208 research outputs found
Etude du PCV et du TSWV au Sénégal
Le virus du rabougrissement de l'arachide (PCV) sévit sur la totalité du territoire sénégalais ; son incidence est trÚs marquée dans les zones irriguées et cultivées en permanence ; plusieurs souches, aux propriétés sérologiques différentes, ont été observées. Le mode de transmission de ce virus exige un contrÎle sévÚre de l'état des semences. Le virus de la maladie bronzée de la tomate atteint l'arachide au Sénégal ; confirmation en est donnée par l'étude sérologique. (Résumé d'auteur
Preferred sizes and ordering in surface nanobubble populations
Two types of homogeneous surface nanobubble populations, created by different
means, are analyzed statistically on both their sizes and spatial positions. In
the first type (created by droplet-deposition, case A) the bubble size R is
found to be distributed according to a generalized gamma law with a preferred
radius R*=20 nm. The radial distribution function shows a preferred spacing at
~5.5 R*. These characteristics do not show up in comparable Monte-Carlo
simulations of random packings of hard disks with the same size distribution
and the same density, suggesting a structuring effect in the nanobubble
formation process. The nanobubble size distribution of the second population
type (created by ethanol-water exchange, case B) is a mixture of two clearly
separated distributions, hence, with two preferred radii. The local ordering is
less significant, due to the looser packing of the nanobubbles.Comment: 5 pages, 5 figure
Bubble size prediction in co-flowing streams
In this paper, the size of bubbles formed through the breakup of a gaseous
jet in a co-axial microfluidic device is derived. The gaseous jet surrounded by
a co-flowing liquid stream breaks up into monodisperse microbubbles and the
size of the bubbles is determined by the radius of the inner gas jet and the
bubble formation frequency. We obtain the radius of the gas jet by solving the
Navier-Stokes equations for low Reynolds number flows and by minimization of
the dissipation energy. The prediction of the bubble size is based on the
system's control parameters only, i.e. the inner gas flow rate , the outer
liquid flow rate , and the tube radius . For a very low gas-to-liquid
flow rate ratio () the bubble radius scales as , independently of the inner to outer viscosity
ratio and of the type of the velocity profile in the gas, which
can be either flat or parabolic, depending on whether high-molecular-weight
surfactants cover the gas-liquid interface or not. However, in the case in
which the gas velocity profiles are parabolic and the viscosity ratio is
sufficiently low, i.e. , the bubble diameter scales as
, with smaller than 1/2
Inverse lift: a signature of the elasticity of complex fluids?
To understand the mechanics of a complex fluid such as a foam we propose a
model experiment (a bidimensional flow around an obstacle) for which an
external sollicitation is applied, and a local response is measured,
simultaneously. We observe that an asymmetric obstacle (cambered airfoil
profile) experiences a downards lift, opposite to the lift usually known (in a
different context) in aerodynamics. Correlations of velocity, deformations and
pressure fields yield a clear explanation of this inverse lift, involving the
elasticity of the foam. We argue that such an inverse lift is likely common to
complex fluids with elasticity.Comment: 4 pages, 4 figures, revised version, submitted to PR
Microbubble shape oscillations excited through ultrasonic parametric driving\ud
An air bubble driven by ultrasound can become shape-unstable through a parametric instability. We report time-resolved optical observations of shape oscillations (mode n=2 to 6) of micron-sized single air bubbles. The observed mode number n was found to be linearly related to the ambient radius of the bubble. Above the critical driving pressure threshold for shape oscillations, which is minimal at the resonance of the volumetric radial mode, the observed mode number n is independent of the forcing pressure amplitude. The microbubble shape oscillations were also analyzed numerically by introducing a small nonspherical linear perturbation to a Rayleigh-Plesset-type equation, capturing the experimental observations in detail.\ud
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Groundnut viral diseases in West Africa
This paper describes groundnut viral diseases observed in West Africa. Six viruses are identified and their main properties are reported here: peanut Clump, groundnut rosette, groundnut eyespot, groundnut crinkle, tomato spotted wilt and groundnut chlorotic spotting viruses. Four other diseases are described in part: groundnut streak, groundnut mosaic, groundnut flecking and groundnut golden mosaic diseases. Some of them are economically very important such as the two strains of rosette, peanut clump and tomato spotted wilt diseases. Others are apparently of minor importance though they occur relatively frequently and show a wide distribution, such as groundnut eyespot, groundnut crinkle, groundnut streak and groundnut golden mosaic diseases. The others appear occasionally but are nevertheless described: some which are very infectious, as groundnut chlorotic spotting disease could become very important within a few years. (Résumé d'auteur
Isolation of phytoplasma DNA from the coconut palms (Cocos nucifera L.) collected from Ghana
This study aimed to verify the presence of the causative agent of Lethal Yellowing which is phytoplasma in samples provided from infected coconut trees. Study was carried out by using various samples like zygotic embryo, young leaves and immature & mature inflorescences. These materials were collected from trees at the stage 1 and 2 of the disease development.. Stage 1 of disease development is characterized by leaf yellowing and the start of the falling nuts while at the stage 2 of disease development, the trees has not bear nuts longer. From infected material, DNA was extracted by three different processes and isolated DNA was amplified by PCR. 16S rRNA gene was amplified by two specific primers of phytoplama viz P1/P2 and Ghana 813/AKSR. Among the various tested materials presence of phytoplasma was reported from the mature inflorescences while the presence of the phytoplasma was not reported from the leaves and embryos of the coconut
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