84,270 research outputs found
Induced-charge Electrokinetic Phenomena: Theory and Microfluidic Applications
We give a general, physical description of ``induced-charge electro-osmosis''
(ICEO), the nonlinear electrokinetic slip at a polarizable surface, in the
context of some new techniques for microfluidic pumping and mixing. ICEO
generalizes ``AC electro-osmosis'' at micro-electrode arrays to various
dielectric and conducting structures in weak DC or AC electric fields. The
basic effect produces micro-vortices to enhance mixing in microfluidic devices,
while various broken symmetries -- controlled potential, irregular shape,
non-uniform surface properties, and field gradients -- can be exploited to
produce streaming flows. Although we emphasize the qualitative picture of ICEO,
we also briefly describe the mathematical theory (for thin double layers and
weak fields) and apply it to a metal cylinder with a dielectric coating in a
suddenly applied DC field.Comment: 4 pages, 4 figs; revsion with more refs, one new fig, and more
emphasis on microfluidic
The ePetri dish, an on-chip cell imaging platform based on subpixel perspective sweeping microscopy (SPSM)
We report a chip-scale lensless wide-field-of-view microscopy imaging technique, subpixel perspective sweeping microscopy, which can render microscopy images of growing or confluent cell cultures autonomously. We demonstrate that this technology can be used to build smart Petri dish platforms, termed ePetri, for cell culture experiments. This technique leverages the recent broad and cheap availability of high performance image sensor chips to provide a low-cost and automated microscopy solution. Unlike the two major classes of lensless microscopy methods, optofluidic microscopy and digital in-line holography microscopy, this new approach is fully capable of working with cell cultures or any samples in which cells may be contiguously connected. With our prototype, we demonstrate the ability to image samples of area 6 mm × 4 mm at 660-nm resolution. As a further demonstration, we showed that the method can be applied to image color stained cell culture sample and to image and track cell culture growth directly within an incubator. Finally, we showed that this method can track embryonic stem cell differentiations over the entire sensor surface. Smart Petri dish based on this technology can significantly streamline and improve cell culture experiments by cutting down on human labor and contamination risks
Structure of the twin-arginine signal-binding protein DmsD from Escherichia coli
The translocation of folded proteins via the twin-arginine translocation (Tat) pathway is regulated to prevent the futile export of inactive substrate. DmsD is part of a class of cytoplasmic chaperones that play a role in preventing certain redox proteins from premature transport. DmsD from Escherichia coli has been crystallized in space group P4_12_12, with unit-cell parameters a = b = 97.45, c = 210.04 Å, in the presence of a small peptide. The structure has been solved by molecular replacement to a resolution of 2.4 Å and refined to an R factor of 19.4%. There are four molecules in the asymmetric unit that may mimic a higher order structure in vivo. There appears to be density for the peptide in a predicted binding pocket, which lends support to its role as the signal-recognition surface for this class of proteins
Sociotropic Voting and the Media
The literature on economic voting notes that voters' subjective evaluations
of the overall state of the economy are correlated with vote choice, whereas
personal economic experiences are not (Kinder and Kiewiet 1979, 1981). Missing
from this literature is a description of how voters acquire information about
the general state of the economy and use that information to form perceptions.
To begin understanding this process, we asked a series of questions on the 2006
ANES Pilot Study about respondents' perceptions of the average price of gas
and the unemployment rate in their home states. In this chapter, we analyze
both the determinants and political consequences of respondents' perceptions
of these economic variables.
