Simulations of a weakly conducting droplet under the influence of an alternating electric field

We investigate the electrohydrodynamics of an initially spherical droplet under the influence of an external alternating electric field by conducting axisymmetric numerical simulations using a charge-conservative volume-of-fluid based finite volume flow solver. The mean amplitude of shape oscillations of a droplet subjected to an alternating electric field for leaky dielectric fluids is the same as the steady-state deformation under an equivalent root mean squared direct electric field for all possible electrical conductivity ratio $(K_r)$ and permittivity ratio $(S)$ of the droplet to the surrounding fluid. In contrast, our simulations for weakly conducting media show that this equivalence between alternating and direct electric fields does not hold for $K_r \ne S$. Moreover, for a range of parameters, the deformation obtained using the alternating and direct electric fields is qualitatively different, i.e. for low $K_r$ and high $S$, the droplet becomes prolate under alternating electric field but deforms to an oblate shape in the case of the equivalent direct electric field. A parametric study is conducted by varying the time period of the applied alternating electric field, the permittivity and the electrical conductivity ratios. It is observed that while increasing $K_r$ has a negligible effect on the deformation dynamics of the droplet for $K_r<S$, it enhances the deformation of the droplet when $K_r>S$ for both alternating and direct electric fields. We believe that our results may be of immense consequence in explaining the morphological evolution of droplets in a plethora of scenarios ranging from nature to biology.Comment: 10 pages, 8 figure

Planar Rayleigh scattering results in helium-air mixing experiments in a Mach-6 wind tunnel

Planar Rayleigh scattering measurements with an argon—fluoride excimer laser are performed to investigate helium mixing into air at supersonic speeds. The capability of the Rayleigh scattering technique for flow visualization of a turbulent environment is demonstrated in a large-scale, Mach-6 facility. The detection limit obtained with the present setup indicates that planar, quantitative measurements of density can be made over a large cross-sectional area (5 cm × 10 cm) of the flow field in the absence of clusters

Ramsey Goodness of Bounded Degree Trees

Given a pair of graphs G and H, the Ramsey number R(G, H) is the smallest N such that every red-blue coloring of the edges of the complete graph KN contains a red copy of G or a blue copy of H. If a graph G is connected, it is well known and easy to show that R(G, H) ≥ (|G| − 1)(χ(H) − 1) + σ(H), where χ(H) is the chromatic number of H and σ(H) is the size of the smallest color class in a χ(H)-coloring of H. A graph G is called H-good if R(G, H) = (|G| − 1)(χ(H) − 1) + σ(H). The notion of Ramsey goodness was introduced by Burr and Erd˝os in 1983 and has been extensively studied since then. In this paper we show that if n ≥ Ω(|H| log4 |H|) then every n-vertex bounded degree tree T is H-good. The dependency between n and |H| is tight up to log factors. This substantially improves a result of Erd˝os, Faudree, Rousseau, and Schelp from 1985, who proved that n-vertex bounded degree trees are H-good when n ≥ Ω(|H| 4 ). MSC: 05C05, 05C5

A remark on Hamilton cycles with few colors

Akbari, Etesami, Mahini, and Mahmoody conjectured that every proper edge colouring of Kn with n colours contains a Hamilton cycle with ≤O(logn) colours. They proved that there is always a Hamilton cycle with ≤8n−−√ colours. In this note we improve this bound to O(log3n)

Évaluation du niveau de pollution par les métaux lourds des lacs Bini et Dang, Région de l’Adamaoua, Cameroun

