2,478 research outputs found
The wall shear rate distribution for flow in random sphere packings
The wall shear rate distribution P(gamma) is investigated for pressure-driven
Stokes flow through random arrangements of spheres at packing fractions 0.1 <=
phi <= 0.64. For dense packings, P(gamma) is monotonic and approximately
exponential. As phi --> 0.1, P(gamma) picks up additional structure which
corresponds to the flow around isolated spheres, for which an exact result can
be obtained. A simple expression for the mean wall shear rate is presented,
based on a force-balance argument.Comment: 4 pages, 3 figures, 1 table, RevTeX 4; significantly revised with
significantly extended scop
Apospory appears to accelerate onset of meiosis and sexual embryo sac formation in sorghum ovules
<p>Abstract</p> <p>Background</p> <p>Genetically unreduced (2<it>n</it>) embryo sacs (ES) form in ovules of gametophytic apomicts, the 2<it>n </it>eggs of which develop into embryos parthenogenetically. In many apomicts, 2<it>n </it>ES form precociously during ovule development. Whether meiosis and sexual ES formation also occur precociously in facultative apomicts (capable of apomictic and sexual reproduction) has not been studied. We determined onset timing of meiosis and sexual ES formation for 569 <it>Sorghum bicolor </it>genotypes, many of which produced 2<it>n </it>ES facultatively.</p> <p>Results</p> <p>Genotype differences for onset timing of meiosis and sexual ES formation, relative to ovule development, were highly significant. A major source of variation in timing of sexual germline development was presence or absence of apomictic ES, which formed from nucellar cells (apospory) in some genotypes. Genotypes that produced these aposporous ES underwent meiosis and sexual ES formation precociously. Aposporous ES formation was most prevalent in subsp. <it>verticilliflorum </it>and in breeding lines of subsp. <it>bicolor</it>. It was uncommon in land races.</p> <p>Conclusions</p> <p>The present study adds meiosis and sexual ES formation to floral induction, apomictic ES formation, and parthenogenesis as processes observed to occur precociously in apomictic plants. The temporally diverse nature of these events suggests that an epigenetic memory of the plants' apomixis status exists throughout its life cycle, which triggers, during multiple life cycle phases, temporally distinct processes that accelerate reproduction.</p
Drying of complex suspensions
We investigate the 3D structure and drying dynamics of complex mixtures of
emulsion droplets and colloidal particles, using confocal microscopy. Air
invades and rapidly collapses large emulsion droplets, forcing their contents
into the surrounding porous particle pack at a rate proportional to the square
of the droplet radius. By contrast, small droplets do not collapse, but remain
intact and are merely deformed. A simple model coupling the Laplace pressure to
Darcy's law correctly estimates both the threshold radius separating these two
behaviors, and the rate of large-droplet evacuation. Finally, we use these
systems to make novel hierarchical structures.Comment: 4 pages, 4 figure
Aplication of Gene Transfer in Aquaculture
Recently, global food security has become a hot issue by the public in national as well as out of the country. Aquacultural output will need to be increased several fold in order to meet the rising demands for fish in coming years as the increasing of mankind population. The intensity and capacity of production is expected to increase using biotechnology approach. One of the advances biotechnologies that expected to be a powerful approach for aquaculture development is transgenic technique. This technique has been applied to several commercially valuable species. This review describes various techniques of gene transfer, persistence and expression of transferred gene, application and future aspect of gene transfer research in aquaculture. Key words: Gene transfer, gene expression, biotechnology, aquaculture ABSTRAK Saat ini, keamanan pangan telah menjadi isu hangat di masyarakat baik di dalam maupun di luar negeri. Produksi akuakultur diharapkan dapat ditingkatkan beberapa kali lipat untuk memenuhi kebutuhan pangan berupa ikan dimasa-masa mendatang akibat peningkatan populasi manusia. Intensitas dan kapasitas produksi diharapkan meningkat dengan menggunakan pendekatan bioteknologi. Salah satu teknik modern yang diduga akan menjadi sarana yang berguna dalam pengembangan akuakultur adalah teknologi transfer gen. Teknik ini telah diaplikasikan pada spesies-spesies yang memiliki nilai ekonomis. Ulasan ini menggambarkan variasi metode transfer gen, persistensi dan ekspressi dari gen yang ditransfer, aplikasi dan prospeknya ke depan dari penelitian transfer gen dalam akuakultur. Kata kunci : transfer gen, ekspressi gen, bioteknologi, akuakultu
Exclusive electromagnetic production of strangeness on the nucleon : review of recent data in a Regge approach
In view of the numerous experimental results recently released, we provide in
this letter an update on the performance of our simple Regge model for
strangeness electroproduction on the nucleon. Without refitting any parameters,
a decent description of all measured observables and channels is achieved. We
also give predictions for spin transfer observables, recently measured at
Jefferson Lab which have high sensitivity to discriminate between different
theoretical approaches.Comment: 5 pages, 5 figure
Generation of ultra-short light pulses by a rapidly ionizing thin foil
A thin and dense plasma layer is created when a sufficiently strong laser
pulse impinges on a solid target. The nonlinearity introduced by the
time-dependent electron density leads to the generation of harmonics. The pulse
duration of the harmonic radiation is related to the risetime of the electron
density and thus can be affected by the shape of the incident pulse and its
peak field strength. Results are presented from numerical
particle-in-cell-simulations of an intense laser pulse interacting with a thin
foil target. An analytical model which shows how the harmonics are created is
introduced. The proposed scheme might be a promising way towards the generation
of attosecond pulses.
PACS number(s): 52.40.Nk, 52.50.Jm, 52.65.RrComment: Second Revised Version, 13 pages (REVTeX), 3 figures in ps-format,
submitted for publication to Physical Review E, WWW:
http://www.physik.tu-darmstadt.de/tqe
The Effect of Air on Granular Size Separation in a Vibrated Granular Bed
Using high-speed video and magnetic resonance imaging (MRI) we study the
motion of a large sphere in a vertically vibrated bed of smaller grains. As
previously reported we find a non-monotonic density dependence of the rise and
sink time of the large sphere. We find that this density dependence is solely
due to air drag. We investigate in detail how the motion of the intruder sphere
is influenced by size of the background particles, initial vertical position in
the bed, ambient pressure and convection. We explain our results in the
framework of a simple model and find quantitative agreement in key aspects with
numerical simulations to the model equations.Comment: 14 pages, 16 figures, submitted to PRE, corrected typos, slight
change
Integrin activation - the importance of a positive feedback
Integrins mediate cell adhesion and are essential receptors for the
development and functioning of multicellular organisms. Integrin activation is
known to require both ligand and talin binding and to correlate with cluster
formation but the activation mechanism and precise roles of these processes are
not yet resolved. Here mathematical modeling, with known experimental
parameters, is used to show that the binding of a stabilizing factor, such as
talin, is alone insufficient to enable ligand-dependent integrin activation for
all observed conditions; an additional positive feedback is required.Comment: in press in Bulletin of Mathematical Biolog
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