Questions about gas prices and unemployment show differences in respondents'
sources of information about these two economic variables. We found
evidence consistent with the idea that information about unemployment rates
comes from media sources, and is biased by partisan factors, and that information
about gas prices comes only from everyday experiences. While information
about both indicators shows effects from demographics, only estimates of
unemployment rates are correlated with a respondent's political outlook. Moreover,
perceptions of unemployment rates can be used to isolate the effect of
economics on partisan preferences
Search for a Light Higgs Boson Decaying to Long-Lived Weakly Interacting Particles in Proton-Proton Collisions at √s = 7 TeV with the ATLAS Detector
A search for the decay of a light Higgs boson (120–140 GeV) to a pair of weakly interacting, long-lived particles in 1.94 fb^(-1) of proton-proton collisions at √s=7 TeV recorded in 2011 by the ATLAS detector is presented. The search strategy requires that both long-lived particles decay inside the muon spectrometer. No excess of events is observed above the expected background and limits on the Higgs boson production times branching ratio to weakly interacting, long-lived particles are derived as a function of the particle proper decay length
On the measurement of a weak classical force coupled to a quantum-mechanical oscillator. I. Issues of principle
The monitoring of a quantum-mechanical harmonic oscillator on which a classical force acts is important in a variety of high-precision experiments, such as the attempt to detect gravitational radiation. This paper reviews the standard techniques for monitoring the oscillator, and introduces a new technique which, in principle, can determine the details of the force with arbitrary accuracy, despite the quantum properties of the oscillator. The standard method for monitoring the oscillator is the "amplitude-and-phase" method (position or momentum transducer with output fed through a narrow-band amplifier). The accuracy obtainable by this method is limited by the uncertainty principle ("standard quantum limit"). To do better requires a measurement of the type which Braginsky has called "quantum nondemolition." A well known quantum nondemolition technique is "quantum counting," which can detect an arbitrarily weak classical force, but which cannot provide good accuracy in determining its precise time dependence. This paper considers extensively a new type of quantum nondemolition measurement—a "back-action-evading" measurement of the real part X_1 (or the imaginary part X_2) of the oscillator's complex amplitude. In principle X_1 can be measured "arbitrarily quickly and arbitrarily accurately," and a sequence of such measurements can lead to an arbitrarily accurate monitoring of the classical force. The authors describe explicit Gedanken experiments which demonstrate that X_1 can be measured arbitrarily quickly and arbitrarily accurately. In these experiments the measuring apparatus must be coupled to both the position (position transducer) and the momentum (momentum transducer) of the oscillator, and both couplings must be modulated sinusoidally. For a given measurement time the strength of the coupling determines the accuracy of the measurement; for arbitrarily strong coupling the measurement can be arbitrarily accurate. The "momentum transducer" is constructed by combining a "velocity transducer" with a "negative capacitor" or "negative spring." The modulated couplings are provided by an external, classical generator, which can be realized as a harmonic oscillator excited in an arbitrarily energetic, coherent state. One can avoid the use of two transducers by making "stroboscopic measurements" of X_1, in which one measures position (or momentum) at half-cycle intervals. Alternatively, one can make "continuous single-transducer" measurements of X_1 by modulating appropriately the output of a single transducer (position or momentum), and then filtering the output to pick out the information about X_1 and reject information about X_2. Continuous single-transducer measurements are useful in the case of weak coupling. In this case long measurement times are required to achieve good accuracy, and continuous single-transducer measurements are almost as good as perfectly coupled two-transducer measurements. Finally, the authors develop a theory of quantum nondemolition measurement for arbitrary systems. This paper (Paper I) concentrates on issues of principle; a sequel (Paper II) will consider issues of practice
The Panchromatic Hubble Andromeda Treasury
The Panchromatic Hubble Andromeda Treasury (PHAT) is an on-going HST
Multicycle Treasury program to image ~1/3 of M31's star forming disk in 6
filters, from the UV to the NIR. The full survey will resolve the galaxy into
more than 100 million stars with projected radii from 0-20 kpc over a
contiguous 0.5 square degree area in 828 orbits, producing imaging in the F275W
and F336W filters with WFC3/UVIS, F475W and F814W with ACS/WFC, and F110W and
F160W with WFC3/IR. The resulting wavelength coverage gives excellent
constraints on stellar temperature, bolometric luminosity, and extinction for
most spectral types. The photometry reaches SNR=4 at F275W=25.1, F336W=24.9,
F475W=27.9, F814W=27.1, F110W=25.5, and F160W=24.6 for single pointings in the
