136 research outputs found
The local order of supercooled water in solution with LiCl studied by NMR proton chemical shift
We study by means of Nuclear Magnetic Resonance (NMR) spectroscopy the local order of water molecules in solution with lithium chloride at eutectic concentration. In particular, by measuring the proton chemical shift as a function of the temperature in the interval 203K < T < 320K, we observe a net change at about 235 K. We ascribe this result to the increase of the hydrogen bond interaction that on decreasing the temperature favors the formation of the network that characterizes the low density liquid phase of water. Furthermore, the Gaussian deconvolution of the NMR peak allows the investigation of the mutual difference between the chemical shift of water solvating lithium and chlorine individually. The thermal behavior of this quantity confirms previous results about the role of the temperature in the solvation mechanisms down to about 225 K. This temperature coincides with that of the so-called Widom line for water supporting the liquid-liquid transition hypothesis
Experimental Evidence For Self-Similar Structures In The Aggregation Of Porphyrins In Aqueous Solutions
We have studied the aggregation of the porphyrin t-HâPagg in aqueous solution by light scattering. The intensity profile of the elastically scattered light, studied in the exchanged wave-vector range 0.2 †q †31.4 ÎŒ mâ»Âč, indicates that the aggregation produces large monodisperse clusters having a fractal structure, and is driven by diffusion-limited aggregation kinetics. Additional measurements performed at different q values confirm such a picture giving a hydrodynamic radius R(H) consistent with the radius of gyration R(g) measured by elastic scattering. This is explained taking into account the qÂČ dependence observed in the mean decay rate of the intensity-intensity correlation function and the effect on this latter dynamical quantity of anisotropies in the cluster structure
Static and dynamic heterogeneities in irreversible gels and colloidal gelation
We compare the slow dynamics of irreversible gels, colloidal gels, glasses
and spin glasses by analyzing the behavior of the so called non-linear
dynamical susceptibility, a quantity usually introduced to quantitatively
characterize the dynamical heterogeneities. In glasses this quantity typically
grows with the time, reaches a maximum and then decreases at large time, due to
the transient nature of dynamical heterogeneities and to the absence of a
diverging static correlation length. We have recently shown that in
irreversible gels the dynamical susceptibility is instead an increasing
function of the time, as in the case of spin glasses, and tends asymptotically
to the mean cluster size. On the basis of molecular dynamics simulations, we
here show that in colloidal gelation where clusters are not permanent, at very
low temperature and volume fractions, i.e. when the lifetime of the bonds is
much larger than the structural relaxation time, the non-linear susceptibility
has a behavior similar to the one of the irreversible gel, followed, at higher
volume fractions, by a crossover towards the behavior of glass forming liquids.Comment: 9 pages, 3 figure
Comparative simulation study of colloidal gels and glasses
Using computer simulations, we identify the mechanisms causing aggregation
and structural arrest of colloidal suspensions interacting with a short-ranged
attraction at moderate and high densities. Two different non-ergodicity
transitions are observed. As the density is increased, a glass transition takes
place, driven by excluded volume effects. In contrast, at moderate densities,
gelation is approached as the strength of the attraction increases. At high
density and interaction strength, both transitions merge, and a logarithmic
decay in the correlation function is observed. All of these features are
correctly predicted by mode coupling theory
Knowledge, attitudes and behaviors regarding influenza vaccination among Hygiene and Preventive Medicine residents in Calabria and Sicily.
Vaccinating health care workers is considered to be one of the most important steps in preventing the transmission of the influenza virus to vulnerable patients. Public Health physicians are the main promoters and executors of influenza vaccination campaigns for both healthcare workers and the general population.
The objective of the present survey was to analyze the knowledge, attitudes and practices regarding influenza vaccination among Hygiene and Preventive Medicine Residents.
64% of the participants had not been vaccinated against the influenza virus in the past 5 years, and 29% had been vaccinated only occasionally , with only 7.2% of the study popu-lation having been vaccinated every year. 20.3% of those surveyed were vaccinated in the 2010/2011 season. The best strategy to increase vaccination rates among health care workers according to the study participants was the participation of future public health operators to multidisciplinary training (34.8%). the main factors associated with influenza vaccination compliance were having been vaccinated in the previous season for 2011/2012 (OR [95%]: 41.14 [7.56 - 223.87]) and having received the vaccination always or occasionally during the previous 5 years for both 2010/2011 (p-value <0.0001) and 2011/2012 (p-value <0.0001).
The findings of this study suggest that future public health physicians with a history of refusing influenza vaccination in previous years usually tend to maintain their beliefs over time. Changing this trend among Hygiene and Preventive Medicine residents is the real challenge for the future, and it can be achieved through organization of multidisciplinary training, improvement of university education and increasing the involvement of Hygiene and Preventive Medicine residents in influenza vaccination campaigns both for the gen-eral population and health care workers
Glasses in hard spheres with short-range attraction
We report a detailed experimental study of the structure and dynamics of
glassy states in hard spheres with short-range attraction. The system is a
suspension of nearly-hard-sphere colloidal particles and non-adsorbing linear
polymer which induces a depletion attraction between the particles. Observation
of crystallization reveals a re-entrant glass transition. Static light
scattering shows a continuous change in the static structure factors upon
increasing attraction. Dynamic light scattering results, which cover 11 orders
of magnitude in time, are consistent with the existence of two distinct kinds
of glasses, those dominated by inter-particle repulsion and caging, and those
dominated by attraction. Samples close to the `A3 point' predicted by mode
coupling theory for such systems show very slow, logarithmic dynamics.Comment: 22 pages, 18 figure
Statistical Mechanics of Glass Formation in Molecular Liquids with OTP as an Example
We extend our statistical mechanical theory of the glass transition from
examples consisting of point particles to molecular liquids with internal
degrees of freedom. As before, the fundamental assertion is that super-cooled
liquids are ergodic, although becoming very viscous at lower temperatures, and
are therefore describable in principle by statistical mechanics. The theory is
based on analyzing the local neighborhoods of each molecule, and a statistical
mechanical weight is assigned to every possible local organization. This
results in an approximate theory that is in very good agreement with
simulations regarding both thermodynamical and dynamical properties
Higher order glass-transition singularities in colloidal systems with attractive interactions
The transition from a liquid to a glass in colloidal suspensions of particles
interacting through a hard core plus an attractive square-well potential is
studied within the mode-coupling-theory framework. When the width of the
attractive potential is much shorter than the hard-core diameter, a reentrant
behavior of the liquid-glass line, and a glass-glass-transition line are found
in the temperature-density plane of the model. For small well-width values, the
glass-glass-transition line terminates in a third order bifurcation point, i.e.
in a A_3 (cusp) singularity. On increasing the square-well width, the
glass-glass line disappears, giving rise to a fourth order A_4 (swallow-tail)
singularity at a critical well width. Close to the A_3 and A_4 singularities
the decay of the density correlators shows stretching of huge dynamical
windows, in particular logarithmic time dependence.Comment: 19 pages, 12 figures, Phys. Rev. E, in prin
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