1,457 research outputs found
Position clamping of optically trapped microscopic non-spherical probes
We investigate the degree of control that can be exercised over an optically trapped microscopic non-spherical force probe. By position clamping translational and rotational modes in different ways, we are able to dramatically improve the position resolution of our probe with no reduction in sensitivity. We also demonstrate control over rotational-translational coupling, and exhibit a mechanism whereby the average centre of rotation of the probe can be displaced away from its centre
A compact holographic optical tweezers instrument
Holographic optical tweezers have found many applications including the construction of complex micron-scale 3D structures and the control of tools and probes for position, force, and viscosity measurement. We have developed a compact, stable, holographic optical tweezers instrument which can be easily transported and is compatible with a wide range of microscopy techniques, making it a valuable tool for collaborative research. The instrument measures approximately 30×30×35 cm and is designed around a custom inverted microscope, incorporating a fibre laser operating at 1070 nm. We designed the control software to be easily accessible for the non-specialist, and have further improved its ease of use with a multi-touch iPad interface. A high-speed camera allows multiple trapped objects to be tracked simultaneously. We demonstrate that the compact instrument is stable to 0.5 nm for a 10 s measurement time by plotting the Allan variance of the measured position of a trapped 2 μm silica bead. We also present a range of objects that have been successfully manipulated
Community-based survey versus sentinel site sampling in determining the nutritional status of rural children: Implications for nutritional surveillance and the development of nutritional programmes
A study of the anthropometric status of under-5-year-olds was conducted in the Nqutu district of Kwazulu by means of a representative community-based sample and concurrent samples from primary health care clinics, preschools and primary schools.The first objective of this exercise was to determine the extent of acute nutritional stress in the district as an indication of the appropriateness of food relief efforts in the region. The absence of wasting and the high prevalence of stunting (37,5%) in the community-based sample suggested that the main problem is chronic socioeconomic underdevelopment, rather than a severe or immediate lack of food. The fact that fewer than 20% of households are in any way reliant on domestic production for their maize requirements explains why the recent drought has not had a greater impact on the nutritional status of this vulnerable group. This study confirms that the more recent emphasis of the National Nutrition and Social Development Programme on social development is appropriate.An equally important objective of this study was to evaluate the usefulness of clinics, preschools and schools as sites for the collection of anthropometric data and the development of nutritional programmes. This preliminary attempt to develop the methodology for district-based nutrition surveillance suggested that all these sites have limitations both in respect of data collection and community access. The implications of using these sites and the developments needed to improve their usefulness in a future nutrition surveillance system are discussed
Constructing 3D crystal templates for photonic band gap materials using holographic optical tweezers
A simple and robust method is presented for the construction of 3-dimensional crystals from silica and polystyrene microspheres. The crystals are suitable for use as templates in the production of three-dimensional photonic band gap (PBG) materials. Manipulation of the microspheres was achieved using a dynamic holographic assembler (DHA) consisting of computer controlled holographic optical tweezers. Attachment of the microspheres was achieved by adjusting their colloidal interactions during assembly. The method is demonstrated by constructing a variety of 3-dimensional crystals using spheres ranging in size from 3 µm down to 800 nm. A major advantage of the technique is that it may be used to build structures that cannot be made using self-assembly. This is illustrated through the construction of crystals in which line defects have been deliberately included, and by building simple cubic structures
High rates of suicide and attempted suicide using pesticides in Nickerie, Suriname, South America
Suicide and attempted suicide are identified as a serious mental health problem in Suriname, especially in the district of Nickerie. An epidemiological study in the Nickerie catchment area revealed high rates of suicide (48 per 100,000) and attempted suicide (207 per 100,000) on average in the years 2000–2004. Particularly remarkable is the high number of attempted suicides among males (49%), and the use of pesticides in both fatal (55%) and nonfatal suicidal behavior (44%). Probably this high incidence of suicidal behavior reflects the very poor economic situation of the district, poverty of most of the population, high levels of alcohol misuse, domestic violence, the rigidity of Hindustani culture regarding family traditions, the accessibility of pesticides, and the lack of future perspectives. Health care alone will not be sufficient to tackle this problem. One of the most urgent measures to prevent suicides is to stow away pesticides in locked cabinets with the key held by the proprietor
Triton photodisintegration in three-dimensional approach
Two- and three- particles photodisintegration of the triton is investigated
in a three-dimensional (3D) Faddeev approach. For this purpose the Jacobi
