277 research outputs found
Pressure dependence of the Boson peak in glassy As2S3 studied by Raman Scattering
A detailed pressure-dependence study of the low-energy excitations of glassy
As2S3 is reported over a wide pressure range, up to 10 GPa. The spectral
features of Boson peak are analysed as a function of pressure. Pressure effects
on the Boson peak are manifested as an appreciable shift of its frequency to
higher values, a suppression of its intensity, as well as a noticeable change
of its asymmetry leading to a more symmetric shape at high pressures. The
pressure-induced Boson peak frequency shift agrees very well with the
predictions of the soft potential model over the whole pressure range studied.
As regards the pressure dependence of the Boson peak intensity, the situation
is more complicated. It is proposed that in order to reach proper conclusions
the corresponding dependence of the Debye density of states must also be
considered. Employing a comparison of the low energy modes of the crystalline
counterpart of As2S3 as well as the experimental data concerning the pressure
dependencies of the Boson peak frequency and intensity, structural or
glass-to-glass transition seems to occur at the pressure ~4 GPa related to a
change of local structure. Finally, the pressure-induced shape changes of the
Boson peak can be traced back to the very details of the excess (over the Debye
contribution) vibrational density of states.Comment: To appear in J. Non-Cryst. Solids (Proceedings of the 5th IDMRCS,
Lille, July 2005
On the origin of the -transition in liquid Sulphur
Developing a novel experimental technique, we applied photon correlation
spectroscopy using infrared radiation in liquid Sulphur around ,
i.e. in the temperature range where an abrupt increase in viscosity by four
orders of magnitude is observed upon heating within few degrees. This allowed
us - overcoming photo-induced and absorption effects at visible wavelengths -
to reveal a chain relaxation process with characteristic time in the ms range.
These results do rehabilitate the validity of the Maxwell relation in Sulphur
from an apparent failure, allowing rationalizing the mechanical and
thermodynamic behavior of this system within a viscoelastic scenario.Comment: 5 pages, 4 eps figures, accepted in Phys. Rev. Let
Dynamics of proteins: Light scattering study of dilute and dense colloidal suspensions of eye lens homogenates
We report a dynamic light scattering study on protein suspensions of bovine
lens homogenates at conditions (pH and ionic strength) similar to the
physiological ones. Light scattering data were collected at two temperatures,
20 oC and 37 oC, over a wide range of concentrations from the very dilute limit
up to the dense regime approaching to the physiological lens concentration. A
comparison with experimental data from intact bovine lenses was advanced
revealing differences between dispersions and lenses at similar concentrations.
In the dilute regime two scattering entities were detected and identified with
the long-time, self-diffusion modes of alpha-crystallins and their aggregates,
which naturally exist in lens nucleus. Self-diffusion coefficients are
temperature insensitive, whereas the collective diffusion coefficient depends
strongly on temperature revealing a reduction of the net repulsive
interparticle forces with lowering temperature. While there are no rigorous
theoretical approaches on particle diffusion properties for multi-component,
non-ideal hard-sphere, polydispersed systems, as the suspensions studied here,
a discussion of the volume fraction dependence of the long-time, self-diffusion
coefficient in the context of existing theoretical approaches was undertaken.
This study is purported to provide some insight into the complex light
scattering pattern of intact lenses and the interactions between the
constituent proteins that are responsible for lens transparency. This would
lead to understand basic mechanisms of specific protein interactions that lead
to lens opacification (cataract) under pathological conditions.Comment: To appear in J. Chem. Phy
Stress distribution and the fragility of supercooled melts
We formulate a minimal ansatz for local stress distribution in a solid that
includes the possibility of strongly anharmonic short-length motions. We
discover a broken-symmetry metastable phase that exhibits an aperiodic,
frozen-in stress distribution. This aperiodic metastable phase is characterized
by many distinct, nearly degenerate configurations. The activated transitions
between the configurations are mapped onto the dynamics of a long range
classical Heisenberg model with 6-component spins and anisotropic couplings. We
argue the metastable phase corresponds to a deeply supercooled non-polymeric,
non-metallic liquid, and further establish an order parameter for the
glass-to-crystal transition. The spin model itself exhibits a continuous range
of behaviors between two limits corresponding to frozen-in shear and uniform
compression/dilation respectively. The two regimes are separated by a
continuous transition controlled by the anisotropy in the spin-spin
interaction, which is directly related to the Poisson ratio of the
material. The latter ratio and the ultra-violet cutoff of the theory determine
the liquid configurational entropy. Our results suggest that liquid's fragility
depends on the Poisson ratio in a non-monotonic way. The present ansatz
provides a microscopic framework for computing the configurational entropy and
relaxational spectrum of specific substances.Comment: 11 pages, 5 figures, Final version published in J Phys Chem
Prehospital induction of mild hypothermia with cold normal saline for cardiac arrest: more harm than good?
