21,318 research outputs found
Modeling one-dimensional island growth with mass-dependent detachment rates
We study one-dimensional models of particle diffusion and
attachment/detachment from islands where the detachment rates gamma(m) of
particles at the cluster edges increase with cluster mass m. They are expected
to mimic the effects of lattice mismatch with the substrate and/or long-range
repulsive interactions that work against the formation of long islands.
Short-range attraction is represented by an overall factor epsilon<<1 in the
detachment rates relatively to isolated particle hopping rates [epsilon ~
exp(-E/T), with binding energy E and temperature T]. We consider various
gamma(m), from rapidly increasing forms such as gamma(m) ~ m to slowly
increasing ones, such as gamma(m) ~ [m/(m+1)]^b. A mapping onto a column
problem shows that these systems are zero-range processes, whose steady states
properties are exactly calculated under the assumption of independent column
heights in the Master equation. Simulation provides island size distributions
which confirm analytic reductions and are useful whenever the analytical tools
cannot provide results in closed form. The shape of island size distributions
can be changed from monomodal to monotonically decreasing by tuning the
temperature or changing the particle density rho. Small values of the scaling
variable X=epsilon^{-1}rho/(1-rho) favour the monotonically decreasing ones.
However, for large X, rapidly increasing gamma(m) lead to distributions with
peaks very close to and rapidly decreasing tails, while slowly increasing
gamma(m) provide peaks close to /2$ and fat right tails.Comment: 16 pages, 6 figure
Caging dynamics in a granular fluid
We report an experimental investigation of the caging motion in a uniformly
heated granular fluid, for a wide range of filling fractions, . At low
the classic diffusive behavior of a fluid is observed. However, as
is increased, temporary cages develop and particles become increasingly
trapped by their neighbors. We statistically analyze particle trajectories and
observe a number of robust features typically associated with dense molecular
liquids and colloids. Even though our monodisperse and quasi-2D system is known
to not exhibit a glass transition, we still observe many of the precursors
usually associated with glassy dynamics. We speculate that this is due to a
process of structural arrest provided, in our case, by the presence of
crystallization.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Let
Broad Iron Emission from Gravitationally Lensed Quasars Observed by Chandra
Recent work has demonstrated the potential of gravitationally lensed quasars
to extend measurements of black hole spin out to high-redshift with the current
generation of X-ray observatories. Here we present an analysis of a large
sample of 27 lensed quasars in the redshift range 1.0<z<4.5 observed with
Chandra, utilizing over 1.6 Ms of total observing time, focusing on the
rest-frame iron K emission from these sources. Although the X-ray
signal-to-noise (S/N) currently available does not permit the detection of iron
emission from the inner accretion disk in individual cases in our sample, we
find significant structure in the stacked residuals. In addition to the narrow
core, seen almost ubiquitously in local AGN, we find evidence for an additional
underlying broad component from the inner accretion disk, with a clear red wing
to the emission profile. Based on simulations, we find the detection of this
broader component to be significant at greater than the 3-sigma level. This
implies that iron emission from the inner disk is relatively common in the
population of lensed quasars, and in turn further demonstrates that, with
additional observations, this population represents an opportunity to
significantly extend the sample of AGN spin measurements out to high-redshift.Comment: 5 pages, 2 figures, accepted for publication in Ap
Influence of seismic activity on the atmospheric electric field in Lisbon (Portugal) from 1955 to 1991
In the present study, we considered the influence of seismic activity on the atmospheric electric field recorded at Portela meteorological station (Lisbon, Portugal) for the period from 1955 to 1991. To this end, an exploratory method was developed, which involved the selection of events for which the distance from the atmospheric electrical field sensor to the earthquake epicenter is smaller than the preparation radius of the event. This enabled the correlation of the atmospheric electric field variations with a quantity S, defined basically as the ratio of the earthquake preparation radius to the distance between the sensor and the event epicenter. The first results
show promising perspectives, but clearly a more profound study is required, in which a careful analysis of the weather conditions and other variables, like atmospheric radon levels, must be considered
On the search for the chiral anomaly in Weyl semimetals: The negative longitudinal magnetoresistance
Recently, the existence of massless chiral (Weyl) fermions has been
postulated in a class of semi-metals with a non-trivial energy dispersion.These
materials are now commonly dubbed Weyl semi-metals (WSM).One predicted property
of Weyl fermions is the chiral or Adler-Bell-Jackiw anomaly, a chirality
imbalance in the presence of parallel magnetic and electric fields. In WSM, it
is expected to induce a negative longitudinal magnetoresistance (NMR), the
chiral magnetic effect.Here, we present experimental evidence that the
observation of the chiral magnetic effect can be hindered by an effect called
"current jetting". This effect also leads to a strong apparent NMR, but it is
characterized by a highly non-uniform current distribution inside the sample.
