633 research outputs found
Propagation of Surface Plasmons in Ordered and Disordered Chains of Metal Nanospheres
We report a numerical investigation of surface plasmon (SP) propagation in
ordered and disordered linear chains of metal nanospheres. In our simulations,
SPs are excited at one end of a chain by a near-field tip. We then find
numerically the SP amplitude as a function of propagation distance. Two types
of SPs are discovered. The first SP, which we call the ordinary or quasistatic,
is mediated by short-range, near-field electromagnetic interaction in the
chain. This excitation is strongly affected by Ohmic losses in the metal and by
disorder in the chain. These two effects result in spatial decay of the
quasistatic SP by means of absorptive and radiative losses, respectively. The
second SP is mediated by longer range, far-field interaction of nanospheres. We
refer to this SP as the extraordinary or non-quasistatic. The non-quasistatic
SP can not be effectively excited by a near-field probe due to the small
integral weight of the associated spectral line. Because of that, at small
propagation distances, this SP is dominated by the quasistatic SP. However, the
non-quasistatic SP is affected by Ohmic and radiative losses to a much smaller
extent than the quasistatic one. Because of that, the non-quasistatic SP
becomes dominant sufficiently far from the exciting tip and can propagate with
little further losses of energy to remarkable distances. The unique physical
properties of the non-quasistatic SP can be utilized in all-optical integrated
photonic systems
Age-dependent decrease in glutamine synthetase expression in the hippocampal astroglia of the triple transgenic Alzheimer's disease mouse model : mechanism for deficient glutamatergic transmission?
Astrocytes are fundamental for brain homeostasis and the progression and outcome of many neuropathologies including Alzheimer's disease (AD). In the triple transgenic mouse model of AD (3xTg-AD) generalised hippocampal astroglia atrophy precedes a restricted and specific beta-amyloid (A beta) plaque-related astrogliosis. Astrocytes are critical for CNS glutamatergic transmission being the principal elements of glutamate homeostasis through maintaining its synthesis, uptake and turnover via glutamate-glutamine shuttle. Glutamine synthetase (GS), which is specifically expressed in astrocytes, forms glutamine by an ATP-dependent amination of glutamate. Here, we report changes in GS astrocytic expression in two major cognitive areas of the hippocampus (the dentate gyrus, DG and the CA1) in 3xTg-AD animals aged between 9 and 18 months. We found a significant reduction in Nv (number of cell/mm(3)) of GS immunoreactive (GS-IR) astrocytes starting from 12 months (28.59%) of age in the DG, and sustained at 18 months (31.65%). CA1 decrease of GS-positive astrocytes Nv (33.26%) occurs at 18 months. This Nv reduction of GSIR astrocytes is paralleled by a decrease in overall GS expression (determined by its optical density) that becomes significant at 18 months (21.61% and 19.68% in DG and CA1, respectively). GS-IR Nv changes are directly associated with the presence of A beta deposits showing a decrease of 47.92% as opposed to 23.47% in areas free of A beta. These changes in GS containing astrocytes and GS-immunoreactivity indicate AD-related impairments of glutamate homeostatic system, at the advanced and late stages of the disease, which may affect the efficacy of glutamatergic transmission in the diseased brain that may contribute to the cognitive deficiency.The present study was supported by Alzheimer's Research Trust Programme Grant (ART/PG2004A/1) to JJR and AV. Grant Agency of the Czech Republic (GACR 309/09/1696 and GACR 304/11/0184) to JJR and (GACR 305/08/1381; GACR 305/08/1384) to AV. The Spanish Government, Plan Nacional de I+D+I 2008-2011 and ISCIII-Subdireccion General de Evaluacion y Fomento de la investigacion (PI10/02738) to JJR and AV and the Government of the Basque Country grant (AE-2010-1-28; AEGV10/16) to JJR. The authors would also like to thank BBSRC for the Ph.D. studentship to H.N. Noristani
Bubble-Driven Inertial Micropump
The fundamental action of the bubble-driven inertial micropump is
investigated. The pump has no moving parts and consists of a thermal resistor
placed asymmetrically within a straight channel connecting two reservoirs.
