208 research outputs found
Dynamics of 8CB confined into porous silicon probed by incoherent neutron backscattering experiments
Confinement in the nanochannels of porous silicon strongly affects the phase
behavior of the archetype liquid-crystal 4-n-octyl-4-cyanobiphenyl (8CB). A
very striking phenom- enon is the development of a short-range smectic order,
which occurs on a very broad temperature range. It suggests in this case that
quenched disorder effects add to usual finite size and surface interaction
effects. We have monitored the temperature variation of the molecular dynamics
of the confined fluid by incoherent quasielastic neutron scat- tering. A
strongly reduced mobility is observed at the highest temperatures in the liquid
phase, which suggests that the interfacial molecular dynamics is strongly
hindered. A continuously increasing slowdown appears on cooling together with a
progressive growth of the static correlation lengt
Rich polymorphism of a rod-like liquid crystal (8CB) confined in two types of unidirectional nanopores
We present a neutron and X-rays scattering study of the phase transitions of
4-n-octyl-4'-cyanobiphenyl (8CB) confined in unidirectional nanopores of porous
alumina and porous silicon (PSi) membranes with an average diameter of 30 nm.
Spatial confinement reveals a rich polymorphism, with at least four different
low temperature phases in addition to the smectic A phase. The structural study
as a function of thermal treatments and conditions of spatial confinement
allows us to get insights into the formation of these phases and their relative
stability. It gives the first description of the complete phase behavior of 8CB
confined in PSi and provides a direct comparison with results obtained in bulk
conditions and in similar geometric conditions of confinement but with reduced
quenched disorder effects using alumina anopore membranesComment: Accepted in EPJ E - Soft Matte
ESR of MnO embedded in silica nanoporous matrices with different topologies
Electron spin resonance (ESR) experiments were performed with
antiferromagnetic MnO confined within a porous vycor-type glass and within
MCM-type channel matrices. A signal from confined MnO shows two components from
crystallized and amorphous MnO and depends on the pore topology. Crystallized
MnO within a porous glass shows a behavior having many similarities to the
bulk. In contrast with the bulk the strong ESR signal due to disordered
"surface" spins is observed below the magnetic transition. With the decrease of
channel diameter the fraction of amorphous MnO increases while the amount of
crystallized MnO decreases. The mutual influence of amorphous and crystalline
MnO is observed in the matrices with a larger channel diameter. In the matrices
with a smaller channel diameter the ESR signal mainly originates from amorphous
MnO and its behavior is typical for the highly disordered magnetic system.Comment: 7 pages pdf file, 5 figure
Molecular dynamics of glycerol and glycerol-trehalose bioprotectant solutions nanoconfined in porous silicon
Glycerol and trehalose-glycerol binary solutions are glass-forming liquids
with remarkable bioprotectant properties. Incoherent quasielastic neutron
scattering (QENS) is used to reveal the different effects of nanoconfinement
and addition of trehalose on the molecular dynamics in the normal liquid and
supercooled liquid phases, on a nanosecond timescale. Confinement has been
realized in straight channels of diameter D=8 nm formed by porous silicon. It
leads to a faster and more inhomogeneous relaxation dynamics deep in the liquid
phase. This confinement effect remains at lower temperature where it affects
the glassy dynamics. The glass transitions of the confined systems are shifted
to low temperature with respect to the bulk ones. Adding trehalose tends to
slow down the overall glassy dynamics and increases the non-exponential
character of the structural relaxation. Unprecedented results are obtained for
the binary bioprotectant solution, which exhibits an extremely non-Debye
relaxation dynamics as a result of the combination of the effects of
confinement and mixing of two constituents
Molecular dynamics of a short range ordered smectic phase nanoconfined into porous silicon
4-n-octyl-4-cyanobiphenyl (8CB) has been recently shown to display an unusual
sequence of phases when confined into porous silicon (PSi). The gradual
increase of oriented short-range smectic (SRS) correlations in place of a phase
transition has been interpreted as a consequence of the anisotropic quenched
disorder induced by confinement in PSi. Combining two quasielastic neutron
scattering experiments with complementary energy resolutions, we present the
first investigation of the individual molecular dynamics of this system. A
large reduction of the molecular dynamics is observed in the confined liquid
phase, as a direct consequence of the dynamical boundary conditions imposed by
the confinement. Temperature fixed window scans (FWS) reveal a continuous
'glass-like' reduction of the molecular dynamics of the confined liquid and SRS
phases on cooling down to 250 K, where a solid-like behavior is finally reached
by a two steps crystallization process
Neutron scattering and molecular correlations in a supercooled liquid
We show that the intermediate scattering function for neutron
scattering (ns) can be expanded naturely with respect to a set of molecular
correlation functions that give a complete description of the translational and
orientational two-point correlations in the liquid. The general properties of
this expansion are discussed with special focus on the -dependence and hints
for a (partial) determination of the molecular correlation functions from
neutron scattering results are given. The resulting representation of the
static structure factor is studied in detail for a model system using
data from a molecular dynamics simulation of a supercooled liquid of rigid
diatomic molecules. The comparison between the exact result for and
different approximations that result from a truncation of the series
representation demonstrates its good convergence for the given model system. On
the other hand it shows explicitly that the coupling between translational
(TDOF) and orientational degrees of freedom (ODOF) of each molecule and
rotational motion of different molecules can not be neglected in the
supercooled regime.Further we report the existence of a prepeak in the
ns-static structure factor of the examined fragile glassformer, demonstrating
