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

Sex-specific effects of gender identification on pain study recruitment

Epidemiological, clinical and laboratory studies show sex differences in pain responses, with women more sensitive to nociceptive stimulation and more vulnerable to long term pain conditions than men. Given evidence that males are culturally reinforced for the ability to endure (or under-report) pain, some of these findings might be explained by socio-cultural beliefs about gender-appropriate behaviour. One potential manifestation of these effects might be differential participation in pain studies, with males adhering to stereotypical masculine roles viewing participation as a way to demonstrate their masculinity. To test this possibility, we assessed gender identification in 137 healthy participants. At the end of the assessment, they were asked if they would like to participate in other research studies. Interested participants were then asked to participate in a study involving administration of pain-evoking stimulation. We compared individuals who agreed to participate in the pain study to those who declined. We observed a significant sex by participation interaction in masculine gender identification, such that males (but not females) who agreed to participate identified significantly more with masculine gender. Among masculine gender traits examined, we found that high levels of aggression and competitiveness were the strongest predictors of pain study participation. Our results suggest that male samples in pain studies might have higher levels of masculine gender identification than the wider male population. Taken together with previous findings of lower pain sensitivity (or reporting) in masculine-identifying males, these results suggest an explanation for some of the sex-related differences observed in pain responses. Perspective : To examine whether sex and gender affect willingness to participate in pain studies, we assessed gender identification in male and female participants, then attempted to recruit them to participate in a pain study. Males who agree to participate in pain studies are significantly higher in masculine gender identification than males who decline to participate or females who agree to participate. Males who agreed to participate were particularly high in aggressiveness and competitiveness

Caracterização dos solos em áreas manejadas com bacurizeiros nativos nas mesorregiões do Nordeste paraense e Marajó.

A distribuição geográfica de bacurizeiros (Platonia insignis Mart.) é controlada por fatores naturais e antrópicos que contribuem para sua ocorrência e concentração de populações manejadas desta espécie, nas mesorregiões do Nordeste Paraense e Marajó. Com base nessa hipótese, este trabalho teve como objetivo a caracterização dos solos de sítios com bacurizeiros manejados nestas duas mesorregiões..

Identificação e controle com antibióticos de bactérias endofíticas contaminantes em explantes de batata micropropagados.

Este trabalho teve por objetivos isolar, caracterizar e identificar bactérias endofíticas contaminantes encontradas em tecidos de batata durante a micropropagação e selecionar antibióticos para o controle in vitro desses microrganismos por meio da determinação da concentração bactericida mínima inibitória. Brotações de batata apresentando contaminação bacteriana durante a etapa de multiplicação in vitro, foram superficialmente esterilizadas e os internódios transferidos para placas de Petri com ágar nutriente, onde permaneceram incubadas a 28°C por até cinco dias. Após purificação, as bactérias foram caracterizadas e identificadas por testes taxonômicos. Um total de oito estirpes bacterianas foram isoladas e identificadas como pertencentes às famílias Acetobacteriaceae (1) e Enterobacteriaceae (2) e aos gêneros Corynebacterium (3), Pseudomonas (1) e Xanthomonas (1). Os melhores resultados para a inibição do crescimento bacteriano foram obtidos com os antibióticos ampicilina, cloranfenicol, estreptomicina e tetraciclina em concentrações que variaram de 32 a 256 mg L-1