Influence of velocity in nanoscale friction processes

Force-microscopy images of boric acid crystals were obtained experimentally and simulated with the use of a two-dimensional mechanical model. An analysis of the stick and slip movement of the microscope tip shows that the energy-dissipation mechanism is strongly influenced by the non-linear dynamics of the sliding system. The contributions of stick and viscous forces on the energy dissipation (or friction forces) are studied as a function of the relative scanning velocity. At low relative velocities, the stick forces are shown to be responsible for the energy dissipation. This energy is velocity-dependent, due to the coupling between the two degrees of freedom of the sliding system. As the scanning velocity increases the stick forces are damped; the viscous force is then predominant in the energy-dissipation process. Received: 30 October 2001 / Accepted: 17 May 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +55-21/2295-9397, E-mail: prioli@vdg.fis.puc-rio.br     

On the paper “Symmetry analysis of wave equation on sphere” by H. Azad and M.T. Mustafa

Using the scalar curvature of the product manifold S²×R and the complete group classification of nonlinear Poisson equation on (pseudo) Riemannian manifolds, we extend the previous results on symmetry analysis of homogeneous wave equation obtained by H. Azad and M.T. Mustafa [H. Azad, M.T. Mustafa, Symmetry analysis of wave equation on sphere, J. Math. Anal. Appl. 333 (2007) 1180-1188] to nonlinear Klein-Gordon equations on the two-dimensional sphere.     

catena-Poly­[[di­aqua(1,10-phen­anthroline-N,N′)­manganese(II)]-μ-(thiosulfato-O:S)] and bis(2,2′-bipyridyl-N,N′)(tetra­thionato-O,O′)manganese(II)

The structures of [Mn(S₂O₃)₂(C₁₂H₈N₂)(H₂O)₂] and [Mn(S₄O₆)(C₁₀H₈N₂)₂] are presented. The former consists of pairs of polymeric chains formed by manganese polyhedra bridged by bidentate thio­sulfate anions, which are in turn related to each other by a pseudo-twofold screw axis. The latter has crystallographic twofold symmetry and consists of monomers in which manganese displays its typical octahedral coordination provided by the bidentate bites of two bi­pyridine bases and a tetra­thionate anion, which is, to our knowledge, the first chelating tetra­thionate to be reported in the literature.     

The mobility in disordered molecular systems with energies given by a charge-induced dipoles interaction

We investigate the field dependence of the mobility in a model for a disordered molecular system containing spatial and energetic disorders. In this model we assign an isotropic polarizability to each site and take the site energies to be the site polarization energies, the interaction energy of a charge in the given site with the induced dipoles in the neighboring sites. This model was shown, in a previous publication, to contain short-ranged energetic correlations and we show in this work that this correlation produces a charge mobility proportional to the exponential of the square root of the applied field, the Poole-Frenkel dependence observed in various disordered organic materials, over a significant range of fields. We present an expression for the field dependence of the mobility in terms of the average intersite separation and of the isotropic polarizability of the electronic states, the two model parameters.     

Colonization of cashew plants by Lasiodiplodia theobromae: microscopical features

Lasiodiplodia theobromae is a phytopathogenic fungus causing gummosis, a threatening disease for cashew plants in Brazil. In an attempt to investigate the ultrastructural features of the pathogen colonization and its response to immunofluorescence labeling, light, confocal and electron microscope studies were conducted on different severity scale patterns of diseased plants. Lasiodiplodia-antisera was checked for cross reactivity against common cashew plants fungi. Optical microscopy analysis revealed a longitudinally sectioned hyphae located within the xylem vessels, showing an extensive hyphal development in the secondary xylem tissue. SEM images demonstrated that the fungus was found in some asymptomatic samples, particularly within the xylem vessels as confirmed by the optical images. Symptomatic sample images showed an extensive distribution of the fungus along the secondary xylem, within the vessels, infecting xylem parenchyma. A closer look in the secondary xylem parenchyma reveals a heavy and profuse invasion of the cells with a distinguishable cell wall disintegration and fully hyphae dispersal. There was no reactivity of Lasiodiplodia-antisera against mycelial extracts of Colletotrichum gloeosporioides, Phomopsis anardii and Pestalotiopsis guepinii. Following incubation of sections with the polyclonal antisera, the hyphae were intensely and regularly labeled. Rays, vessels and parenchyma cells were the preferred pathway for L. theobromae colonization. Artificial infection provides the information that the vascular cylinder is undoubtedly employed and used by the fungus for hyphae distribution. Immunofluorescence assay employed in situ was applied and the polyclonal antisera produced was able to recognize the fungus and proved to be a sensitive technique to detect it.     

Combination of Raman, Infrared, and X-Ray Energy-Dispersion Spectroscopies and X-Ray Diffraction to Study a Fossilization Process

X-ray diffraction was combined with X-ray energy-dispersion, Fourier-transform infrared, and Raman spectroscopies to study the fossilization of a Cretaceous specimen of the plant Brachyphyllum castilhoi, a fossil from the Ipubi Formation, in the Araripe Sedimentary Basin, Northeastern Brazil. Among the possible fossilization processes, which could involve pyrite, silicon oxide, calcium oxide, or other minerals, we were able to single out pyritization as the central mechanism producing the fossil, more than 100 million years ago. In addition to expanding the knowledge of the Ipubi Formation, this study shows that, when combined with other experimental techniques, Raman spectroscopy is a valuable tool at the paleontologist’s disposal.     

Monodromy,center–focus and integrability problems for quasi-homogeneous polynomial systems

This paper deals with planar quasi-homogeneous polynomial vector fields, and addresses three major questions: the monodromy, the center–focus and the integrability problems. We characterize the monodromic planar quasi-homogeneous polynomial vector fields, and we give a condition to distinguish between a center and a focus in this case. Also, we provide conditions which characterize the integrability of quasi-homogeneous polynomial systems under non-resonance conditions. The results obtained allow us to analyse two monodromic planar systems with degenerate linear part: one of them with nilpotent linearization, and another one with null linear part.     

Excess thermodynamics properties of propyl propanoate+hexane+cyclohexane at 298.15 K

Summary In this paper we present excess molar volumes and excess molar enthalpies of binary and ternary mixtures containing propyl propanoate, hexane and cyclohexane as components at 298.15 K. Excess molar volumes were calculated from the density of the pure liquids and mixtures. The density was measured using an Anton Paar DMA 60/602 vibrating-tube densimeter. Excess molar enthalpies were obtained using a Calvet microcalorimeter     

Weak compactness of wave maps and harmonic maps

We show that a weak limit of a sequence of wave maps in (1 + 2) dimensions with uniformly bounded energy is again a wave map. Essential ingredients in the proof are Hodge structures related to harmonic maps, ¹ estimates for Jacobians, ¹-BMO duality, a “monotonicity” formula in the hyperbolic context and the concentration compactness method. Application of similar ideas in the elliptic context yields a drastically shortened proof of recent results by Bethuel on Palais-Smale sequences for the harmonic map functional on two dimensional domains and on limits of almost H-surfaces.