303 research outputs found
FATTY ACIDS AND MERCURY IN SEVENTY SEVEN SPECIES OF COMMERCIALLY AVAILABLE FINFISH IN THE UNITED STATES
Finfish are consumed across the United States and constitute an important part of the American diet. However, seafood consumption can be a tenuous topic, with supporters highlighting the health benefits of eicosapentaenoic acid (EPA, C20:5n3) and docosahexaenoic acid (DHA, C22:6n3) and opponents emphasizing the neurotoxicity of methylmercury. Because all fish contain varying amounts of EPA, DHA, and methylmercury, the need for clear and unbiased information is essential to alleviate the confusion experienced by many consumers and empower them to make informed decisions regarding seafood consumption. As the market changes and more fish originate from aquaculture sources, where diets are controlled, consumer intakes of EPA, DHA, and methylmercury are changing. Thus, the goal of this project was to examine fatty acid and methylmercury content in 77 commercially available finfish species commonly consumed across the U.S.
EPA and DHA are important for the development of neurological function and eyesight in fetuses and infants, as well as heart health and the retention of cognitive abilities in aging populations. In accordance with these benefits, fatty acid profiles were determined for all collected species. EPA plus DHA content varied widely both within and between species. The fatty acid profiles of farmed species differed markedly from those of wild-caught species. Farmed species, including channel catfish, salmon (Atlantic and Chinook), and sturgeon (green and white), exhibited high concentrations of saturated fatty acids, monounsaturated fatty acids, and n-6 fatty acids. These differences stem from the lower costs associated with incorporating these fatty acids into farm-fed diets as compared to EPA and DHA. Some farmed species (rainbow trout, salmon, and sturgeon) were found to contain high levels of EPA and DHA, though the ratio of EPA plus DHA to other fatty acids was generally lower in farmed species than wild-caught species.
In contrast to the health benefits offered by EPA and DHA, methylmercury exposure may adversely affect neurological development. For most adults consuming moderate amounts of fish, methylmercury is not a significant health hazard. Fetuses and developing infants, however, are considerably more sensitive to the neurotoxic effects of methymercury. Therefore, pregnant and nursing women should exercise caution when consuming seafood. The second half of this project examined the mercury content of all finfish collected. Total mercury content was low in most species, including salmon, Alaskan pollock, Atlantic cod, tilapia, channel catfish, and pangasius/swai, which are among the top ten species consumed in the U.S. Total mercury content was also low in all farmed species studied, though wide variations were still observed within and between species. In order to keep blood mercury levels below the USEPA RfD of 0.1 μg/kg bw-day, 27 species examined in this study should be avoided by sensitive populations. In addition, swordfish (1107 ppb) and king mackerel (1425 ppb) contained mercury levels above the FDA Action Level of 1000 ppb, meaning that consumers are not being adequately protected from high mercury species entering the marketplace
Fluctuations of topological disclination lines in nematics: renormalization of the string model
The fluctuation eigenmode problem of the nematic topological disclination
line with strength is solved for the complete nematic tensor order
parameter. The line tension concept of a defect line is assessed, the line
tension is properly defined. Exact relaxation rates and thermal amplitudes of
the fluctuations are determined. It is shown that within the simple string
model of the defect line the amplitude of its thermal fluctuations is
significantly underestimated due to the neglect of higher radial modes. The
extent of universality of the results concerning other systems possessing line
defects is discussed.Comment: 6 pages, 3 figure
Nematic cells with defect-patterned alignment layers
Using Monte Carlo simulations of the Lebwohl--Lasher model we study the
director ordering in a nematic cell where the top and bottom surfaces are
patterned with a lattice of point topological defects of lattice
spacing . We find that the nematic order depends crucially on the ratio of
the height of the cell to . When the system is very
well--ordered and the frustration induced by the lattice of defects is relieved
by a network of half--integer defect lines which emerge from the point defects
and hug the top and bottom surfaces of the cell. When the
system is disordered and the half--integer defect lines thread through the cell
joining point defects on the top and bottom surfaces. We present a simple
physical argument in terms of the length of the defect lines to explain these
results. To facilitate eventual comparison with experimental systems we also
