1,618 research outputs found
In utero DDT and DDE exposure and obesity status of 7-year-old Mexican-American children in the CHAMACOS cohort.
BackgroundIn utero exposure to endocrine disrupting compounds including dichlorodiphenyltrichloroethane (DDT) and dichlorodiphenyldichloroethylene (DDE) has been hypothesized to increase risk of obesity later in life.ObjectivesThe Center for the Health Assessment of Mothers and Children of Salinas (CHAMACOS) study is a longitudinal birth cohort of low-income Latinas living in a California agricultural community. We examined the relation of in utero DDT and DDE exposure to child obesity at 7 years of age. We also examined the trend with age (2, 3.5, 5, and 7 years) in the exposure-obesity relation.MethodsWe included 270 children with o,p´-DDT, p,p´-DDT, and p,p´-DDE concentrations measured in maternal serum during pregnancy (nanograms per gram lipid) and complete 7-year follow-up data including weight (kilograms) and height (centimeters). Body mass index (BMI; kilograms per meter squared) was calculated and obesity was defined as ≥ 95th percentile on the sex-specific BMI-for-age Centers for Disease Control and Prevention 2000 growth charts.ResultsAt 7 years, 96 (35.6%) children were obese. A 10-fold increase in o,p´-DDT, p,p´-DDT, or p,p´-DDE, was nonsignificantly associated with increased odds (OR) of obesity [o,p´-DDT adjusted (adj-) OR = 1.17, 95% CI: 0.75, 1.82; p,p´-DDT adj-OR = 1.19, 95% CI: 0.81, 1.74; p,p´-DDE adj-OR = 1.22, 95% CI: 0.72, 2.06]. With increasing age at follow-up, we observed a significant trend toward a positive association between DDT and DDE exposure and odds of obesity.ConclusionWe did not find a significant positive relation between in utero DDT and DDE exposure and obesity status of 7-year-old children. However, given the observed trend with age, continued follow-up will be informative
Magnetic coupling in colloidal clusters for hierarchical self-assembly
Manipulating the way in which colloidal particles self-organise is a central
challenge in the design of functional soft materials. Meeting this challenge
requires the use of building blocks that interact with one another in a highly
specific manner. Their fabrication, however, is limited by the complexity of
the available synthesis procedures. Here, we demonstrate that, starting from
experimentally available magnetic colloids, we can create a variety of complex
building blocks suitable for hierarchical self-organisation using a simple
scalable process. Using computer simulations, we compress spherical and cubic
magnetic colloids in spherical confinement, and investigate their suitability
to form small clusters with reproducible structural and magnetic properties. We
find that, while the structure of these clusters is highly reproducible, their
magnetic character depends on the particle shape. Only spherical particles have
the rotational degrees of freedom to produce consistent magnetic
configurations, whereas cubic particles frustrate the minimisation of the
cluster energy, resulting in various magnetic configurations. To highlight
their potential for self-assembly, we demonstrate that already clusters of
three magnetic particles form highly nontrivial Archimedean lattices, namely
staggered kagome, bounce and honeycomb, when viewing different aspects of the
same monolayer structure. The work presented here offers a conceptually
different way to design materials by utilizing pre-assembled magnetic building
blocks that can readily self-organise into complex structures.Comment: Main text: 13 pages, 6 figures. SI:14 pages, 11 figure
Extended Wertheim theory predicts the anomalous chain length distributions of divalent patchy particles under extreme confinement
Colloidal patchy particles with divalent attractive interaction can
self-assemble into linear polymer chains. Their equilibrium properties in 2D
and 3D are well described by Wertheim's thermodynamic perturbation theory which
predicts a well-defined exponentially decaying equilibrium chain length
distribution. In experimental realizations, due to gravity, particles sediment
to the bottom of the suspension forming a monolayer of particles with a
gravitational height smaller than the particle diameter. In accordance with
experiments, an anomalously high monomer concentration is observed in
simulations which is not well understood. To account for this observation, we
interpret the polymerization as taking place in a highly confined quasi-2D
plane and extend the Wertheim thermodynamic perturbation theory by defining
addition reactions constants as functions of the chain length. We derive the