momentum vectors for three particles system and spin-isospin quantum numbers of
the individual nucleons are considered. Based on this picture the three-nucleon
Faddeev integral equations with the two-nucleon interaction are formulated
without employing the partial wave decomposition. The single nucleon current as
well as and like exchange currents are used in an appropriate
form to be employed in 3D approach. The exchange currents are derived from AV18
NN force. The two-body t-matrix, Deuteron and Triton wave functions are
calculated in the 3D approach by using AV18 potential. Benchmarks are presented
to compare the total cross section for the two- and three- particles
photodisintegration in the range of . The 3D Faddeev
approach shows promising results
A Parametric Study of Erupting Flux Rope Rotation. Modeling the "Cartwheel CME" on 9 April 2008
The rotation of erupting filaments in the solar corona is addressed through a
parametric simulation study of unstable, rotating flux ropes in bipolar
force-free initial equilibrium. The Lorentz force due to the external shear
field component and the relaxation of tension in the twisted field are the
major contributors to the rotation in this model, while reconnection with the
ambient field is of minor importance. Both major mechanisms writhe the flux
rope axis, converting part of the initial twist helicity, and produce rotation
profiles which, to a large part, are very similar in a range of shear-twist
combinations. A difference lies in the tendency of twist-driven rotation to
saturate at lower heights than shear-driven rotation. For parameters
characteristic of the source regions of erupting filaments and coronal mass
ejections, the shear field is found to be the dominant origin of rotations in
the corona and to be required if the rotation reaches angles of order 90
degrees and higher; it dominates even if the twist exceeds the threshold of the
helical kink instability. The contributions by shear and twist to the total
rotation can be disentangled in the analysis of observations if the rotation
and rise profiles are simultaneously compared with model calculations. The
resulting twist estimate allows one to judge whether the helical kink
instability occurred. This is demonstrated for the erupting prominence in the
"Cartwheel CME" on 9 April 2008, which has shown a rotation of \approx 115
degrees up to a height of 1.5 R_sun above the photosphere. Out of a range of
initial equilibria which include strongly kink-unstable (twist Phi=5pi), weakly
kink-unstable (Phi=3.5pi), and kink-stable (Phi=2.5pi) configurations, only the
evolution of the weakly kink-unstable flux rope matches the observations in
their entirety.Comment: Solar Physics, submitte
3D MHD Flux Emergence Experiments: Idealized models and coronal interactions
This paper reviews some of the many 3D numerical experiments of the emergence
of magnetic fields from the solar interior and the subsequent interaction with
the pre-existing coronal magnetic field. The models described here are
idealized, in the sense that the internal energy equation only involves the
adiabatic, Ohmic and viscous shock heating terms. However, provided the main
aim is to investigate the dynamical evolution, this is adequate. Many
interesting observational phenomena are explained by these models in a
self-consistent manner.Comment: Review article, accepted for publication in Solar Physic
Processing of aluminum-graphite particulate metal matrix composites by advanced shear technology
Copyright @ 2009 ASM International. This paper was published in Journal of Materials Engineering and Performance 18(9) and is made available as an electronic reprint with the permission of ASM International. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplications of any material in this paper for a fee or for commercial purposes, or modification of the content of this paper are prohibited.To extend the possibilities of using aluminum/graphite composites as structural materials, a novel process is developed. The conventional methods often produce agglomerated structures exhibiting lower strength and ductility. To overcome the cohesive force of the agglomerates, a melt conditioned high-pressure die casting
(MC-HPDC) process innovatively adapts the well-established, high-shear dispersive mixing action of a twin screw mechanism. The distribution of particles and properties of composites are quantitatively evaluated.
The adopted rheo process significantly improved the distribution of the reinforcement in the matrix with a strong interfacial bond between the two. A good combination of improved ultimate tensile strength (UTS) and tensile elongation (e) is obtained compared with composites produced by conventional processes.EPSR
Observations of Coronal Mass Ejections with the Coronal Multichannel Polarimeter
The Coronal Multichannel Polarimeter (CoMP) measures not only the
polarization of coronal emission, but also the full radiance profiles of
coronal emission lines. For the first time, CoMP observations provide
high-cadence image sequences of the coronal line intensity, Doppler shift and
line width simultaneously in a large field of view. By studying the Doppler
shift and line width we may explore more of the physical processes of CME
initiation and propagation. Here we identify a list of CMEs observed by CoMP
and present the first results of these observations. Our preliminary analysis
shows that CMEs are usually associated with greatly increased Doppler shift and
enhanced line width. These new observations provide not only valuable
information to constrain CME models and probe various processes during the
initial propagation of CMEs in the low corona, but also offer a possible
cost-effective and low-risk means of space weather monitoring.Comment: 6 figures. Will appear in the special issue of Coronal Magnetism,
Sol. Phy
- …