15,604 research outputs found
Phase transitions and crossovers in reaction-diffusion models with catalyst deactivation
The activity of catalytic materials is reduced during operation by several
mechanisms, one of them being poisoning of catalytic sites by chemisorbed
impurities or products. Here we study the effects of poisoning in two
reaction-diffusion models in one-dimensional lattices with randomly distributed
catalytic sites. Unimolecular and bimolecular single-species reactions are
considered, without reactant input during the operation. The models show
transitions between a phase with continuous decay of reactant concentration and
a phase with asymptotic non-zero reactant concentration and complete poisoning
of the catalyst. The transition boundary depends on the initial reactant and
catalyst concentrations and on the poisoning probability. The critical system
behaves as in the two-species annihilation reaction, with reactant
concentration decaying as t^{-1/4} and the catalytic sites playing the role of
the second species. In the unimolecular reaction, a significant crossover to
the asymptotic scaling is observed even when one of those parameters is 10% far
from criticality. Consequently, an effective power-law decay of concentration
may persist up to long times and lead to an apparent change in the reaction
kinetics. In the bimolecular single-species reaction, the critical scaling is
followed by a two-dimensional rapid decay, thus two crossovers are found.Comment: 8 pages, 7 figure
Disorder Averaging and Finite Size Scaling
We propose a new picture of the renormalization group (RG) approach in the
presence of disorder, which considers the RG trajectories of each random sample
(realization) separately instead of the usual renormalization of the averaged
free energy. The main consequence of the theory is that the average over
randomness has to be taken after finding the critical point of each
realization. To demonstrate these concepts, we study the finite-size scaling
properties of the two-dimensional random-bond Ising model. We find that most of
the previously observed finite-size corrections are due to the sample-to-sample
fluctuation of the critical temperature and scaling is more adequate in terms
of the new scaling variables.Comment: 4 pages, 6 figures include
Some features of flow and particle transport in porous structures
There has been a growing interest in the study of porous and complex flow structures due to its impact in technology. This concerns not only environmental but also diagnostic and therapeutic exposure in medical research. Physics of flow within porous structures is especially important to model transport and deposition of viruses, pollutants and drugs deep in these structures. In this work we analyze numerically low and medium Reynolds number flows in axisymmetric cylindrical duct surrounded by a torus. We also consider three different particle sizes (0.02, 0.1 and 20 micron) for possible physiological and environmental applications
Desempenho e avaliação das carcaças de borregos castrados ou inteiros terminados em confinamento.
Este trabalho teve como objetivo avaliar o desempenho e a qualidade da carcaça de borregos castrados e inteiros terminados em confinamento
A sociodemographic and neuropsychological characterization of an illiterate population
The objectives of this article are to characterize the performance and to discuss the performance differences between literate and illiterate participants in a well-defined study population. We describe the participant-selection procedure used to investigate this population. Three groups with similar sociocultural backgrounds living in a relatively homogeneous fishing community in southern Portugal were characterized in terms of socioeconomic and sociocultural background variables and compared on a simple neuropsychological test battery; specifically, a literate group with more than 4 years of education (n = 9), a literate group with 4 years of education (n = 26), and an illiterate group (n = 31) were included in this study. We compare and discuss our results with other similar studies on the effects of literacy and illiteracy. The results indicate that naming and identification of real objects, verbal fluency using ecologically relevant semantic criteria, verbal memory, and orientation are not affected by literacy or level of formal education. In contrast, verbal working memory assessed with digit span, verbal abstraction, long-term semantic memory, and calculation (i.e., multiplication) are significantly affected by the level of literacy. We indicate that it is possible, with proper participant-selection procedures, to exclude general cognitive impairment and to control important sociocultural factors that potentially could introduce bias when studying the specific effects of literacy and level of formal education on cognitive brain function
Does time constrain affect consumer intention to purchase? A case study with orange/pomegranate juice.
The influence of time pressure on consumer perception may be particularly relevant for the design of food packages. The aim of the present work was to evaluate the influence of time-constraint on results from a rating-based conjoint task, using pomegranate/orange juice as case study. One hundred consumers evaluated the intention to purchase of images of juice bottles, which were designed by manipulating the following variables: bottle design, front-of-pack nutritional information, nutrition claim and processing claim. They were randomly divided into two experimental conditions: control and time-constraint. Consumers with a time-constraint gave significantly higher intention to purchase scores than those in control condition, but time-constraint did not largely modify the way in which consumers evaluated their intention to purchase. In both experimental conditions consumers gave higher intention to purchase scores for bottles with traditional design, which included processing claim and front-of-pack nutritional information. Result suggests that graphic design could help consumers to make healthier food choices.Food: the tree that sustains life. 1648
- …