theory, test it on simple square well potentials, and apply it to the
experimental case of synthetic colloidal patchy particles immersed in a binary
liquid mixture that are described by an accurate effective critical Casimir
patchy particle potential. The important interaction parameters entering the
theory are explicitly computed using the integral method in combination with
Monte Carlo sampling. Without any adjustable parameter, the predictions of the
chain length distribution are in excellent agreement with explicit simulations
of self-assembling particles. We discuss generality of the approach, and its
application range.Comment: The following article has been submitted to The Journal of Chemical
Physic
Shape and Interaction Decoupling for Colloidal Pre-Assembly
Creating materials with a structural hierarchy that is independently
controllable at a range of scales requires breaking naturally occurring
hierarchies. Breaking natural hierarchies is possible if building block
attributes can be decoupled from the structure of pre-assembled, mesoscale
building blocks that form the next level in the structural hierarchy. Here, we
show that pre-assembled colloidal structures achieving geometric and
interaction decoupling can be prepared in emulsions of silica superballs, which
are cubic-like particles with rounded edges. We show that, for clusters of up
to nine particles, colloidal superballs pack consistently like spheres, despite
the presence of shape anisotropy and facets in the cubic-like particles. We
compare our results with clusters prepared with magnetic superballs and find
good qualitative agreement, suggesting that the cluster geometries are solely
determined by the shape of the constituent particles. Our findings demonstrate
that highly shape-anisotropic building blocks, under suitable conditions, can
be pre-assembled into structures that are not found in bulk, thereby achieving
a decoupling that can be further exploited for hierarchical materials
development.Comment: 22 pages, 7 figure
Incentivizing High Quality Crowdwork
We study the causal effects of financial incentives on the quality of
crowdwork. We focus on performance-based payments (PBPs), bonus payments
awarded to workers for producing high quality work. We design and run
randomized behavioral experiments on the popular crowdsourcing platform Amazon
Mechanical Turk with the goal of understanding when, where, and why PBPs help,
identifying properties of the payment, payment structure, and the task itself
that make them most effective. We provide examples of tasks for which PBPs do
improve quality. For such tasks, the effectiveness of PBPs is not too sensitive
to the threshold for quality required to receive the bonus, while the magnitude
of the bonus must be large enough to make the reward salient. We also present
examples of tasks for which PBPs do not improve quality. Our results suggest
that for PBPs to improve quality, the task must be effort-responsive: the task
must allow workers to produce higher quality work by exerting more effort. We
also give a simple method to determine if a task is effort-responsive a priori.
Furthermore, our experiments suggest that all payments on Mechanical Turk are,
to some degree, implicitly performance-based in that workers believe their work
may be rejected if their performance is sufficiently poor. Finally, we propose
a new model of worker behavior that extends the standard principal-agent model
from economics to include a worker's subjective beliefs about his likelihood of
being paid, and show that the predictions of this model are in line with our
experimental findings. This model may be useful as a foundation for theoretical
studies of incentives in crowdsourcing markets.Comment: This is a preprint of an Article accepted for publication in WWW
\c{opyright} 2015 International World Wide Web Conference Committe
Colloidal aggregation in microgravity by critical Casimir forces
By using the critical Casimir force, we study the attractive strength
dependent aggregation of colloids with and without gravity by means of Near
Field scattering. Significant differences were seen between microgravity and
ground experiments, both in the structure of the formed fractal aggregates as
well as the kinetics of growth. Ground measurements are severely affected by
sedimentation resulting in reaction limited behavior. In microgravity, a purely
diffusive behavior is seen reflected both in the measured fractal dimensions
for the aggregates as well as the power law behavior in the rate of growth.
Formed aggregates become more open as the attractive strength increases.Comment: 4 pages, 3 figure
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