702 research outputs found
Investigation of two-dimensional acoustic resonant modes in a particle separator
Within an acoustic standing wave particles experience acoustic radiation forces, a phenomenon which is exploited in particle or cell manipulation devices. When developing such devices, one-dimensional acoustic characteristics corresponding to the transducer(s) are typically of most importance and determine the primary radiation forces acting on the particles. However, radiation forces have also been observed to act in the lateral direction, perpendicular to the primary radiation force, forming striated patterns. These lateral forces are due to lateral variations in the acoustic field influenced by the geometry and materials used in the resonator. The ability to control them would present an advantage where their effect is either detrimental or beneficial to the particle manipulation process.The two-dimensional characteristics of an ultrasonic separator device have been modelled within a finite element analysis (FEA) package. The fluid chamber of the device, within which the standing wave is produced, has a width to height ratio of approximately 30:1 and it is across the height that a half-wavelength standing wave is produced to control particle movement. Two-dimensional modal analyses have calculated resonant frequencies which agree well with both the one-dimensional modelling of the device and experimentally measured frequencies. However, these two-dimensional analyses also reveal that these modes exhibit distinctive periodic variations in the acoustic pressure field across the width of the fluid chamber. Such variations lead to lateral radiation forces forming particle bands (striations) and are indicative of enclosure modes.The striation spacings predicted by the FEA simulations for several modes compare well with those measured experimentally for the ultrasonic particle separator device. It is also shown that device geometry and materials control enclosure modes and therefore the strength and characteristics of lateral radiation forces, suggesting the potential use of FEA in designing for the control of enclosure modes in similar particle manipulator devices
Resonant Ultrasonic Particle Manipulators and their Applications in Sensor Systems
The paper describes the use of ultrasonic standing waves as bulk acoustic wave actuators, exploiting the acoustic radiation forces within the standing wave to move biological cells or other particles. This is a technology with the potential to enhance many forms of microflow-based sensors. Example applications discussed include half-wavelength filters, flow-through chambers which move cells from one fluid medium into another (washing the cells), and quarter wavelength chambers that attract cells to a solid boundary such as the face of a sensor. Microfabricated devices are described, including resonators with multiple sub-wavelength resonances, which are driven by multilayer thick film PZT actuators
Diabatic and Adiabatic Collective Motion in a Model Pairing System
Large amplitude collective motion is investigated for a model pairing
Hamiltonian containing an avoided level crossing. A classical theory of
collective motion for the adiabatic limit is applied utilising either a
time-dependent mean-field theory or a direct parametrisation of the
time-dependent Schr\"odinger equation. A modified local harmonic equation is
formulated to take account of the Nambu-Goldstone mode. It turns out that in
some cases the system selects a diabatic path. Requantizing the collective
Hamiltonian, a reasonable agreement with an exact calculation for the low-lying
levels are obtained for both weak and strong pairing force. This improves on
results of the conventional Born-Oppenheimer approximation.Comment: 23 pages, 7 ps figures. Latex, uses revtex and graphic
Net ecosystem exchange from two formerly afforested peatlands undergoing restoration in the Flow Country of northern Scotland
Northern peatlands are important in the global carbon (C) cycle as they help regulate local, regional and global C budgets through high atmospheric carbon dioxide (CO2) uptake and low net CO2 losses to the atmosphere. Since the 1900s (but particularly the 1950s) land-use change has affected many peatland areas, driven in part by attempts to improve their commercial value. During this period, many peatlands in the UK were drained and planted with non-native conifer plantations. Efforts are now underway to restore the ecosystem functioning of these peatlands to those characteristic of pristine peatlands, in particular C flux dynamics. A lack of ecosystem level measurements means that the timescales of restoration and the degree to which they are successful remains poorly determined. In this research, we present the first year-round study of net ecosystem CO2 exchange (NEE) from peatlands undergoing restoration from forestry. Annual NEE was measured from two sites between March 2014 and June 2015, where restoration commenced 10 years and 16 years prior to the start of this study, and the results were then compared to existing measurements from a near-pristine peatland. Existing NEE data (expressed as CO2-C) from the near-pristine peatland indicated a C sink of 114 g m-2 yr-1, and our estimates suggest that the older restored site (16 years) was also a NEE sink (71 g m-2 yr-1). In contrast, the younger site (10 years) was a NEE source (80 g m-2 yr-1). We critically assess the confidence of these measurements and also present these data in relation to other northern hemisphere peatlands to better understand the timeframe in which a peatland site can turn from a C source to a C sink after restoration
