933 research outputs found
Crystal nucleation mechanism in melts of short polymer chains under quiescent conditions and under shear flow
We present a molecular dynamics simulation study of crystal nucleation from
undercooled melts of n-alkanes, and we identify the molecular mechanism of
homogeneous crystal nucleation under quiescent conditions and under shear flow.
We compare results for n-eicosane(C20) and n-pentacontahectane(C150), i.e. one
system below the entanglement length and one above. Under quiescent conditions,
we observe that entanglement does not have an effect on the nucleation
mechanism. For both chain lengths, the chains first align and then straighten
locally. Then the local density increases and finally positional ordering sets
in. At low shear rates the nucleation mechanism is the same as under quiescent
conditions, while at high shear rates the chains align and straighten at the
same time. We report on the effects of shear rate and temperature on the
nucleation rates and estimate the critical shear rates, beyond which the
nucleation rates increase with the shear rate. We show that the viscosity of
the system is not affected by the crystalline nuclei.Comment: 9 page
Odkrycie i zastosowanie pegwisomantu: antagonisty hormonu wzrostu
Growth hormone (GH) is a well established participant in several complex physiological processes including growth, differentiation,
and metabolism. Recombinant human GH is a drug that has been approved for use for several clinical conditions
where the action of GH is diminished or completely lacking. Thus there is considerable interest in developing novel
drugs that modify the function of GH. Only in the last several decades have the detailed structural features of GH along
with its interaction with its receptor been elucidated. In this review we summarise the basic structural and functional
properties of GH, its receptor and their interaction. In addition, we discuss the discovery and development of an effective
GH receptor antagonist, pegvisomant, and summarise potential therapeutic uses of this drug.
(Pol J Endocrinol 2007; 58 (4): 322-329)Hormon wzrostu (GH, growth hormone) uczestniczy w wielu fizjologicznych procesach dotyczących wzrastania, różnicowania
i metabolizmu. Leczenie rekombinowanym ludzkim GH jest akceptowane w wielu schorzeniach wiążących się
z całkowitym brakiem lub zmniejszeniem działania GH. Wynika stąd znaczne zainteresowanie rozwojem nowych leków
mogących modyfikować czynność GH. Dopiero niedawno wyjaśniono dokładną strukturę GH i jego interakcje z receptorem.
W niniejszej pracy autorzy podsumowują wiedzę dotyczącą podstawowej budowy GH, jego receptora i interakcji
między nimi. Ponadto, omówiono odkrycie i rozwój skutecznego antagonisty receptora GH, pegvisomantu i przedstawiono
potencjalne możliwości zastosowania terapeutycznego tego leku
Geometry of Frictionless and Frictional Sphere Packings
We study static packings of frictionless and frictional spheres in three
dimensions, obtained via molecular dynamics simulations, in which we vary
particle hardness, friction coefficient, and coefficient of restitution.
Although frictionless packings of hard-spheres are always isostatic (with six
contacts) regardless of construction history and restitution coefficient,
frictional packings achieve a multitude of hyperstatic packings that depend on
system parameters and construction history. Instead of immediately dropping to
four, the coordination number reduces smoothly from as the friction
coefficient between two particles is increased.Comment: 6 pages, 9 figures, submitted to Phys. Rev.
Nonlinear equation for anomalous diffusion: unified power-law and stretched exponential exact solution
The nonlinear diffusion equation is analyzed here, where , and , and are real parameters.
This equation unifies the anomalous diffusion equation on fractals ()
and the spherical anomalous diffusion for porous media (). Exact
point-source solution is obtained, enabling us to describe a large class of
subdiffusion (), normal diffusion () and
superdiffusion (). Furthermore, a thermostatistical basis
for this solution is given from the maximum entropic principle applied to the
Tsallis entropy.Comment: 3 pages, 2 eps figure
Mirror Energy Differences at Large Isospin Studied through Direct Two-Nucleon Knockout
The first spectroscopy of excited states in 52Ni (Tz=2) and 51Co (Tz=-3/2)
has been obtained using the highly selective two-neutron knockout reaction.