that prepeaks can occur even in the most simple molecular liquids. Besides
examining the dependence of the prepeak on the scattering length and the
temperature we use the expansion of into molecular correlation
functions to point out intermediate range orientational order as its principle
origin.Comment: 13 pages, 7 figure
Large-scale STI services in Avahan improve utilization and treatment seeking behaviour amongst high-risk groups in India: an analysis of clinical records from six states
<p>Abstract</p> <p>Background</p> <p>Avahan, the India AIDS Initiative, implemented a large HIV prevention programme across six high HIV prevalence states amongst high risk groups consisting of female sex workers, high risk men who have sex with men, transgenders and injecting drug users in India. Utilization of the clinical services, health seeking behaviour and trends in syndromic diagnosis of sexually transmitted infections amongst these populations were measured using the individual tracking data.</p> <p>Methods</p> <p>The Avahan clinical monitoring system included individual tracking data pertaining to clinical services amongst high risk groups. All clinic visits were recorded in the routine clinical monitoring system using unique identification numbers at the NGO-level. Visits by individual clinic attendees were tracked from January 2005 to December 2009. An analysis examining the limited variables over time, stratified by risk group, was performed.</p> <p>Results</p> <p>A total of 431,434 individuals including 331,533 female sex workers, 10,280 injecting drug users, 82,293 men who have sex with men, and 7,328 transgenders visited the clinics with a total of 2,700,192 visits. Individuals made an average of 6.2 visits to the clinics during the study period. The number of visits per person increased annually from 1.2 in 2005 to 8.3 in 2009. The proportion of attendees visiting clinics more than four times a year increased from 4% in 2005 to 26% in 2009 (p<0.001). The proportion of STI syndromes diagnosed amongst female sex workers decreased from 39% in 2005 to 11% in 2009 (p<0.001) while the proportion of STI syndromes diagnosed amongst high risk men who have sex with men decreased from 12% to 3 % (p<0.001). The proportion of attendees seeking regular STI check-ups increased from 12% to 48% (p<0.001). The proportion of high risk groups accessing clinics within two days of onset of STI-related symptoms and acceptability of speculum and proctoscope examination increased significantly during the programme implementation period.</p> <p>Conclusions</p> <p>The programme demonstrated that acceptable and accessible services with marginalised and often difficultâto-reach populations can be brought to a very large scale using standardized approaches. Utilization of these services can dramatically improve health seeking behaviour and reduce STI prevalence.</p
Dynamics in a supercooled molecular liquid: Theory and Simulations
We report extensive simulations of liquid supercooled states for a simple
three-sites molecular model, introduced by Lewis and Wahnstr"om [L. J. Lewis
and G. Wahnstr"om, Phys. Rev. E 50, 3865 (1994)] to mimic the behavior of
ortho-terphenyl. The large system size and the long simulation length allow to
calculate very precisely --- in a large q-vector range --- self and collective
correlation functions, providing a clean and simple reference model for
theoretical descriptions of molecular liquids in supercooled states. The time
and wavevector dependence of the site-site correlation functions are compared
with detailed predictions based on ideal mode-coupling theory, neglecting the
molecular constraints. Except for the wavevector region where the dynamics is
controlled by the center of mass (around 9 nm-1), the theoretical predictions
compare very well with the simulation data.
Physical Origin of the Boson Peak Deduced from a Two-Order-Parameter Model of Liquid
We propose that the boson peak originates from the (quasi-) localized
vibrational modes associated with long-lived locally favored structures, which
are intrinsic to a liquid state and are randomly distributed in a sea of
normal-liquid structures. This tells us that the number density of locally
favored structures is an important physical factor determining the intensity of
the boson peak. In our two-order-parameter model of the liquid-glass
transition, the locally favored structures act as impurities disturbing
crystallization and thus lead to vitrification. This naturally explains the
dependence of the intensity of the boson peak on temperature, pressure, and
fragility, and also the close correlation between the boson peak and the first
sharp diffraction peak (or prepeak).Comment: 5 pages, 1 figure, An error in the reference (Ref. 7) was correcte
Validity of measures of pain and symptoms in HIV/AIDS infected households in resources poor settings: results from the Dominican Republic and Cambodia
BACKGROUND: HIV/AIDS treatment programs are currently being mounted in many developing nations that include palliative care services. While measures of palliative care have been developed and validated for resource rich settings, very little work exists to support an understanding of measurement for Africa, Latin America or Asia. METHODS: This study investigates the construct validity of measures of reported pain, pain control, symptoms and symptom control in areas with high HIV-infected prevalence in Dominican Republic and Cambodia Measures were adapted from the POS (Palliative Outcome Scale). Households were selected through purposive sampling from networks of people living with HIV/AIDS. Consistencies in patterns in the data were tested used Chi Square and Mantel Haenszel tests. RESULTS: The sample persons who reported chronic illness were much more likely to report pain and symptoms compared to those not chronically ill. When controlling for the degrees of pain, pain control did not differ between the chronically ill and non-chronically ill using a Mantel Haenszel test in both countries. Similar results were found for reported symptoms and symptom control for the Dominican Republic. These findings broadly support the construct validity of an adapted version of the POS in these two less developed countries. CONCLUSION: The results of the study suggest that the selected measures can usefully be incorporated into population-based surveys and evaluation tools needed to monitor palliative care and used in settings with high HIV/AIDS prevalence
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