Simón Bolívar University

About: Simón Bolívar University is a education organization based out in Caracas, Venezuela. It is known for research contribution in the topics: Population & Crystallization. The organization has 5912 authors who have published 8294 publications receiving 126152 citations.

Heterogeneous nucleation and self-nucleation of poly(p-dioxanone)

Abstract: The changes in nucleation behaviour upon addition of Boron Nitride (BN), Talc and Hydroxyapatite (HA) to poly(p-dioxanone) (PPDX) were monitored by DSC and Polarised Optical Microscopy (PM). Self-nucleation DSC studies evidenced the existence of the usual three self-nucleation domains depending on the self-nucleation temperature (T s) employed. By far the best nucleation agents for PPDX were its own self-nuclei and this result was independent of the presence or absence of any of the other nucleating agents employed; once Domain II was reached, self-nucleation dominated the nucleation process. BN and Talc were able to nucleate PPDX, thereby increasing its nucleation density, its dynamic crystallisation temperature upon cooling from the melt (T c) and its enthalpy of crystallisation (ΔH c). BN was a better nucleating agent than talc. HA on the other hand caused an “antinucleation” effect on PPDX characterised by a decrease in its nucleation density, a decrease in its T c and in ΔH c. Isothermally crystallised PPDX exhibited large banded spherulites whose morphology changed as a function of crystallisation temperature from single banded structures with a very clear Maltese cross to double banded spherulites. PPDX also shows a change in growth regime upon increasing crystallisation temperature (from Regime III to Regime II) according to the kinetic interpretation of growth rate data. BN did not cause any significant modification of the spherulitic growth kinetics (in Regime II) except for a small decrease in surface free energy of PPDX crystals (σe). On the other hand HA was found to increase the spherulitic growth rate and the overall crystallisation rate of PPDX, this increase was caused by a degradation process experienced by the polymer during the treatments involved in isothermal crystallisation that was only present in the samples with HA. It is postulated that the interaction between the phosphate groups on the surface of HA and the ester groups of PPDX are responsible for both the antinucleation effect and the catalysis of the hydrolytic degradation of PPDX.

Monte Carlo study of a mixed spin-3/2 and spin-1/2 Ising ferrimagnetic model

Abstract: The magnetic properties of a mixed Ising ferrimagnetic model on a square lattice, in which the two interpenetrating square sublattices have spins that can take two values, $\ensuremath{\sigma}=\ifmmode\pm\else\textpm\fi{}1/2,$ alternated with spins that can take four values, $S=\ifmmode\pm\else\textpm\fi{}3/2,\ifmmode\pm\else\textpm\fi{}1/2,$ are studied. This model can be relevant for understanding the magnetic behavior of the new class of organometallic ferrimagnetic materials that exhibit spontaneous magnetic moments at room temperature. We carried out exact ground-state calculations for the model and employ a Monte Carlo algorithm to obtain the finite-temperature phase diagram for both the transition and compensation temperatures. The role of the different interactions in the Hamiltonian is explored. When only the nearest-neighbor interaction and the crystal-field term are included our results indicate no compensation point at finite temperature. When the next-nearest-neighbor interaction between the spins $\ensuremath{\sigma}=\ifmmode\pm\else\textpm\fi{}1/2$ exceeds a minimum value that depends on the other parameters in the Hamiltonian, a compensation point appears. The interaction between the $S=\ifmmode\pm\else\textpm\fi{}3/2$ spins, next-nearest neighbors in the lattice, has the effect of changing the compensation temperature.

Pollen flow in the distylous Palicourea fendleri (Rubiaceae): an experimental test of the Disassortative Pollen Flow Hypothesis

Abstract: The Disassortative Pollen Flow Hypothesis proposed by Darwin postulates that the relative position of anthers and stigmas in distylous flowers enhances pollen flow between flowers of different morphs (legitimate pollination), in comparison to flow between flowers of the same morph (illegitimate pollination). In order to test this hypothesis, we measured pollen transport, mediated by a trained Copper-rumped Hummingbird (Amazilia tobaci), between flowers of the distylous Palicourea fendleri under laboratory conditions. In individual tests, we offered to the hummingbird a pollen donor flower and two emasculated recipient flowers in a controlled sequence. After each foraging bout, we counted the number of pollen grains transported from the donor flower to the stigmas of both recipient flowers. In agreement with Darwin's hypothesis, we found that hummingbirds transport pollen of "pin" flowers in significantly higher numbers to legitimate "thrum" stigmas, even if previously visiting a "pin" flower. However, "thrum" pollen was deposited in greater numbers on illegitimate "thrum" stigmas. We interpret this asymmetry largely as the consequence of floral morphology; pollen flow was greater between anthers and stigmas that exhibit greater spatial matching. In P. fendleri, the position of floral organs along the corolla tube does not always precisely correspond. In our experimental system, the probability that the pollinator extracts a pollen grain from the anther and the probability of self-pollination were both dependent on the type of floral morph. We discuss the relevance of the latter findings in relation to other studies of pollen flow in heterostylous species.

Microbial diversity in a Venezuelan orthoquartzite cave is dominated by the Chloroflexi (Class Ktedonobacterales) and Thaumarchaeota Group I.1c.

Abstract: The majority of caves are formed within limestone rock and hence our understanding of cave microbiology comes from carbonate-buffered systems. In this paper, we describe the microbial diversity of Roraima Sur Cave, an orthoquartzite (SiO4) cave within Roraima Tepui, Venezuela. The cave contains a high level of microbial activity when compared with other cave systems, as determined by an ATP-based luminescence assay and cell counting. Molecular phylogenetic analysis of microbial diversity within the cave demonstrate the dominance of Actinomycetales and Alphaproteobacteria in endolithic bacterial communities close to the entrance, while communities from deeper in the cave are dominated (82-84%) by a unique clade of Ktedonobacterales within the Chloroflexi. While members of this phylum are commonly found in caves, this is the first identification of members of the Class Ktedonobacterales. An assessment of archaeal species demonstrates the dominance of phylotypes from the Thaumarchaeota Group I.1c (100%), which have previously been associated with acidic environments. While the Thaumarchaeota have been seen in numerous cave systems, the dominance of Group I.1c in Roraima Sur Cave is unique and a departure from the traditional archaeal community structure. Geochemical analysis of the cave environment suggests that water entering the cave, rather than the nutrient-limited orthoquartzite rock, provides the carbon and energy necessary for microbial community growth and subsistence, while the poor buffering capacity of quartzite or the low pH of the environment may be selecting for this unusual community structure. Together these data suggest that pH, imparted by the geochemistry of the host rock, can play as important a role in niche-differentiation in caves as in other environmental systems.

Geometrical origin of gauge theories

Abstract: The identification of a basic geometric group structure acting on the manifold of paths $P(M)$ of some manifold $M$ enables us to derive all the kinematical elements of a gauge theory from the single postulate of invariance of the formulation under this group.