Using numerical simulations, the net flow is studied as a function of channel
geometry, resistor location, vapor bubble strength, fluid viscosity, and
surface tension. Two major regimes of behavior are identified: axial and
non-axial. In the axial regime, the drive bubble either remains inside the
channel or continues to grow axially when it reaches the reservoir. In the
non-axial regime the bubble grows out of the channel and in all three
dimensions while inside the reservoir. The net flow in the axial regime is
parabolic with respect to the hydraulic diameter of the channel cross-section
but in the non-axial regime it is not. From numerical modeling, it is
determined that the net flow is maximal when the axial regime crosses over to
the non-axial regime. To elucidate the basic physical principles of the pump, a
phenomenological one-dimensional model is developed and solved. A linear array
of micropumps has been built using silicon-SU8 fabrication technology, and
semi-continuous pumping across a 2 mm-wide channel has been demonstrated
experimentally. Measured variation of the net flow with fluid viscosity is in
excellent agreement with simulation results.Comment: 18 pages, 18 figures, single colum
Effects of Size Polydispersity on the Extinction Spectra of Colloidal Nanoparticle Aggregates
We investigate the effect of particle polydispersity on the optical extinction spectra of colloidal aggregates of spherical metallic (silver) nanoparticles, taking into account the realistic interparticle gaps caused by layers of stabilizing polymer adsorbed on the metal surface (adlayers). The spectra of computer-generated aggregates are computed using two different methods. The coupled-multipole method is used in the quasistatic approximation and the coupled-dipole method beyond the quasistatics. The latter approach is applicable if the interparticle gaps are sufficiently wide relative to the particle radii. Simulations are performed for two different particle size distribution functions (bimodal and Gaussian), varying the number of particles per aggregate, and different distribution functions of the interparticle gap width. The strong influence of the latter factor on the spectra is demonstrated and investigated in detail
Comparative Analysis of Impact Attenuation Properties from Soccer Headgear
Athletes suffering from long-term neurocognitive deficiency due to subconcussive impacts is a major concern for football and soccer players today. Football players wear helmets that can help reduce injury risks like skull fractures, and these helmets must meet standard criteria that determinines how well a functional helmet should reduce accelerations of the player’s head. Currently no standard exists for testing soccer headgear despite studies demonstrating soccer players experience similar magnitudes of impacts. In this study, a modal impact hammer was used in conjunction with a Hybrid III 50th percentile test dummy head to simulate impacts experienced by soccer players to quantify the effectiveness of headgear in attenuating head acceleration due to direct impacts. The study found one device to be functional, and able to reduce the translational acceleration for an average hit experienced by a soccer player by 20%. Devices need to be developed and common testing standards need to be established to allow for a more widespread implementation of similar devices to protect players from short and long-term injuries due to impacts
Additively manufactured lattice structures with controlled transverse isotropy for orthopedic porous implants
Additively manufactured lattice structures enable the design of tissue scaffolds with tailored mechanical properties, which can be implemented in porous biomaterials. The adaptation of bone to physiological loads results in anisotropic bone tissue properties which are optimized for site-specific loads; therefore, some bone sites are stiffer and stronger along the principal load direction compared to other orientations. In this work, a semi-analytical model was developed for the design of transversely isotropic lattice structures that can mimic the anisotropy characteristics of different types of bone tissue. Several design possibilities were explored, and a particular unit cell, which was best suited for additive manufacturing was further analyzed. The design of the unit cell was parameterized and in-silico analysis was performed via Finite Element Analysis. The structures were manufactured additively in metal and tested under compressive loads in different orientations. Finite element analysis showed good correlation with the semi-analytical model, especially for elastic constants with low relative densities. The anisotropy measured experimentally showed a variable accuracy, highlighting the deviations from designs to additively manufactured parts. Overall, the proposed model enables to exploit the anisotropy of lattice structures to design lighter scaffolds with higher porosity and increased permeability by aligning the scaffold with the principal direction of the load
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