9,398 research outputs found
Piezoelectric copolymer hydrophones for ultrasonic field characterization
Hydrophones to be used in the characterization of medical ultrasonic transducers have
been fabricated using a new polyvinylidene fluoride/trifluoroethylene (VF2/VF3)
copolymer. The copolymer has an advantage over VF2 in that it does not require
prestretching before poling. Thin copolymer films can be cast from solution and then
poled using the corona discharge method. As there is a need for smallâdiameter
hydrophones to provide good spatial resolution in measuring highly focused ultrasonic
beams, hydrophones with diameter as small as 0.1 mm have been made. Both
needleâtype and line hydrophones have been tested and their performance reported. In
the case of line hydrophones, the output signal is proportional to the line integral of the
acoustic pressure and a computer tomographic technique has been used to reconstruct the
beam profiles
Light-Front Holography: A First Approximation to QCD
Starting from the Hamiltonian equation of motion in QCD, we identify an
invariant light-front coordinate which allows the separation of the
dynamics of quark and gluon binding from the kinematics of constituent spin and
internal orbital angular momentum. The result is a single variable light-front
Schrodinger equation for QCD which determines the eigenspectrum and the
light-front wavefunctions of hadrons for general spin and orbital angular
momentum. This light-front wave equation is equivalent to the equations of
motion which describe the propagation of spin- modes on anti-de Sitter (AdS)
space.Comment: 4 pages. The limits of validity of the model are further discussed.
To appear in Physical Review Letter
Three Green Bonnets
[Verse 1]Three green bonnets at church one day, (Dulcie and Daisy and Dorothy May); Three green bonnets that nod in a row, Each bonnet tied with a green ribbon bow; One pair of blue eyes, And one pair of gray, And one pair of brown eyes For, Dorothy May.
[Verse 2]Three little heads at the close of day, (Dulcie and Daisy and Dorothy May); Three little heads of clustering curls, Three little beds, and three little girls; Brown eyes are sleeping, and blue eyes, and gray, But angels are peeping at Dorothy May.
[Verse 3]Three green bonnets have had their day,(Dulcie and Daisy and Dorothy may); Three green bonnets grown old unawares Hang on three pegs at the front of the stairs, And blue eyes are swollen, And so are the gray, For angels have stolen Dear Dorothy May
An Efficient and Robust Method for Simulating Two-Phase Gel Dynamics
We develop a computational method for simulating models of gel dynamics where the gel is described by two phases: a networked polymer and a fluid solvent. The models consist of transport equations for the two phases, two coupled momentum equations, and a volume-averaged incompressibility constraint, which we discretize with finite differences/volumes. The momentum and incompressibility equations present the greatest numerical challenges since (i) they involve partial derivatives with variable coefficients that can vary quite significantly throughout the domain (when the phases separate), and (ii) their approximate solution requires the âinversionâ of a large linear system of equations. For solving this system, we propose a box-type multigrid method to be used as a preconditioner for the generalized minimum residual (GMRES) method. Through numerical experiments of a model problem, which exhibits phase separation, we show that the computational cost of the method scales nearly linearly with the number of unknowns and performs consistently well over a wide range of parameters. For solving the transport equation, we use a conservative finite-volume method for which we derive stability bounds
Lasing from a circular Bragg nanocavity with an ultra-small modal volume
We demonstrate single-mode lasing at telecommunication wavelengths from a
circular nanocavity employing a radial Bragg reflector. Ultra-small modal
volume and Sub milliwatt pump threshold level are observed for lasers with
InGaAsP quantum well active membrane. The electromagnetic field is shown to be
tightly confined within the 300nm central pillar of the cavity. The quality
factors of the resonator modal fields are estimated to be on the order of a few
thousands.Comment: 3 pages, 4 figures Submitted to AP
Radioiodination studies of tumour cell-surface proteins after different disaggregation procedures.