Some remarks on the low-energy excitations in glasses: interpretation of Boson peak data
EXPERIMENTAL STUDY ON THE EFFECTIVENESS OF AN ENERGY DISSIPATION CONFIGURATION IN AN OPEN CHANNEL OF STEEP SLOPE
Στην παρούσα εργασία μελετάται πειραματικά η ροή σε σήραγγα υπό συνθήκες ελεύθερης επιφάνειας και έντονης κλίσης πυθμένα 1:10. Διερευνάται η δυνατότητα μείωσης της ταχύτητας ροής μέσω συνδυασμού κατακόρυφων, πλευρικών στοιχείων τραχύτητας και λεκανών καταστροφής ενέργειας με οδοντώσεις. Η μελετώμενη διάταξη στηρίζεται στην ύπαρξη επαναλαμβανομένων τμημάτων (modules) εντός των οποίων επιτυγχάνεται επαναληψιμότητα της ροής. Η αποτελεσματικότητα του σχεδιασμού διερευνήθηκε σε υδραυλικό ομοίωμα κλίμακας 1:12,5 που βασίσθηκε σε συνθήκες δυναμικής ομοιότητας κατά Froude για χαρακτηριστικές τιμές παροχής. Η επεξεργασία των μετρήσεων έδειξε ότι με κατάλληλη διάταξη πλευρικών στοιχείων τραχύτητας και διαμόρφωση της λεκάνης καταστροφής ενέργειας ελέγχεται η τιμή της ταχύτητας, ικανοποιείται η απαίτηση μεγίστου βάθους ροής σε σχέση με τις διαστάσεις της σήραγγας και επιτυγχάνεται επαναληψιμότητα της ροής σε κάθε module.The free-surface flow in a tunnel of steep bed slope 1:10 is studied experimentally. The effectiveness of vertical roughness elements on the side walls and energy dissipation basins with blocks is investigated, with the aim to reduce flow velocity in the tunnel. The design is based on the concept of repeated modules in order to achieve flow repeatability. The scale of the physical model was 1:12.5 under Froude similarity conditions. The analysis of measurements indicates that a suitable arrangement of vertical roughness elements on the side walls and a suitable design of the energy dissipation basin can control the flow velocity magnitude, satisfy the maximum flow depth requirement with respect to the tunnel dimensions, and achieve flow repeatability in each modul
A counterbalanced cross-over study of the effects of visual, auditory and no feedback on performance measures in a simulated cardiopulmonary resuscitation
<p>Abstract</p> <p>Background</p> <p>Previous research has demonstrated that trained rescuers have difficulties achieving and maintaining the correct depth and rate of chest compressions during both in and out of hospital cardiopulmonary resuscitation (CPR). Feedback on rate and depth mitigate decline in performance quality but not completely with the residual performance decline attributed to rescuer fatigue. The purpose of this study was to examine the effects of feedback (none, auditory only and visual only) on the quality of CPR and rescuer fatigue.</p> <p>Methods</p> <p>Fifteen female volunteers performed 10 minutes of 30:2 CPR in each of three feedback conditions: none, auditory only, and visual only. Visual feedback was displayed continuously in graphic form. Auditory feedback was error correcting and provided by a voice assisted CPR manikin. CPR quality measures were collected using SkillReporter<sup>® </sup>software. Blood lactate (mmol/dl) and perceived exertion served as indices of fatigue. One-way and two way repeated measures analyses of variance were used with alpha set <it>a priori </it>at 0.05.</p> <p>Results</p> <p>Visual feedback yielded a greater percentage of correct compressions (78.1 ± 8.2%) than did auditory (65.4 ± 7.6%) or no feedback (44.5 ± 8.1%). Compression rate with auditory feedback (87.9 ± 0.5 compressions per minute) was less than it was with both visual and no feedback (p < 0.05). CPR performed with no feedback (39.2 ± 0.5 mm) yielded a shallower average depth of compression and a lower percentage (55 ± 8.9%) of compressions within the accepted 38-50 mm range than did auditory or visual feedback (p < 0.05). The duty cycle for auditory feedback (39.4 ± 1.6%) was less than it was with no feedback (p < 0.05). Auditory feedback produced lower lactate concentrations than did visual feedback (p < 0.05) but there were no differences in perceived exertion.</p> <p>Conclusions</p> <p>In this study feedback mitigated the negative effects of fatigue on CPR performance and visual feedback yielded better CPR performance than did no feedback or auditory feedback. The perfect confounding of sensory modality and periodicity of feedback (visual feedback provided continuously and auditory feedback provided to correct error) leaves unanswered the question of optimal form and timing of feedback.</p
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