coefficients
within each frequency band. The performances of the spectrum analyzer have been
investigated by performing Dynamic Light Scattering experiments on colloidal
suspensions and by comparing the measured spectra with the correlation
functions data obtained with a traditional multi tau correlator. In order to
asses the potentialities of the spectrum analyzer in the investigation of
processes involving a wide range of timescales, we have performed measurements
on a model system where fluctuations in the scattered intensities are generated
by the number fluctuations in a dilute colloidal suspension illuminated by a
wide beam. This system is characterized by a power-law spectrum with exponent
-3/2 in the scattered intensity fluctuations. The spectrum analyzer allows to
recover the power spectrum with a dynamic range spanning about 8 decades. The
advantages of wavelet analysis versus correlation analysis in the investigation
of processes characterized by a wide distribution of time scales and
non-stationary processes are briefly discussed.Comment: 12 pages, 6 figure
Nonequilibrium solid-solid phase transition in a lattice of liquid jets
Solid-solid phase transitions are commonly encountered at the atomic scale in alloys and in superatomic mesoscopic systems of colloidal particles. Here we investigate a solid-solid phase transition occurring at the macroscopic scale between lattices of liquid jets with different symmetries generated by convection in a horizontal layer of a binary liquid mixture. In the absence of a shear stress, upwelling and downwelling jets arrange into two staggered square lattices with a spacing of approximately 3 mm. Applying a shear stress triggers a phase separation of the square patterns into two centered-rectangular lattices drifting into opposite directions, each lattice being made either by upwelling or downwelling jets. This structural phase transition is reversible. The macroscopic nature of the system allows us to investigate the kinetics of the transition by direct visualization with shadowgraphy. The mechanism of the transition depends on the path followed. It occurs through a nucleation and growth mechanism when the shear stress is imposed, and through a martensitic transformation of the lattice when the stress is removed
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