simulate optical textures and study the switching behavior in the presence of
an electric field
Two Experimental Tests of the Halperin-Lubensky-Ma Effect at the Nematic-Smectic-A Phase Transition
We have conducted two quantitative tests of predictions based on the
Halperin-Lubensky-Ma (HLM) theory of fluctuation-induced first-order phase
transitions. First, we explore the effect of an external magnetic field on the
nematic-smectic-A (NA) transition in a liquid crystal. Second, we examine the
dependence of the first-order discontinuity as a function of mixture
concentration in pure 8CB and three 8CB-10CB mixtures. We find the first
quantitative evidence for deviations from the HLM theory.Comment: 4 pages, 2 figure
Bulk and surface biaxiality in nematic liquid crystals
Nematic liquid crystals possess three different phases: isotropic, uniaxial,
and biaxial. The ground state of most nematics is either isotropic or uniaxial,
depending on the external temperature. Nevertheless, biaxial domains have been
frequently identified, especially close to defects or external surfaces. In
this paper we show that any spatially-varying director pattern may be a source
of biaxiality. We prove that biaxiality arises naturally whenever the symmetric
tensor \Sb=(\grad \nn)(\grad \nn)^T possesses two distinct nonzero
eigenvalues. The eigenvalue difference may be used as a measure of the expected
biaxiality. Furthermore, the corresponding eigenvectors indicate the directions
in which the order tensor \QQ is induced to break the uniaxial symmetry about
the director \nn. We apply our general considerations to some examples. In
particular we show that, when we enforce homeotropic anchoring on a curved
surface, the order tensor become biaxial along the principal directions of the
surface. The effect is triggered by the difference in surface principal
curvatures
Escape configuration lattice near the nematic-isotropic transition: Tilt analogue of blue phases
We predict the possible existence of a new phase of liquid crystals near the
nematic-isotropic () transition. This phase is an achiral, tilt-analogue
of the blue phase and is composed of a lattice of {\em double-tilt},
escape-configuration cylinders. We discuss the structure and the stability of
this phase and provide an estimate of the lattice parameter.Comment: 5 pages, 6 figures (major revision, typos corrected, references
added
Spatiotemporal rheochaos in nematic hydrodynamics
Motivated by the observation of rheochaos in sheared wormlike micelles
[Bandyopadhyay et al., Phys. Rev. Lett, 84 2022, (2000); Europhys. Lett. 56,
447 (2001); Pramana 53, 223 (1999)] we study the coupled nonlinear partial
differential equations for the hydrodynamic velocity and order parameter fields
in a sheared nematogenic fluid. In a suitable parameter range, we find
irregular, dynamic shear-banding and establish by decisive numerical tests that
the chaos we observe in the model is spatiotemporal in nature.Comment: Slight changes in text, references and Fig. 5 inset; 6 eps figures
(figs 2,3,4 at lower resolution to reduce file size; full files available on
request); accepted for publication in Phys Rev Let
Ring-Pattern Dynamics in Smectic-C* and Smectic-C_A* Freely Suspended Liquid Crystal Films
Ring patterns of concentric 2pi-solitons in molecular orientation, form in
freely suspended chiral smectic-C films in response to an in-plane rotating
electric field. We present measurements of the zero-field relaxation of ring
patterns and of the driven dynamics of ring formation under conditions of
synchronous winding, and a simple model which enables their quantitative
description in low polarization DOBAMBC. In smectic C_A* TFMHPOBC we observe an
odd-even layer number effect, with odd number layer films exhibiting order of
magnitude slower relaxation rates than even layer films. We show that this rate
difference is due to much larger spontaneous polarization in odd number layer
films.Comment: 4 RevTeX pgs, 4 eps figures, submitted to Phys. Rev. Let
The Complex Ginzburg-Landau Equation for Beginners
Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe
Ordered droplet structures at the liquid crystal surface and elastic-capillary colloidal interactions
We demonstrate a variety of ordered patterns, including hexagonal structures
and chains, formed by colloidal particles (droplets) at the free surface of a
nematic liquid crystal (LC). The surface placement introduces a new type of
particle interaction as compared to particles entirely in the LC bulk. Namely,
director deformations caused by the particle lead to distortions of the
interface and thus to capillary attraction. The elastic-capillary coupling is
strong enough to remain relevant even at the micron scale when its
buoyancy-capillary counterpart becomes irrelevant.Comment: 10 pages, 3 figures, to be published in Physical Review Letter
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