27 research outputs found
Growing length and time scales in glass forming liquids
We study the growing time scales and length scales associated with dynamical
slow down for a realistic glass former, using computer simulations. We perform
finite size scaling to evaluate a length scale associated with dynamical
heterogeneity which grows as temperature decreases. However, relaxation times
which also grow with decreasing temperature, do not show the same kind of
scaling behavior with system size as the dynamical heterogeneity, indicating
that relaxation times are not solely determined by the length scale of
dynamical heterogeneity. We show that relaxation times are instead determined,
for all studied system sizes and temperatures, by configurational entropy, in
accordance with the Adam-Gibbs relation, but in disagreement with theoretical
expectations based on spin-glass models that configurational entropy is not
relevant at temperatures substantially above the critical temperature of mode
coupling theory. The temperature dependence of the heterogeneity length scale
shows significant deviations from theoretical expectations, and the length
scale one may extract from the system size dependence of the configurational
entropy has much weaker temperature dependence compared to the heterogeneity
length scale at all studied temperatures. Our results provide new insights into
the dynamics of glass-forming liquids and pose serious challenges to existing
theoretical descriptions
Short-time -relaxation in glass-forming liquids is cooperative in nature
Temporal relaxation of density fluctuations in supercooled liquids near the
glass transition occurs in multiple steps. The short-time -relaxation is
generally attributed to spatially local processes involving the rattling motion
of a particle in the transient cage formed by its neighbors. Using molecular
dynamics simulations for three model glass-forming liquids, we show that the
-relaxation is actually cooperative in nature. Using finite-size scaling
analysis, we extract a growing length-scale associated with -relaxation
from the observed dependence of the -relaxation time on the system size.
Remarkably, the temperature dependence of this length scale is found to be the
same as that of the length scale that describes the spatial heterogeneity of
local dynamics in the long-time -relaxation regime. These results show
that the conventional interpretation of -relaxation as a local process
is too simplified and provide a clear connection between short-time dynamics
and long-time structural relaxation in glass-forming liquids
The Adam-Gibbs relation for glass-forming liquids in 2, 3 and 4 dimensions
The Adam-Gibbs relation between relaxation times and the configurational
entropy has been tested extensively for glass formers using experimental data
and computer simulation results. Although the form of the relation contains no
dependence on the spatial dimensionality in the original formulation,
subsequent derivations of the Adam-Gibbs relation allow for such a possibility.
We test the Adam-Gibbs relation in 2, 3, and 4 spatial dimensions using
computer simulations of model glass formers. We find that the relation is valid
in 3 and 4 dimensions. But in 2 dimensions, the relation does not hold, and
interestingly, no single alternate relation describes the results for the
different model systems we study.Comment: Submitted to Phys. Rev. Let
Breakdown of the Stokes-Einstein relation in two, three and four dimensions
The breakdown of the Stokes-Einstein (SE) relation between diffusivity and
viscosity at low temperatures is considered to be one of the hallmarks of
glassy dynamics in liquids. Theoretical analyses relate this breakdown with the
presence of heterogeneous dynamics, and by extension, with the fragility of
glass formers. We perform an investigation of the breakdown of the SE relation
in 2, 3 and 4 dimensions, in order to understand these interrelations. Results
from simulations of model glass formers show that the degree of the breakdown
of the SE relation decreases with increasing spatial dimensionality. The
breakdown itself can be rationalized via the difference between the activation
free energies for diffusivity and viscosity (or relaxation times) in the
Adam-Gibbs relation in three and four dimensions. The behavior in two
dimensions also can be understood in terms of a generalized Adam-Gibbs relation
that is observed in previous work. We calculate various measures of
heterogeneity of dynamics and find that the degree of the SE breakdown and
measures of heterogeneity of dynamics are generally well correlated but with
some exceptions. The two dimensional systems we study show deviations from the
pattern of behavior of the three and four dimensional systems both at high and
low temperatures. The fragility of the studied liquids is found to increase
with spatial dimensionality, contrary to the expectation based on the
association of fragility with heterogeneous dynamics
Glass Transition in Supercooled Liquids with Medium Range Crystalline Order
The origins of rapid dynamical slow down in glass forming liquids in the
growth of static length scales, possibly associated with identifiable
structural ordering, is a much debated issue. Growth of medium range
crystalline order (MRCO) has been observed in various model systems to be
associated with glassy behaviour. Such observations raise the question about