The surface of single cells isolated from solid tumours by either a mechanical or an enzymatic method have been compared, using lactoperoxidase-catalyzed radioiodination of the tyrosine-containing proteins. Qualitatively, the patterns of surface labelling were similar, and duplicate experiments indicated that each method of isolation gave reproducible results. Analysis of incorporated label into 4 defined sections of the electrophoretic pattern illustrated quantitative differences. When the cells were isolated mechanically, the incorporation into low-mol.- wt. components was considerably reduced, whereas that into the high-mol.-wt. components was unaffected. Treatment of enzymatically isolated cells with trypsin also reduced incorporation into low-mol.-wt components
Ensemble v-representable ab-initio density functional calculation of energy and spin in atoms: atest of exchange-correlation approximations
The total energies and the spin states for atoms and their first ions with Z
= 1-86 are calculated within the the local spin-density approximation (LSDA)
and the generalized-gradient approximation (GGA) to the exchange-correlation
(xc) energy in density-functional theory. Atoms and ions for which the
ground-state density is not pure-state v-representable, are treated as ensemble
v- representable with fractional occupations of the Kohn-Sham system. A newly
developed algorithm which searches over ensemble v-representable densities [E.
Kraisler et al., Phys. Rev. A 80, 032115 (2009)] is employed in calculations.
It is found that for many atoms the ionization energies obtained with the GGA
are only modestly improved with respect to experimental data, as compared to
the LSDA. However, even in those groups of atoms where the improvement is
systematic, there remains a non-negligible difference with respect to the
experiment. The ab-initio electronic configuration in the Kohn-Sham reference
system does not always equal the configuration obtained from the spectroscopic
term within the independent-electron approximation. It was shown that use of
the latter configuration can prevent the energy-minimization process from
converging to the global minimum, e.g. in lanthanides. The spin values
calculated ab-initio fit the experiment for most atoms and are almost
unaffected by the choice of the xc-functional. Among the systems with
incorrectly obtained spin there exist some cases (e.g. V, Pt) for which the
result is found to be stable with respect to small variations in the
xc-approximation. These findings suggest a necessity for a significant
modification of the exchange-correlation functional, probably of a non-local
nature, to accurately describe such systems. PACS numbers: 31.15.
Neuromechanical Mechanisms of Gait Adaptation in C. elegans: Relative Roles of Neural and Mechanical Coupling
Understanding principles of neurolocomotion requires the synthesis of neural
activity, sensory feedback, and biomechanics. The nematode \textit{C. elegans}
is an ideal model organism for studying locomotion in an integrated
neuromechanical setting because its neural circuit has a well-characterized
modular structure and its undulatory forward swimming gait adapts to the
surrounding fluid with a shorter wavelength in higher viscosity environments.
This adaptive behavior emerges from the neural modules interacting through a
combination of mechanical forces, neuronal coupling, and sensory feedback
mechanisms. However, the relative contributions of these coupling modes to gait
adaptation are not understood. The model consists of repeated neuromechanical
modules that are coupled through the mechanics of the body, short-range
proprioception, and gap-junctions. The model captures the experimentally
observed gait adaptation over a wide range of mechanical parameters, provided
that the muscle response to input from the nervous system is faster than the
body response to changes in internal and external forces. The modularity of the
model allows the use of the theory of weakly coupled oscillators to identify
the relative roles of body mechanics, gap-junctional coupling, and
proprioceptive coupling in coordinating the undulatory gait. The analysis shows
that the wavelength of body undulations is set by the relative strengths of
these three coupling forms. In a low-viscosity fluid environment, the
competition between gap-junctions and proprioception produces a long wavelength
undulation, which is only achieved in the model with sufficiently strong
gap-junctional coupling.The experimentally observed decrease in wavelength in
response to increasing fluid viscosity is the result of an increase in the
relative strength of mechanical coupling, which promotes a short wavelength.Comment: Pages 25, Figures 14. Submitted to SIAM Journal on Applied Dynamical
System
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