La présente étude a pour objectif principal d’évaluer le niveau de pollution métallique des lacs Bini et Dang (Ngaoundéré, Cameroun) à travers l’analyse des eaux et des sédiments de surface. La concentration des métaux lourds (Ni, Cr, Fe, Pb, Cd, Zn) a été mesurée par spectrophotométrie d’absorption atomique. Des résultats obtenus, il ressort que les éléments Ni, Fe, Cr, Pb et Cd ont des teneurs élevées dans les eaux des deux lacs comparés aux normes OMS sur les eaux de boisson et aux normes canadiennes sur la protection de la vie aquatique. Afin d’évaluer le niveau de contamination des sédiments des deux lacs, le facteur d’enrichissement (FE) et l’indice de géoaccumulation (I-géo) ont été calculés.Ainsi, le cadmium présente des FE forts à très forts alors que le plomb affiche des FE modérés à forts. L’Igéo varie d’une forte contamination à une contamination extrême pour le cadmium. Cet index est modéré à fort pour le plomb et modéré pour le fer. Les sédiments du lac de Dang sont dans l’ensemble les plus contaminés en métaux lourds par rapport à ceux du lac Bini. Les activités agricoles, les rejets des eaux usées domestiques et les décharges incontrôlées sont considérés comme principales sources de pollution des deux lacs.Mots-clés : bini, dang, métaux lourds, lac, sédiment

Food security and sustainability - chances and limitations of agriculture

The increasing population of the world may induce various problems. There are two major fields; food security and adequate fresh water supply, which are predominant for all Global economic structures. On the other hand all activities implemented in favour of meeting the demand of the population, increase the ecological footprint and may risk sustainability of both mankind and its environment. The present study deals with an assessment of future trends on the basis of the present state of alimentation. Agriculture has a basic role in providing food for the human race. All sort of activities of that are in relation with the environment and at the same time they are driven by economic and social aspects. Sustainable agriculture can only be implemented if agricultural production can be run in an environment which is socially bearable, and economically viable. The scheme has to take into consideration that the society has to be equitable economically continuously. Whenever any of these interrelations cannot be manifested, the whole system may turn to be non-sustainable, or inefficient regarding food security

Food security and sustainability - chances and limitations of agriculture

The increasing population of the world may induce various problems. There are two major fields; food security and adequate fresh water supply, which are predominant for all Global economic structures. On the other hand all activities implemented in favour of meeting the demand of the population, increase the ecological footprint and may risk sustainability of both mankind and its environment. The present study deals with an assessment of future trends on the basis of the present state of alimentation. Agriculture has a basic role in providing food for the human race. All sort of activities of that are in relation with the environment and at the same time they are driven by economic and social aspects. Sustainable agriculture can only be implemented if agricultural production can be run in an environment which is socially bearable, and economically viable. The scheme has to take into consideration that the society has to be equitable economically continuously. Whenever any of these interrelations cannot be manifested, the whole system may turn to be non-sustainable, or inefficient regarding food security

Cultura do girassol: tecnologia de produção.

Escolha de area e preparo do solo; Correcao da acidez; Adubacao; Epoca de plantio; Plantio; Densidade; Controle de plantas daninhas; Doencas e pragas; Colheita.bitstream/item/61288/1/Documentos-67-1996.pdf2. ed. rev. aum

Cultura do girassol: tecnologia de produção.

Escolha da area e preparo do solo; Correcao da acidez; Adubacao; Epoca de plantio; Plantio; Densidade; Controle de plantas daninhas; Doencas e pragas; Colheita.bitstream/item/60796/1/Documentos-67.pd

Charge carrier interaction with a purely electronic collective mode: Plasmarons and the infrared response of elemental bismuth

We present a detailed optical study of single crystal bismuth using infrared reflectivity and ellipsometry. Colossal changes in the plasmon frequency are observed as a function of temperature due to charge transfer between hole and electron Fermi pockets. In the optical conductivity, an anomalous temperature dependent mid-infrared absorption feature is observed. An extended Drude model analysis reveals that it can be connected to a sharp upturn in the scattering rate, the frequency of which exactly tracks the temperature dependent plasmon frequency. We interpret this absorption and increased scattering as the first direct optical evidence for a charge carrier interaction with a collective mode of purely electronic origin; here electron-plasmon scattering. The observation of a \emph{plasmaron} as such is made possible only by the unique coincidence of various energy scales and exceptional properties of semi-metal bismuth.Comment: 4 pages, 4 figure