the eventual state reached by a glass former, if allowed to relax for
sufficiently long times. Is a slowly growing crystalline order responsible for
slow dynamics? Are the molecular mechanisms for glass transition in liquids
with and without MRCO the same? If yes, glass formers with MRCO provide a
paradigm for understanding glassy behaviour generically. If not, systems with
MRCO form a new class of glass forming materials whose molecular mechanism for
slow dynamics may be easier to understand in terms of growing crystalline
order, and should be approached in that manner, even while they will not
provide generic insights. In this study we perform extensive molecular dynamics
simulations of a number of glass forming liquids in two dimensions and show
that the static and dynamic properties of glasses with MRCO are different from
other glass forming liquids with no predominant local order. We also resolve an
important issue regarding the so-called Point-to-set method for determining
static length scales, and demonstrate it to be a robust, order agnostic, method
for determining static correlation lengths in glass formers
Standardizing data exchange for clinical research protocols and case report forms: An assessment of the suitability of the Clinical Data Interchange Standards Consortium (CDISC) Operational Data Model (ODM)
AbstractEfficient communication of a clinical study protocol and case report forms during all stages of a human clinical study is important for many stakeholders. An electronic and structured study representation format that can be used throughout the whole study life-span can improve such communication and potentially lower total study costs. The most relevant standard for representing clinical study data, applicable to unregulated as well as regulated studies, is the Operational Data Model (ODM) in development since 1999 by the Clinical Data Interchange Standards Consortium (CDISC). ODM’s initial objective was exchange of case report forms data but it is increasingly utilized in other contexts. An ODM extension called Study Design Model, introduced in 2011, provides additional protocol representation elements.Using a case study approach, we evaluated ODM’s ability to capture all necessary protocol elements during a complete clinical study lifecycle in the Intramural Research Program of the National Institutes of Health. ODM offers the advantage of a single format for institutions that deal with hundreds or thousands of concurrent clinical studies and maintain a data warehouse for these studies. For each study stage, we present a list of gaps in the ODM standard and identify necessary vendor or institutional extensions that can compensate for such gaps. The current version of ODM (1.3.2) has only partial support for study protocol and study registration data mainly because it is outside the original development goal. ODM provides comprehensive support for representation of case report forms (in both the design stage and with patient level data). Inclusion of requirements of observational, non-regulated or investigator-initiated studies (outside Food and Drug Administration (FDA) regulation) can further improve future revisions of the standard
Under-Five Mortality in High Focus States in India: A District Level Geospatial Analysis
<div><h3>Background</h3><p>This paper examines if, when controlling for biophysical and geographical variables (including rainfall, productivity of agricultural lands, topography/temperature, and market access through road networks), socioeconomic and health care indicators help to explain variations in the under-five mortality rate across districts from nine high focus states in India. The literature on this subject is inconclusive because the survey data, upon which most studies of child mortality rely, rarely include variables that measure these factors. This paper introduces these variables into an analysis of 284 districts from nine high focus states in India.</p> <h3>Methodology/Principal Findings</h3><p>Information on the mortality indicator was accessed from the recently conducted Annual Health Survey of 2011 and other socioeconomic and geographic variables from Census 2011, District Level Household and Facility Survey (2007–08), Department of Economics and Statistics Divisions of the concerned states. Displaying high spatial dependence (spatial autocorrelation) in the mortality indicator (outcome variable) and its possible predictors used in the analysis, the paper uses the Spatial-Error Model in an effort to negate or reduce the spatial dependence in model parameters. The results evince that the coverage gap index (a mixed indicator of district wise coverage of reproductive and child health services), female literacy, urbanization, economic status, the number of newborn care provided in Primary Health Centers in the district transpired as significant correlates of under-five mortality in the nine high focus states in India. The study identifies three clusters with high under-five mortality rate including 30 districts, and advocates urgent attention.</p> <h3>Conclusion</h3><p>Even after controlling the possible biophysical and geographical variables, the study reveals that the health program initiatives have a major role to play in reducing under-five mortality rate in the high focus states in India.</p> </div