uncrowded outer disk; however, the optical and NIR data are crowding limited,
and the deepest reliable magnitudes are up to 5 magnitudes brighter in the
inner bulge. All pointings are dithered and produce Nyquist-sampled images in
F475W, F814W, and F160W. We describe the observing strategy, photometry,
astrometry, and data products, along with extensive tests of photometric
stability, crowding errors, spatially-dependent photometric biases, and
telescope pointing control. We report on initial fits to the structure of M31's
disk, derived from the density of RGB stars, in a way that is independent of
the assumed M/L and is robust to variations in dust extinction. These fits also
show that the 10 kpc ring is not just a region of enhanced recent star
formation, but is instead a dynamical structure containing a significant
overdensity of stars with ages >1 Gyr. (Abridged)Comment: 48 pages including 22 pages of figures. Accepted to the Astrophysical
Journal Supplements. Some figures slightly degraded to reduce submission siz
Double-heterostructure GaAs-GaAIAs injection lasers on semi-insulating substrates using carrier crowding
GaAs‐GaAlAs double‐heterostructure lasers were fabricated on semi‐insulating substrates. Laser action based on carrier confinement via the crowding effect has been demonstrated. Laser action takes place in a narrow (10–20 μm) region near the edge of the mesa where the current is injected. The threshold current is low and is comparable to that of stripe‐geometry lasers
Coordination of opposing sex-specific and core muscle groups regulates male tail posture during Caenorhabditis elegans male mating behavior
Background
To survive and reproduce, animals must be able to modify their motor behavior in response to changes in the environment. We studied a complex behavior of Caenorhabditis elegans, male mating behavior, which provided a model for understanding motor behaviors at the genetic, molecular as well as circuit level. C. elegans male mating behavior consists of a series of six sub-steps: response to contact, backing, turning, vulva location, spicule insertion, and sperm transfer. The male tail contains most of the sensory structures required for mating, in addition to the copulatory structures, and thus to carry out the steps of mating behavior, the male must keep his tail in contact with the hermaphrodite. However, because the hermaphrodite does not play an active role in mating and continues moving, the male must modify his tail posture to maintain contact. We provide a better understanding of the molecular and neuro-muscular pathways that regulate male tail posture during mating.
Results
Genetic and laser ablation analysis, in conjunction with behavioral assays were used to determine neurotransmitters, receptors, neurons and muscles required for the regulation of male tail posture. We showed that proper male tail posture is maintained by the coordinated activity of opposing muscle groups that curl the tail ventrally and dorsally. Specifically, acetylcholine regulates both ventral and dorsal curling of the male tail, partially through anthelmintic levamisole-sensitive, nicotinic receptor subunits. Male-specific muscles are required for acetylcholine-driven ventral curling of the male tail but dorsal curling requires the dorsal body wall muscles shared by males and hermaphrodites. Gamma-aminobutyric acid activity is required for both dorsal and ventral acetylcholine-induced curling of the male tail and an inhibitory gamma-aminobutyric acid receptor, UNC-49, prevents over-curling of the male tail during mating, suggesting that cross-inhibition of muscle groups helps maintain proper tail posture.
Conclusion
Our results demonstrated that coordination of opposing sex-specific and core muscle groups, through the activity of multiple neurotransmitters, is required for regulation of male tail posture during mating. We have provided a simple model for regulation of male tail posture that provides a foundation for studies of how genes, molecular pathways, and neural circuits contribute to sensory regulation of this motor behavior
The Discovery of an Ultra-Faint Star Cluster in the Constellation of Ursa Minor
We report the discovery of a new ultra-faint globular cluster in the
constellation of Ursa Minor, based on stellar photometry from the MegaCam
imager at the Canada-France-Hawaii Telescope (CFHT). We find that this cluster,
Munoz 1, is located at a distance of 45 +/- 5 kpc and at a projected distance
of only 45 arcmin from the center of the Ursa Minor dSph galaxy. Using a
Maximum Likelihood technique we measure a half-light radius of 0.5 arcmin, or
equivalently 7 pc and an ellipticity consistent with being zero. We estimate
its absolute magnitude to be M_V=-0.4 +/- 0.9, which corresponds to L_V=120
(+160, -65) L_sun and we measure a heliocentric radial velocity of -137 +/- 4
km/s based on Keck/DEIMOS spectroscopy. This new satellite is separate from
Ursa Minor by ~30 kpc and 110 km/s suggesting the cluster is not obviously
associated with the dSph, despite the very close angular separation. Based on
its photometric properties and structural parameters we conclude that Munoz 1
is a new ultra-faint stellar cluster. Along with Segue 3 this is one of the
faintest stellar clusters known to date.Comment: Accepted for publication in ApJ Letter
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