It appears in materials possessing a large field-induced anisotropy of the
resistivity tensor, such as almost compensated high-mobility semimetals due to
the orbital effect.In case of a non-homogeneous current injection, the
potential distribution is strongly distorted in the sample.As a consequence, an
experimentally measured potential difference is not proportional to the
intrinsic resistance.Our results on the MR of the WSM candidate materials NbP,
NbAs, TaAs, TaP exhibit distinct signatures of an inhomogeneous current
distribution, such as a field-induced "zero resistance' and a strong dependence
of the `measured resistance" on the position, shape, and type of the voltage
and current contacts on the sample. A misalignment between the current and the
magnetic-field directions can even induce a "negative resistance".
Finite-element simulations of the potential distribution inside the sample,
using typical resistance anisotropies, are in good agreement with the
experimental findings. Our study demonstrates that great care must be taken
before interpreting measurements of a NMR as evidence for the chiral anomaly in
putative Weyl semimetals.Comment: 13 pages, 6 figure
ON THE MONITORING OF THE GAS-SOLID FLOWS IN INDUSTRIAL FLUIDIZED BEDS BY USING ELECTRICAL CHARGE SENSORS
The fluidized bed technology has been used in many industrial processes. It promotes good rates of heat, mass transfer and chemical reaction by generating high level of gas-solid mixture. However, the assurance of quality and efficiency of these processes requires the monitoring of the gas-solid flow. For this propose, there are some sensing techniques that allows generating dynamic signals from cold or hot fluidized beds. They are based on pressure fluctuations, acoustic and mechanical vibrations, electrical capacitance and on electrical charges. Electrical charge sensors were proposed originally for measuring the flow velocity in pneumatic conveying. They are composed of one or more metallic electrodes that detect electrical charges in the gas-solid flow, which are generated by particle-particle and particle-wall interaction due to triboelectric effect. In this work, such sensors are explored as a robust and inexpensive solution for the monitoring of industrial fluidized beds. However, since research investments are requested specially on the design of the sensor, concerning the flow quantity of interest and the electrification processes acting on the sensor, in this work different configurations were classified from information in literature, and other were proposed in this work concerning their use with industrial fluidized beds. Although the relation between magnitude of the detected charges and some physical quantities of the flow, such as concentration, is still not clear, other important information can be obtained by analyzing dynamic signals, as velocity or bubbles frequency, or even for identifying of the fluidization regime. It was stated that each configuration, with its own shape and arrangement, can promote or not one or other electrification process by contact, friction or induction and, therefore, each one has a different perception of the flow
Analysis of loop heat pipe performance under varying wick load
Loop heat pipes (LHP) are heat transfer devices used to enhance cooling of small spaces and basically consist of sealed
tubes connecting a heat source, the evaporator, whose major part is a porous wick, with a condenser that operates as
heat sink. In this paper we analyse the effect of curvature of the liquid vapor interface upon the vapor pressure within
wick pores. We show how this effect affects start-up by requiring a difference between wick and condenser temperatures as higher as wick pore width becomes smaller. We analysed also transient operation and found that idealy LHP are self-adjusting systems that tend to stable operation. We present a formula to describe the transient regime. The
analysis provides also optimization of wick pore width for maximum heat transfer. Optimal pore width is shown to vary with temperature difference between wick and condenser. It is envisaged how this feature may help in LHP design
Improvements in small scale artisanal cheesemaking via a novel mechanized apparatus
The current research effort consisted on design, building and testing of a customized apparatus – a closed cylindrical, double-jacketed cheese vat, with a 50 L-capacity, aimed at manufacturing Serra da Estrela cheese (or similar ewe’s milk cheeses), provided with knives that play a double role – stirring and cutting, a combined device for draining and pre-moulding, and a cleaning-in-place system. This novel apparatus makes it possible to semi-automate cheesemaking in small scale – including a continuous cycle of hands-free sequential
coagulation, cutting, syneresis and moulding of cheeses. It was successfully applied to ewe’s milk cheese manufacture, and was able to reduce duration of the overall cycle – with concomitant standardization of the process, and a significant (P> 0.05) increase in cheese yield and fat recovery
Numerical Particle Transport in Partitioned Room.
In this work, we solved particle transport in a two-zone enclosure numerically with different airflow pattems, particle properties and algo source positions. A discrete trajectory was employed for the particIe movement. Five isotropic line sources which are alI taken at different positions oftne same zone were festejo It was observed that for alI airflow pattems, residence time increases with decreasing size of particIes. Increase in size and mass increases the chaTIceof particles to get deposited auto the fIoor. Source locations should be chosen in the main stream of the fIow so that particles convect fast through and not contribute indoor polIutant concentration. ConcIusions were drawn for
every numerical experiment in arder to show tendencies of particle dynamics within the encIosure
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