Observations of quasi-periodic solar X-ray emission as a result of MHD oscillations in a system of multiple flare loops
We investigate the solar flare of 20 October 2002. The flare was accompanied
by quasi-periodic pulsations (QPP) of both thermal and nonthermal hard X-ray
emissions (HXR) observed by RHESSI in the 3-50 keV energy range. Analysis of
the HXR time profiles in different energy channels made with the Lomb
periodogram indicates two statistically significant time periods of about 16
and 36 seconds. The 36-second QPP were observed only in the nonthermal HXR
emission in the impulsive phase of the flare. The 16-second QPP were more
pronounced in the thermal HXR emission and were observed both in the impulsive
and in the decay phases of the flare. Imaging analysis of the flare region, the
determined time periods of the QPP and the estimated physical parameters of
magnetic loops in the flare region allow us to interpret the observations as
follows. 1) In the impulsive phase energy was released and electrons were
accelerated by successive acts with the average time period of about 36 seconds
in different parts of two spatially separated, but interacting loop systems of
the flare region. 2) The 36-second periodicity of energy release could be
caused by the action of fast MHD oscillations in the loops connecting these
flaring sites. 3) During the first explosive acts of energy release the MHD
oscillations (most probably the sausage mode) with time period of 16 seconds
were excited in one system of the flare loops. 4) These oscillations were
maintained by the subsequent explosive acts of energy release in the impulsive
phase and were completely damped in the decay phase of the flare.Comment: 14 pages, 4 figure
Nernst Effect in Electron-Doped PrCeCuO
The Nernst effect of PrCeCuO (x=0.13, 0.15, and 0.17) has
been measured on thin film samples between 5-120 K and 0-14 T. In comparison to
recent measurements on hole-doped cuprates that showed an anomalously large
Nernst effect above the resistive T and H
\cite{xu,wang1,wang2,capan}, we find a normal Nernst effect above T and
H for all dopings. The lack of an anomalous Nernst effect in the
electron-doped compounds supports the models that explain this effect in terms
of amplitude and phase fluctuations in the hole-doped cuprates. In addition,
the H(T) determined from the Nernst effect shows a conventional behavior
for all dopings. The energy gap determined from H(0) decreases as the
system goes from under-doping to over-dopingin agreement with the recent
tunnelling experiments
Energy Release During Slow Long Duration Flares Observed by RHESSI
Slow Long Duration Events (SLDEs) are flares characterized by long duration
of rising phase. In many such cases impulsive phase is weak with lack of
typical short-lasting pulses. Instead of that smooth, long-lasting Hard X-ray
(HXR) emission is observed. We analysed hard X-ray emission and morphology of
six selected SLDEs. In our analysis we utilized data from RHESSI and GOES
satellites. Physical parameters of HXR sources were obtained from imaging
spectroscopy and were used for the energy balance analysis. Characteristic time
of heating rate decrease, after reaching its maximum value, is very long, which
explains long rising phase of these flares.Comment: Accepted for publication in Solar Physic
Evolution of active and polar photospheric magnetic fields during the rise of Cycle 24 compared to previous cycles
The evolution of the photospheric magnetic field during the declining phase
and minimum of Cycle 23 and the recent rise of Cycle 24 are compared with the
behavior during previous cycles. We used longitudinal full-disk magnetograms
from the NSO's three magnetographs at Kitt Peak, the Synoptic Optical Long-term
Investigations of the Sun (SOLIS) Vector Spectro-Magnetograph (VSM), the
Spectromagnetograph and the 512-Channel Magnetograph instruments, and
longitudinal full-disk magnetograms from the Mt. Wilson 150-foot tower. We
analyzed 37 years of observations from these two observatories that have been
observing daily, weather permitting, since 1974, offering an opportunity to
study the evolving relationship between the active region and polar fields in
some detail over several solar cycles. It is found that the annual averages of
a proxy for the active region poloidal magnetic field strength, the magnetic
field strength of the high-latitude poleward streams, and the time derivative
of the polar field strength are all well correlated in each hemisphere. These
results are based on statistically significant cyclical patterns in the active
region fields and are consistent with the Babcock-Leighton phenomenological
model for the solar activity cycle. There was more hemispheric asymmetry in the
activity level, as measured by total and maximum active region flux, during
late Cycle 23 (after around 2004), when the southern hemisphere was more
active, and Cycle 24 up to the present, when the northern hemisphere has been
more active, than at any other time since 1974. The active region net proxy
poloidal fields effectively disappeared in both hemispheres around 2004, and
the polar fields did not become significantly stronger after this time. We see
evidence that the process of Cycle 24 field reversal has begun at both poles.Comment: Accepted for publication in Solar Physic
Regulation of pentraxin-3 by antioxidants
Peer reviewedPublisher PD
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