627 research outputs found
Triptycene Structure-Directing Agents in Aluminophosphate Synthesis
The synthesis of aluminophosphates is investigated using a number of triptycene-based organic structure-directing agents (OSDA). These OSDAs are designed to synthesize extra-large pore and/or large cavity-containing molecular sieves. Starting from the hydrophobic triptycene molecule, OSDAs are prepared by introducing three amine-based centers that can be charged either by protonation in the acidic aluminophosphate reaction media or through quaternization. VPI-5 is synthesized using these tripytycene OSDAs, and the OSDAs are occluded inside the pores. This synthesis marks the first time VPI-5 has been made as a single phase with an OSDA occluded inside the framework of the as-made material that is not removed by simple washing with water or other solvents. Additionally, several other aluminophosphates with unknown structures are synthesized using these new OSDAs
The thermal Casimir effect in lipid bilayer tubules
We calculate the thermal Casimir effect for a dielectric tube of radius
and thickness delta formed from a membrane in water. The method uses a
field-theoretic approach in the grand canonical ensemble. The leading
contribution to the Casimir free energy behaves as -k_BTL kappa_C/R giving rise
to an attractive force which tends to contract the tube. We find that kappa_C ~
0.3 for the case of typical lipid membrane t-tubules. We conclude that except
in the case of a very soft membrane this force is insufficient to stabilize
such tubes against the bending stress which tends to increase the radius.Comment: 4 pages no figures RevTe
Effect of symmetry energy on two-nucleon correlation functions in heavy-ion collisions induced by neutron-rich nuclei
Using an isospin-dependent transport model, we study the effects of nuclear
symmetry energy on two-nucleon correlation functions in heavy ion collisions
induced by neutron-rich nuclei. We find that the density dependence of the
nuclear symmetry energy affects significantly the nucleon emission times in
these collisions, leading to larger values of two-nucleon correlation functions
for a symmetry energy that has a stronger density dependence. Two-nucleon
correlation functions are thus useful tools for extracting information about
the nuclear symmetry energy from heavy ion collisions.Comment: Revised version, to appear in Phys. Rev. Let
Molecular Model of the Contractile Ring
We present a model for the actin contractile ring of adherent animal cells.
The model suggests that the actin concentration within the ring and
consequently the power that the ring exerts both increase during contraction.
We demonstrate the crucial role of actin polymerization and depolymerization
throughout cytokinesis, and the dominance of viscous dissipation in the
dynamics. The physical origin of two phases in cytokinesis dynamics ("biphasic
cytokinesis") follows from a limitation on the actin density. The model is
consistent with a wide range of measurements of the midzone of dividing animal
cells.Comment: PACS numbers: 87.16.Ka, 87.16.Ac
http://www.ncbi.nlm.nih.gov/pubmed/16197254
http://www.weizmann.ac.il/complex/tlusty/papers/PhysRevLett2005.pd
Intermittency and Exotic Channels
It is pointed out that accurate measurements of short-range two-particle
correlations in like-charge and in channels should be
very helpful in determining the origin of the \lq\lq intermittency\rq\rq\
phenomenon observed recently for the like-charge pion pairs.Comment: 5 p., plain tex, preprint T94/078(Saclay), LPTHE 94/58(Orsay
Relative space-time asymmetries in pion and nucleon production in non-central nucleus-nucleus collisions at high energies
We propose to use the ratio of the pion-proton correlation functions
evaluated under different conditions to study the relative space-time
asymmetries in pion and proton emission (pion and nucleon source relative
shifts) in high energy heavy ion collision. We address the question of the
non-central collisions, where the sources can be shifted spatially both in the
longitudinal and in the transverse directions in the reaction plane. We use the
RQMD event generator to illustrate the effect and the technique.Comment: RevTex, 4 pages, 3 figures included as eps file
Dynamic study on fusion reactions for Ca+Zr around Coulomb barrier
By using the updated improved Quantum Molecular Dynamics model in which a
surface-symmetry potential term has been introduced for the first time, the
excitation functions for fusion reactions of Ca+Zr at
energies around the Coulomb barrier have been studied. The experimental data of
the fusion cross sections for Ca+Zr have been reproduced
remarkably well without introducing any new parameters. The fusion cross
sections for the neutron-rich fusion reactions of Ca+Zr around
the Coulomb barrier are predicted to be enhanced compared with a
non-neutron-rich fusion reaction. In order to clarify the mechanism of the
enhancement of the fusion cross sections for neutron-rich nuclear fusions, we
pay a great attention to study the dynamic lowering of the Coulomb barrier
during a neck formation. The isospin effect on the barrier lowering is
investigated. It is interesting that the effect of the projectile and target
nuclear structure on fusion dynamics can be revealed to a certain extent in our
approach. The time evolution of the N/Z ratio at the neck region has been
firstly illustrated. A large enhancement of the N/Z ratio at neck region for
neutron-rich nuclear fusion reactions is found.Comment: 21 pages, 7 figures,3 table
Elastic energy of polyhedral bilayer vesicles
In recent experiments [M. Dubois, B. Dem\'e, T. Gulik-Krzywicki, J.-C.
