11,329 research outputs found
Forehead Skin Blood Flow in Normal Neonates during Active and Quiet Sleep, Measured with a Diode Laser Doppler Instrument
Changes in forehead skin blood flow during active and quiet sleep were determined in 16 healthy neonates using a recently developed semi-conductor laser Doppler flow meter without light conducting fibres. Measurements were carried out at a postnatal age varying from 5 hours to 7 days. The two sleep states could be distinguished in 17 recordings. The mean skin blood flow values during active sleep were significantly higher (p<0.01) than those during quiet sleep, the mean increase being 28.1%. The variability of the flow signal, expressed as the coefficient of variation, changed significantly from 23.1% during active sleep to 18.2% during quiet sleep
Scientific, institutional and personal rivalries among Soviet geographers in the late Stalin era
Scientific, institutional and personal rivalries between three key centres of geographical research and scholarship (the Academy of Sciences Institute of Geography and the Faculties of Geography at Moscow and Leningrad State Universities) are surveyed for the period from 1945 to the early 1950s. It is argued that the debates and rivalries between members of the three institutions appear to have been motivated by a variety of scientific, ideological, institutional and personal factors, but that genuine scientific disagreements were at least as important as political and ideological factors in influencing the course of the debates and in determining their final outcome
The whole and its parts : why and how to disentangle plant communities and synusiae in vegetation classification
Most plant communities consist of different structural and ecological subsets, ranging from cryptogams to different tree layers. The completeness and approach with which these subsets are sampled have implications for vegetation classification. Nonâvascular plants are often omitted or sometimes treated separately, referring to their assemblages as âsynusiaeâ (e.g. epiphytes on bark, saxicolous species on rocks). The distinction of complete plant communities (phytocoenoses or holocoenoses) from their parts (synusiae or merocoenoses) is crucial to avoid logical problems and inconsistencies of the resulting classification systems. We here describe theoretical differences between the phytocoenosis as a whole and its parts, and outline consequences of this distinction for practise and terminology in vegetation classification. To implement a clearer separation, we call for modifications of the International Code of Phytosociological Nomenclature and the EuroVegChecklist. We believe that these steps will make vegetation classification systems better applicable and raise the recognition of the importance of nonâvascular plants in the vegetation as well as their interplay with vascular plants
Hydrodynamic flow of expanding Bose-Einstein condensates
We study expansion of quasi-one-dimensional Bose-Einstein condensate (BEC)
after switching off the confining harmonic potential. Exact solution of
dynamical equations is obtained in framework of the hydrodynamic approximation
and it is compared with the direct numerical simulation of the full problem
showing excellent agreement at realistic values of physical parameters. We
analyze the maximum of the current density and estimate the velocity of
expansion. The results of the 1D analysis provides also qualitative
understanding of some properties of BEC expansion observed in experiments.Comment: 5 pages, 3 figures, RevTeX4. To appear in Physical Review
On-line Excited-State Laser Spectroscopy of Trapped Short-Lived Ra Ions
As an important step towards an atomic parity violation experiment in one
single trapped Ra ion, laser spectroscopy experiments were performed with
on-line produced short-lived Ra ions. The isotope shift of
the 6\,^2D\,-\,7\,^2P and
6\,^2D\,-\,7\,^2P transitions and the hyperfine structure
constant of the 7\,^2S and 6\,^2D states in Ra
were measured. These values provide a benchmark for the required atomic theory.
A lower limit of ms for the lifetime of the metastable
6\,^2D state was measured by optical shelving.Comment: 4.2 pages, 6 figures, 2 tables
Sheared bioconvection in a horizontal tube
The recent interest in using microorganisms for biofuels is motivation enough
to study bioconvection and cell dispersion in tubes subject to imposed flow. To
optimize light and nutrient uptake, many microorganisms swim in directions
biased by environmental cues (e.g. phototaxis in algae and chemotaxis in
bacteria). Such taxes inevitably lead to accumulations of cells, which, as many
microorganisms have a density different to the fluid, can induce hydrodynamic
instabilites. The large-scale fluid flow and spectacular patterns that arise
are termed bioconvection. However, the extent to which bioconvection is
affected or suppressed by an imposed fluid flow, and how bioconvection
influences the mean flow profile and cell transport are open questions. This
experimental study is the first to address these issues by quantifying the
patterns due to suspensions of the gravitactic and gyrotactic green
biflagellate alga Chlamydomonas in horizontal tubes subject to an imposed flow.
With no flow, the dependence of the dominant pattern wavelength at pattern
onset on cell concentration is established for three different tube diameters.
For small imposed flows, the vertical plumes of cells are observed merely to
bow in the direction of flow. For sufficiently high flow rates, the plumes
progressively fragment into piecewise linear diagonal plumes, unexpectedly
inclined at constant angles and translating at fixed speeds. The pattern
wavelength generally grows with flow rate, with transitions at critical rates
that depend on concentration. Even at high imposed flow rates, bioconvection is
not wholly suppressed and perturbs the flow field.Comment: 19 pages, 9 figures, published version available at
http://iopscience.iop.org/1478-3975/7/4/04600
First Test of Lorentz Invariance in the Weak Decay of Polarized Nuclei
A new test of Lorentz invariance in the weak interactions has been made by
searching for variations in the decay rate of spin-polarized 20Na nuclei. This
test is unique to Gamow-Teller transitions, as was shown in the framework of a
recently developed theory that assumes a Lorentz symmetry breaking background
field of tensor nature. The nuclear spins were polarized in the up and down
direction, putting a limit on the amplitude of sidereal variations of the form
|(\Gamma_{up} - \Gamma_{down})| / (\Gamma_{up} + \Gamma_{down}) < 3 * 10^{-3}.
This measurement shows a possible route toward a more detailed testing of
Lorentz symmetry in weak interactions.Comment: 11 pages, 6 figure
Isotope Shifts of the 6d\,^2D - 7p\,^2P Transition in Trapped Short-Lived Ra
Laser spectroscopy of short-lived radium isotopes in a linear Paul trap has
been performed. The isotope shifts of the 6d\,^2D -
7p\,^2P transition in Ra were measured, which are
sensitive to the short range part of the atomic wavefunctions. The results are
essential experimental input for improving the precision of atomic structure
calculation. This is indispensable for parity violation in Ra aiming at the
determination of the weak mixing angle.Comment: Accepted for publication in Physical Review A as a Rapid
Communicatio
Enhanced reaction kinetics in biological cells
The cell cytoskeleton is a striking example of "active" medium driven
out-of-equilibrium by ATP hydrolysis. Such activity has been shown recently to
have a spectacular impact on the mechanical and rheological properties of the
cellular medium, as well as on its transport properties : a generic tracer
particle freely diffuses as in a standard equilibrium medium, but also
intermittently binds with random interaction times to motor proteins, which
perform active ballistic excursions along cytoskeletal filaments. Here, we
propose for the first time an analytical model of transport limited reactions
in active media, and show quantitatively how active transport can enhance
reactivity for large enough tracers like vesicles. We derive analytically the
average interaction time with motor proteins which optimizes the reaction rate,
and reveal remarkable universal features of the optimal configuration. We
discuss why active transport may be beneficial in various biological examples:
cell cytoskeleton, membranes and lamellipodia, and tubular structures like
axons.Comment: 10 pages, 2 figure
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