Continuous Forest Fire Propagation in a Local Small World Network Model

This paper presents the development of a new continuous forest fire model implemented as a weighted local small-world network approach. This new approach was designed to simulate fire patterns in real, heterogeneous landscapes. The wildland fire spread is simulated on a square lattice in which each cell represents an area of the land's surface. The interaction between burning and non-burning cells, in the present work induced by flame radiation, may be extended well beyond nearest neighbors. It depends on local conditions of topography and vegetation types. An approach based on a solid flame model is used to predict the radiative heat flux from the flame generated by the burning of each site towards its neighbors. The weighting procedure takes into account the self-degradation of the tree and the ignition processes of a combustible cell through time. The model is tested on a field presenting a range of slopes and with data collected from a real wildfire scenario. The critical behavior of the spreading process is investigated

Self-assembly of triblock copolymers in aqueous solution

Indexación: ScieloThe aggregation of PE4VP-b-PS-b-PE4VP block copolymers was studied in aqueous solution. Triblock copolymers P4VP-b-PS-b-P4VP were synthesized by sequential anionic polymerization of poly(styrene) and poly(4-vinylpirydine) using sodium naphthalene as a bifunctional initiator. Subsequently, the 4-vinylpyridine units were quaternized with ethyl bromide to obtain cationic PE4VP-b-PS-b-PE4VP block copolymers. Both star and crew-cut micelles were formed. The concentrations at which micelles are formed cmc were determined, by steady-state and time-resolved fluorescence probing methods, as a function of quatemization degree. The results indicate that cmc of crew-cut micelles increases with increasing charge density of the PE4VP blocks. For star micelles there is not a clear dependency of cmc with the percentage of quatemization. The lifetime of pyrene fluorescence and the ratio I1/I3 were determined at concentrations of copolymers well above the cmc, and the results show that the location of pyrene into the micelle changes with the charge density of the micelle corona. The micropolarity sensed by pyrene decreases with increasing quatemization degree. The presence of aggregates was confirmed by transmission electronic microscopy.http://www.scielo.cl/scielo.php?script=sci_arttext&pid=S0717-97072008000200013&lng=es&nrm=is

Sum-of-squares of polynomials approach to nonlinear stability of fluid flows: an example of application

With the goal of providing the first example of application of a recently proposed method, thus demonstrating its ability to give results in principle, global stability of a version of the rotating Couette flow is examined. The flow depends on the Reynolds number and a parameter characterising the magnitude of the Coriolis force. By converting the original Navier-Stokes equations to a finite-dimensional uncertain dynamical system using a partial Galerkin expansion, high-degree polynomial Lyapunov functionals were found by sum-of-squares-of-polynomials optimization. It is demonstrated that the proposed method allows obtaining the exact global stability limit for this flow in a range of values of the parameter characterising the Coriolis force. Outside this range a lower bound for the global stability limit was obtained, which is still better than the energy stability limit. In the course of the study several results meaningful in the context of the method used were also obtained. Overall, the results obtained demonstrate the applicability of the recently proposed approach to global stability of the fluid flows. To the best of our knowledge, it is the first case in which global stability of a fluid flow has been proved by a generic method for the value of a Reynolds number greater than that which could be achieved with the energy stability approach

Potential of short wavelength laser ablation of organic materials

Although the literature contains several articles on UV laser ablation of synthetic polymers [1] and human tissue for surgical applications, to our knowledge there is no published record on organic geochemical applications for UV laser pyrolysis–gas chromatography–mass spectrometry (LA-GC-MS). In this study we have demonstrated the use of a 213 nm UV laser beam for ablating kerogens and organic rich rocks to liberate and analyse hydrocarbon signatures and compared the results against IR laser pyrolysis and traditional Py-GC-MS. It is possible to equate laser wavelength to electron volts where 1064 nm (IR) = 1.2 eV and 213 nm (UV) = 5.8 eV. Most chemical bonds have an energy between 2-4 eV and C-C bonds are ~3.6 eV. Organic materials can absorb radiation from a UV laser and chemical bonds can be cleaved cleanly by complex photochemical pathways by a single photon [2]. Ablation occurs with almost no heating of the sample and hence the term laser ablation instead of pyrolysis. Visible or IR lasers have insufficient energy to break bonds with a single photon this results in the heating of sample by the absobtion of energy into the vibrational modes of the molecule which can then result in pyrolysis. A solvent-extracted kerogen consisting mainly of higher plant material (Brownie Butte, Montanna, ~ 70 Ma) was used for initial experiments. A number of other samples have also been analysed. Laser ablation work was performed off-line in a static helium cell followed by solvent extraction of the laser cell. Separate analysis of the same samples using a more traditional flash pyrolysis approach was performed with a CDS pyroprobe and IR laser pyrolysis [3] for comparative purposes. As can be seen in Fig 1 UV laser ablation is able to liberate relatively high molecular weight fragments with no alkenes or other pyrolysis artefacts detected. SEM images of ablation pits indicate there is no obvious thermal alteration of the sample. The results of the pyrolysis techniques (on-line and IR laser pyrolysis) are similar and display a number of artefacts related to the pyrolysis process. Laser ablation of a number of samples has also shown that the distributions of biomarkers are comparable with the solvent extracts. Product yields although not quantified appear to be much higher than traditional pyrolysis technique

Is UV laser ablation a suitable tool for geochemical analysis of organic rich source materials?

Abstract not available

Role of Glia in Sculpting Synaptic Connections at the Drosophila Neuromuscular Junction: A Dissertation

Emerging evidence in both vertebrates and invertebrates is redefining glia as active players in the development and integrity of the nervous system. The formation of functional neuronal circuits requires the precise addition of new synapses. Mounting evidence implicates glial function in synapse remodeling and formation. However, the precise molecular mechanisms governing these functions are poorly understood. My thesis work begins to define the molecular mechanisms by which glia communicate with neurons at the Drosophila neuromuscular junction (NMJ). During development glia play a critical role in remodeling neuronal circuits in the CNS. In order to understand how glia remodel synapses, I manipulated a key component of the glial engulfment machinery, Draper. I found that during normal NMJ growth presynaptic boutons constantly shed membranes or debris. However, a loss of Draper resulted in an accumulation of debris and ghost boutons, which inhibited synaptic growth. I found that glia use the Draper pathway to engulf these excess membranes to sculpt synapses. Surprisingly, I found that muscle cells function as phagocytic cells as well by eliminating immature synaptic ghost boutons. This demonstrates that the combined efforts of glia and muscle are required for the addition of synapses and proper growth. My work establishes that glia play a crucial role in synapse development at the NMJ and suggests that there are other glial-derived molecules that regulate synapse function. I identified one glial derived molecule critical for the development of the NMJ, a TGF-β ligand called Maverick. Presynaptically, Maverick regulates the activation of BMP pathway confirmed by reducing the transcription of the known target gene Trio. Postsynaptically, it regulates the transcription of Glass bottom boat (Gbb) in the muscle suggesting that glia modulate the function of Gbb and consequently the activation of the BMP retrograde pathway at NMJ. Surprisingly, I also found that glial Maverick regulates the transcription of Shaker potassium channel, suggesting that glia potentially could regulate muscle excitability and consequently modulate synaptic transmission. Future work will elucidate such hypothesis. My work has demonstrated two novel roles for glia at the NMJ. First is that glia engulfing activity is important for proper synaptic growth. Second is that the secretion of glial-derived molecules are required to orchestrate synaptic development. This further supports that glia are critical active players in maintaining a functional nervous system