Dedieu, C. Vautrin, S. D\'esert, E. Perez, and T. Zemb, Nature (London) Vol.
411, 672 (2001)] the spontaneous formation of hollow bilayer vesicles with
polyhedral symmetry has been observed. On the basis of the experimental
phenomenology it was suggested [M. Dubois, V. Lizunov, A. Meister, T.
Gulik-Krzywicki, J. M. Verbavatz, E. Perez, J. Zimmerberg, and T. Zemb, Proc.
Natl. Acad. Sci. U.S.A. Vol. 101, 15082 (2004)] that the mechanism for the
formation of bilayer polyhedra is minimization of elastic bending energy.
Motivated by these experiments, we study the elastic bending energy of
polyhedral bilayer vesicles. In agreement with experiments, and provided that
excess amphiphiles exhibiting spontaneous curvature are present in sufficient
quantity, we find that polyhedral bilayer vesicles can indeed be energetically
favorable compared to spherical bilayer vesicles. Consistent with experimental
observations we also find that the bending energy associated with the vertices
of bilayer polyhedra can be locally reduced through the formation of pores.
However, the stabilization of polyhedral bilayer vesicles over spherical
bilayer vesicles relies crucially on molecular segregation of excess
amphiphiles along the ridges rather than the vertices of bilayer polyhedra.
Furthermore, our analysis implies that, contrary to what has been suggested on
the basis of experiments, the icosahedron does not minimize elastic bending
energy among arbitrary polyhedral shapes and sizes. Instead, we find that, for
large polyhedron sizes, the snub dodecahedron and the snub cube both have lower
total bending energies than the icosahedron
Small size boundary effects on two-pion interferometry
The Bose-Einstein correlations of two identically charged pions are derived
when these particles, the most abundantly produced in relativistic heavy ion
collisions, are confined in finite volumes. Boundary effects on single pion
spectrum are also studied. Numerical results emphasize that conventional
formulation usually adopted to describe two-pion interferometry should not be
used when the source size is small, since this is the most sensitive case to
boundary effects. Specific examples are considered for better illustration.Comment: more discussion on Figure4 and diffuse boundar
The field theory of symmetrical layered electrolytic systems and the thermal Casimir effect
We present a general extension of a field-theoretic approach developed in
earlier papers to the calculation of the free energy of symmetrically layered
electrolytic systems which is based on the Sine-Gordon field theory for the
Coulomb gas. The method is to construct the partition function in terms of the
Feynman evolution kernel in the Euclidean time variable associated with the
coordinate normal to the surfaces defining the layered structure. The theory is
applicable to cylindrical systems and its development is motivated by the
possibility that a static van der Waals or thermal Casimir force could provide
an attractive force stabilising a dielectric tube formed from a lipid bilayer,
an example of which are t-tubules occurring in certain muscle cells. In this
context, we apply the theory to the calculation of the thermal Casimir effect
for a dielectric tube of radius and thickness formed from such a
membrane in water. In a grand canonical approach we find that the leading
contribution to the Casimir energy behaves like which gives
rise to an attractive force which tends to contract the tube radius. We find
that for the case of typical lipid membrane t-tubules. We
conclude that except in the case of a very soft membrane this force is
insufficient to stabilise such tubes against the bending stress which tend to
increase the radius. We briefly discuss the role of lipid membrane reservoir
implicit in the approach and whether its nature in biological systems may
possibly lead to a stabilising mechanism for such lipid tubes.Comment: 28 pages, 2 figures, LaTe
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