Mirror energy differences between isobaric analogue states in these nuclei and
their mirror partners are interpreted in terms of isospin nonconserving
effects. A comparison between large scale shell-model calculations and data
provides the most compelling evidence to date that both electromagnetic and an
additional isospin nonconserving interactions for J=2 couplings, of unknown
origin, are required to obtain good agreement.Comment: Accepted for publication in Physical Review Letter
Numerical model for granular compaction under vertical tapping
A simple numerical model is used to simulate the effect of vertical taps on a
packing of monodisperse hard spheres. Our results are in agreement with an
experimantal work done in Chicago and with other previous models, especially
concerning the dynamics of the compaction, the influence of the excitation
strength on the compaction efficiency, and some ageing effects. The principal
asset of the model is that it allows a local analysis of the packings. Vertical
and transverse density profiles are used as well as size and volume
distributions of the pores. An interesting result concerns the appearance of a
vertical gradient in the density profiles during compaction. Furthermore, the
volume distribution of the pores suggests that the smallest pores, ranging in
size between a tetrahedral and an octahedral site, are not strongly affected by
the tapping process, in contrast to the largest pores which are more sensitive
to the compaction of the packing.Comment: 8 pages, 15 figures (eps), to be published in Phys. Rev. E. Some
corrections have been made, especially in paragraph IV
Recent direct reaction experimental studies with radioactive tin beams
Direct reaction techniques are powerful tools to study the single-particle
nature of nuclei. Performing direct reactions on short-lived nuclei requires
radioactive ion beams produced either via fragmentation or the Isotope
Separation OnLine (ISOL) method. Some of the most interesting regions to study
with direct reactions are close to the magic numbers where changes in shell
structure can be tracked. These changes can impact the final abundances of
explosive nucleosynthesis. The structure of the chain of tin isotopes is
strongly influenced by the Z=50 proton shell closure, as well as the neutron
shell closures lying in the neutron-rich, N=82, and neutron-deficient, N=50,
regions. Here we present two examples of direct reactions on exotic tin
isotopes. The first uses a one-neutron transfer reaction and a low-energy
reaccelerated ISOL beam to study states in 131Sn from across the N=82 shell
closure. The second example utilizes a one-neutron knockout reaction on
fragmentation beams of neutron-deficient 106,108Sn. In both cases, measurements
of gamma rays in coincidence with charged particles proved to be invaluable.Comment: 11 pages, 5 figures, Zakopane Conference on Nuclear Physics "Extremes
of the Nuclear Landscape", Zakopane, Poland, August 31 - September 7, 201
Nodal dynamics, not degree distributions, determine the structural controllability of complex networks
Structural controllability has been proposed as an analytical framework for
making predictions regarding the control of complex networks across myriad
disciplines in the physical and life sciences (Liu et al.,
Nature:473(7346):167-173, 2011). Although the integration of control theory and
network analysis is important, we argue that the application of the structural
controllability framework to most if not all real-world networks leads to the
conclusion that a single control input, applied to the power dominating set
(PDS), is all that is needed for structural controllability. This result is
consistent with the well-known fact that controllability and its dual
observability are generic properties of systems. We argue that more important
than issues of structural controllability are the questions of whether a system
is almost uncontrollable, whether it is almost unobservable, and whether it
possesses almost pole-zero cancellations.Comment: 1 Figures, 6 page
Global Equation of State of two-dimensional hard sphere systems
Hard sphere systems in two dimensions are examined for arbitrary density.
Simulation results are compared to the theoretical predictions for both the low
and the high density limit, where the system is either disordered or ordered,
respectively. The pressure in the system increases with the density, except for
an intermediate range of volume fractions , where a
disorder-order phase transition occurs. The proposed {\em global equation of
state} (which describes the pressure {\em for all densities}) is applied to the
situation of an extremely dense hard sphere gas in a gravitational field and
shows reasonable agreement with both experimental and numerical data.Comment: 4 pages, 2 figure
Nonlinear anomalous diffusion equation and fractal dimension: Exact generalized gaussian solution
In this work we incorporate, in a unified way, two anomalous behaviors, the
power law and stretched exponential ones, by considering the radial dependence
of the -dimensional nonlinear diffusion equation where , ,
, and are real parameters and is a time-dependent
source. This equation unifies the O'Shaugnessy-Procaccia anomalous diffusion
equation on fractals () and the spherical anomalous diffusion for
porous media (). An exact spherical symmetric solution of this
nonlinear Fokker-Planck equation is obtained, leading to a large class of
anomalous behaviors. Stationary solutions for this Fokker-Planck-like equation
are also discussed by introducing an effective potential.Comment: Latex, 6 pages. To appear in Phys. Rev.
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